Architecture and Programming of 8051 Micro Controllers

Architecture and Programming of 8051 Microcontrollers | Chapter 1 : I... http://www.mikroe.com/en/books/8051book/ch1/

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Chapter1: Introduction to Microcontrollers

Introduction

1.1 What are microcontrollers and what are they used for?

1.2 What is what in microcontroller?

Introduction

It was electricity in the beginning....The people were happy because they did not know that it was existing all

around them and that it could be utilized. It was fine. And then Faraday came and the stone has started rolling

slowly...

During the time, the first machines using a new sort of energy appeared accompanied with people who knew

something about electricity. A long time has passed since then and just when civilization got used to this

innovation and stopped paying attention to what a new generation of specialists were doing, someone came up with

an idea that electrons could be a very convenient toy being closed in a glass pipe. At first sight, it was only a good

idea, but there was no return, electonics was born and the stone continued rolling down the hill faster and faster...

A new science - new specialists. The blue coats were replaced with white ones and people who knew something

about electronics appeared on stage. While the rest of the humanity were passively watching in disbelief what was

going on, within plotters two fractions appeared- “software-oriented” and “hardware-oriented”. A bit younger than

their teachers, very enthusiastic and full of ideas, both of them kept on working but separate ways. While the first

group had a stable development, hardware-oriented people, driven by success, soon threw caution to the wind and

invented transistor.

Up till that moment, the things could be more or less kept under control, but broad publicity was not aware of

seriousness of the situation and it soon led to a fatal mistake! Being naive in belief that using cheap tricks could

slow down technology development as well as developing of the world, mass market opened its door to the

products of Electronics Industry, closing a magic circle therefore. Components’ prices fell rapidly becoming

available for use to younger population. The stone was falling freely...

The first integrated circuits and processors have soon appeared, which enabled for computers to drop down in

price. The computers have started to develop their own production. The prices droped down again and Electonics

got new adherents. It appeared everywhere. Another circle has been closed! Ordinary people got hold of computers

and computer era has begun...

While this drama was going on, hobbyists and professionals, protected by anonymity, were working hard on their

projects , although split in two big groups. Then, someone has again remembered: Why should not we make an

universal component? It would be a cheap and universal integrated circuit that could be programmed and used in

any field of electronics, device and wherever needed. Technology has been developed enough, market exists, why

not? So it happened, body and spirit are united, the circuit is created and called MICROCONTROLLER.

1.1 What are microcontrollers and what are they used for?

As all other good things, this powerful component is basically very simple and is obtained by uniting tested and

high- quality "ingredients" (components) as per following receipt:

The simplest computer’s processor is used as a "brain" of the future system.1.

Depending on the taste of the producer, it is added : a bit of memory, a few A/D converters, timers,

input/output lines etc.

2.

It is all placed in one of standard packages.3.

A simple software that will be able to control it all and about which everyone will be able to learn has been

developed.

4.

Three things have had a crucial impact on such a success of the microcontrollers:

Table of Contents Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7


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Powerful and intelligently chosen electronics embedded in the microcontrollers can via input/output devices

( switches, push buttons, sensors, LCD displays, relays…) control various processes and devices such as:

industrial automatics, electric current, temperature, engine performance etc.

A very low price enables them to be embedded in such devices in which, until recent time it was not worth

embedding anything. Thanks to that, the world is overwhelmed today with cheap automatic devices and

various “intelligent” appliences.

Prior knowledge is hardly needed for programming. It is sufficient to have any kind of PC (software in use is

not demanding at all and it is easy to learn to work on it) and one simple device (programmer) used for

“transffering” completed programs into the microcontroller.

Therefore, if you are infected with a virus called electronics, there is nothing left for you to do but to learn how to

control its power and how to direct it at the right course.

How does microcontroller operate?

Even though there is a great number of various microcontrollers and even greater number of programs designed for

the microcontrollers’ use only, all of them have many things in common. That means that if you learn to handle one

of them you will be able to handle them all. A typical scenario on whose basis it all functions is as follows:

Power supply is turned off and everything is so still…chip is programmed, every thing is in place, nothing

indicates what is to come…

1.

Power supply connectors are connected to the power supply source and every thing starts to happen at high

speed! The control logic registers what is going on first. It enables only quartz oscillator to operate. While

the first preparations are in progress and parasite capacities are being charged, the first milliseconds go by.

2.

Power supply connectors are connected to the power supply source and every thing starts to happen at high

speed! The control logic registers what is going on first. It enables only quartz oscillator to work. While the

first preparations are in progress and parasite capacities are being charged, the first milliseconds go by.

3.

Voltage level has reached its full value and frequency of oscillator has became stable. The bits are being

written to the SFRs, showing the state of all periph erals and all pins are configured as outputs. Everything

occurs in harmony to the pulses’ rhythm and the overall electronis starts operating. Since this moment the

time is measured in micro and nanoseconds.

4.

Program Counter is reset to zero address of the program memory. Instruction from that address is sent to

instruction decoder where its meaning is recognised and it is executed with immediate effect.

5.

The value of the Program Counter is being incremented by 1 and the whole process is being

repeated...several million times per second.

6.


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1.2 What is what in microcontroller?

Obviously, everything that occurs in the microcontroller occurs at high speed and quite simple, but it would not be

so useful if there are no special interfaces which make it complete. Text below refers to that (in short).

Program Memory (ROM)

The Program Memory is a type of memory which permanently stores a program being executed. Obviously, the

maximal length of the program that can be written to depends on the size of the memory. Program memory can be

built in the microcontroller or added from outside as a separate chip, which depends on type of the microcontroller.

Both variants have advantages and disadvantages: if added from outside, the microcontroller is cheaper and

program can be considerably longer. At the same time, a number of available pins is decremented as the

microcontroller uses its own input/output ports to be connected to the memory. The capacity of Internal Program

Memory is usually smaller and more expensive but such a chip has more possibilities of connecting to peripheral

environment. Program memory size ranges from 512B to 64KB.

Data Memory (RAM)

Data Memory is a type of memory used for temporary storing and keeping different data and constants created and

used during operating process . The content of this memory is erased once the power is off. For example: when the

program performes addition, it is necessary to have a register presenting what in everyday life is called “a sum” .

For that purpose one of the registers in RAM is named as such and serves for storing results of addition. Data

memory size goes up to a few KBs.

EEPROM Memory

The EEPROM Memory is a special type of memory which not all the types of the microcontrollers have.Its content

can be changed during program execution (similar to RAM ), but it is permanently saved even after the power goes

off (similar to ROM). It is used for storing different values created and used during operating process and which

must be saved upon turning off the device ( calibration values, codes, values to count up to etc.). A disadvantage of

this memory is that programming is relatively slow- measured in miliseconds.


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SFRs ( Special Function Registers )

SFRs are a particular part of memory whose purpose is defined in advance by the producer. Each of these registers

have its name and control some of interfaces within the microcontroller. For example: by writing zero or one to the

SFR controlling some input/output port, each of the port pins can be configured as input or output (each bit in this

register controls the purpose of one single pin).

Program Counter

Program Counter is an engine which starts the program and indicates the address in memory where next instruction

to execute is found. Immediately after its execution, the value of the counter is incremented by 1. For this

automatic increment, the program executes one instruction at a time as it is written. However…the program

counter value could be changed at any moment, which will cause “jump” to a new location in the program memory.

This is how subroutines or branch instructions are executed. When finding its new place in the program, the

counter resumes even automatic counting +1, +1, +1…

CPU (Central Processor Unit)

As its name tells, this is "Big Brother" who monitors and controls all operations being performed within the

microcontroller and the user cannot affect its work. It consists of several smaller units. The most important are:

Instruction decoder - a part of electronics which recognizes program instructions and on the basis of which

runs other circuits.

Arithmetical Logical Unit (ALU) - performs all mathematical and logical operations with data. The features


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of this circuit are described in the "instruction set" which differs for each type of the microcontroller..

Accumulator - is a special type of the SFR closely related to operating mode of the ALU. It is a kind of desk

on which all data needed to perform some operation on are set (addition, shift etc.). It also contains a result,

ready to be used further in operation. One of the SFRs, called the Status Register, is closely related to the

accumulator, showing at any time the "status" of a number being in the accumulator (the number is greater

than or less than zero etc.).

Bit - a word invented to confuse people who start handling electronics. In practice ( only in practice) this word

indicates whether the voltage is applied to an electrical conductor or not. In the first case, a logical one is present

on the pin, i.e. the bit’s value is 1. Otherwise, if the voltage level is 0 V, i.e. a logical zero is present on the pin, the

bit’s value is 0. It is more complicated in theory where the bit is actually a digit in a binary system, whereas, a bit

is just a bit whose value amounts 0 or 1 (in decade system we are used to the digits’ value amounting 0, 1 , 2 , 3 ,

…..8 or 9).

Input/output ports (I/O Ports)

The microcontroller cannot be of any use without being connected to peripheral devices. For that reason each

microcontroller has one or more registers connected to its pins (called ports in this case).

Why input/output? Because the

user can change pin’s role

according to his/her own needs.

These are, in fact, the only

registers in the microcontroller

whose state can be checked by

voltmeter !

Oscillator

The oscillator can be compared

with rhythm section of a mini

orchestra. Equalized pulses coming

from this circuit enable

harmonious and synchronic

operating of all other parts of the

microcontroller. It is commonly

configured so as to use

quartz-crystal or ceramics

resonator for frequency

stabilization. Besides, it can often

operate without elements for

frequency stabilization ( like RC

oscillator). It is important to know

that instructions are not executed at

the rate ordered by oscillator but

several times slower. The reason for this is that each instruction is

executed in several steps (In some microcontrollers execution

time of all instructions is equal, while in others microcontrollers

execution time differs for different instructions). Consequently, if

your system uses quartz-crystal of 20MHz, execution time of a

program instruction is not 50nS but 200, 400 or even 800 nS!

Timers/Counters

Most programs use in some way these miniature electronic


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"stopwatches". They are mostly 8- or 16-bit SFRs whose value is automatically incremented with each coming

pulse. Once the register is completely "filled up"- an interrupt is generated !

If the registers use internal oscillator for its operating then it is possible to measure the time between two events ( if

the register value is T1 at the moment measuring has started, and T2 at the moment measuring has finished, then

the time that has passed is equal to the value gained by their subtruction T2-T1 ). If the registers for its operating

use pulses coming from external source then such a timer is converted to counter.

This is a very simple explanation used to describe the essence of the operating. It’s a bit more complicated in

practice.

Register is another name for a memory cell. Beside 8 bits available to the user, each register has also addressing

part usually not visible to the user. It is important to know:

All registers in ROM as well as those in RAM memory identified as general-purpose registers are mutually

equal and nameless. During programming, each register can be assigned a name, which makes operating

much easier.

All SFRs have their own names which are different for different types of the microcontrollers and each of

them has a particular role.

Watchdog timer

Its name tells a lot about its purpose. Watchdog Timer is a timer connected to a particular and totally independent

RC oscillator within the microcontroller.

If enabled to operate, every time it "counts up to end", the microcontroller is reset and program execution starts

from the first instruction. The to keep this from happening by using particular command. The whole idea is based

on the fact that every program circulates, in other words, the program is executed in several longer or shorter loops.

If the instructions which resets the value of the watchdog timer are set at some important program locations,


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besides commands being regularly executed, then the operation of the watchdog timer will not be noticed.

If for any reason (usually electrical disturbances in industry), the program counter "gets stuck" at memory location

from where there is no return, the register’s value being steadily incremented by the watchdog timer will reach the

maximum et voila ! Reset occurs!

Power Supply Circuit

Two things within the circuit that take care of the microcontroller power supply are worth attention :

Brown out is potentially dangerous state

coming up at the moment the

microcontroller is being turned off or in

situations when due to powerful

disturbances, voltage supply comes to the

lowest limit. As the microcontroller

consists of several circuits which have

different operating voltage levels, this can

cause its "out of control" performance. In

order to prevent that, a circuit for brown

out reset is usually embedded. When the

voltage level drops below the lower limit

then this circuit immediately resets the

whole electronics.

Reset pin is usually identified as MCLR

(Master Clear Reset) and serves for "external" reset of the microcontroller by applying logical zero or one

depending on type of the microcontroller. In case the brown out is not embedded a simple external circuit for

brown out reset can be connected to this pin.

Serial communication

Connection between the

microcontroller and peripheral

devices established through I/O

ports is an ideal solution for shorter

distances- up to several meters.

But, when it is needed to enable

communication between two

devices on longer distances or

when for any other reason it is not

possible to use "parallel"

connection ( for example remote control of the aircraft ) it is obvious that something so simple cannot be taken into

account. In such and similar situations, communication through pulses, called serial communication is the most

appropriate to use.

Serial communication problem has been resolved a long time ago and nowadays several different systems enabling

this kind of connection are embedded as a standard equipment into most microcontroller. Which of them will be

used in very situation depends on several factors. The most important are the following:

With how many devices the microcontroller must exchange data ?

How fast the serial communication must be?

What is the distance between devices?

Is there any need to transmit and receive data simultaneously ?

One of the most important things concerning the use of serial communication is to strictly observe the Protocol. It

is a set of rules which must be applied in order to enable devices to recognize the data being exchanged.

Fortunately, the microcontrollers automatically take care of it, which leads to a reduction of the programmer’s

work to simple writing and reading data.


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Byte - 8 bits next to each other make entity called a program word or a byte. If the bit is a digit then it is logical

that bytes are numbers. All mathematical operations can be performed upon them, just like with usual decimal

numbers and they are performed in the ALU. It is important to note that byte ( as each number) has “two sides”,

i.e. digits a byte consists of are not of equal significance. The highest value has a digit on the far left called the

most significant bit ( MSB). A digit on the far right has the least value and is called the least significant bit (LSB).

As 8 digits can be combined in 256 different ways, the greatest decimal number that can present one byte is 255 (

zero is also presented with one combination ).

Program

Unlike other integrated circuits which only need to be connected to other components and then powered on, the

microcontrollers need to be programmed too prior to turning the power on. This is so called "a bitter pill" and the

main reason why hardware-oriented electronics engineers mainly avoid the microcontrollers. It is a trap causing

huge losses because the microcontrollers programming is in fact very simple.

In order to write a program for running microcontrollers, several "low-level" program languages for programming

computers can be used – Assembler , C and Basic ( and their versions ). Besides, writing program procedure

consists of simple giving instructions in order in which they should be executed. There are also many programs

operating in Windows environment used to facilitate work and provide additional- visual tools.

The use of Assembler is described in this book because it is the simplest language with the fastest execution

allowing entire control on what is going on in the circuit.

Interrupt - electronics is usually more faster than physical process in environment it should keep under control.

That’s why the microcontroller spend the most of its time waiting for something to happen or execute.In order to

avoid continuous checking for logical state on input pins and in internal registers, the interrupt is generated. It is a

signal interrupting regular program execution. Since several events can cause interrupt, when it occurs, the

microcontroller immediately stops operating and checks for the cause. If it is needed to perform some action, a

current state of the program counter is pushed on the Stack and the appropriate program is executed (so called

interrupt routine ).

Stack is a part of RAM used for storing the current state of the program counter (address).This address lets the

controller know where to return after the subroutine has been executed. Stack can consist of several levels. This

enables subroutines’ nesting, i.e. calling one subroutine from another.


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Architecture and Programming of 8051 Microcontrollers | Chapter 2 : ... http://www.mikroe.com/en/books/8051book/ch2/

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Chapter 2 : 8051 Microcontroller Architecture

2.1 What is 8051 Standard?

2.2 8051 Microcontroller's pins

2.3 Input/Output Ports (I/O Ports)

2.4 8051 Microcontroller Memory Organisation

2.5 SFRs (Special Function Registers)

2.6 Counters and Timers

2.7 UART (Universal Asynchronous Receiver and Transmitter)

2.8 8051 Microcontroller Interrupts

2.9 8051 Microcontroller Power Consumption Control

2.1 What is 8051 Standard?

Microcontrollers’ producers have been struggling for a long time for attracting more and more choosy customers. Every

couple of days a new chip with a higher operating frequency, more memory and more high-quality A/D converters comes

on the market.

Nevertheless, by analyzing their structure it is concluded that most of them have the same (or at least very similar)

architecture known in the product catalogs as “8051 compatible”. What is all this about?

The whole story began in the far 80s when Intel launched its series of the microcontrollers labelled with MCS 051.

Although, several circuits belonging to this series had quite modest features in comparison to the new ones, they took over

the world very fast and became a standard for what nowadays is ment by a word microcontroller.

The reason for success and such a big popularity is a skillfully chosen configuration which satisfies needs of a great number

of the users allowing at the same time stable expanding ( refers to the new types of the microcontrollers ). Besides, since a

great deal of software has been developed in the meantime, it simply was not profitable to change anything in the

microcontroller’s basic core. That is the reason for having a great number of various microcontrollers which actually are

solely upgraded versions of the 8051 family. What is it what makes this microcontroller so special and universal so that

almost all the world producers manufacture it today under different name ?

As shown on the previous picture, the 8051 microcontroller has nothing impressive at first sight:

4 Kb program memory is not much at all.

128Kb RAM (including SFRs as well) satisfies basic needs, but it is not imposing amount.

4 ports having in total of 32 input/output lines are mostly enough to make connection to peripheral environment and

are not luxury at all.

As it is shown on the previous picture, the 8051 microcontroller have nothing impressive at first sight:



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The whole configuration is obviously envisaged as such to satisfy the needs of most programmers who work on

development of automation devices. One of advantages of this microcontroller is that nothing is missing and nothing is too

much. In other words, it is created exactly in accordance to the average user‘s taste and needs. The other advantage is the

way RAM is organized, the way Central Processor Unit (CPU) operates and ports which maximally use all recourses and

enable further upgrading.

2.2 8051 Microcontroller's pins

Pins 1-8: Port 1 Each of these pins can be configured as input or output.

Pin 9: RS Logical one on this pin stops microcontroller’s operating and erases the contents of most registers. By applying

logical zero to this pin, the program starts execution from the beginning. In other words, a positive voltage pulse on this pin

resets the microcontroller.

Pins10-17: Port 3 Similar to port 1, each of these pins can serve as universal input or output . Besides, all of them have

alternative functions:

Pin 10: RXD Serial asynchronous communication input or Serial synchronous communication output.

Pin 11: TXD Serial asynchronous communication output or Serial synchronous communication clock output.

Pin 12: INT0 Interrupt 0 input

Pin 13: INT1 Interrupt 1 input

Pin 14: T0 Counter 0 clock input

Pin 15: T1 Counter 1 clock input

Pin 16: WR Signal for writing to external (additional) RAM

Pin 17: RD Signal for reading from external RAM

Pin 18, 19: X2, X1 Internal oscillator input and output. A quartz crystal which determines operating frequency is usually

connected to these pins. Instead of quartz crystal, the miniature ceramics resonators can be also used for frequency

stabilization. Later versions of the microcontrollers operate at a frequency of 0 Hz up to over 50 Hz.

Pin 20: GND Ground

Pin 21-28: Port 2 If there is no intention to use external memory then these port pins are configured as universal

inputs/outputs. In case external memory is used then the higher address byte, i.e. addresses A8-A15 will appear on this port.

It is important to know that even memory with capacity of 64Kb is not used ( i.e. note all bits on port are used for memory

addressing) the rest of bits are not available as inputs or outputs.

Pin 29: PSEN If external ROM is used for storing program then it has a logic-0 value every time the microcontroller

reads a byte from memory.

Pin 30: ALE Prior to each reading from external memory, the microcontroller will set the lower address byte (A0-A7) on

P0 and immediately after that activates the output ALE. Upon receiving signal from the ALE pin, the external register

(74HCT373 or 74HCT375 circuit is usually embedded ) memorizes the state of P0 and uses it as an address for memory

chip. In the second part of the microcontroller’s machine cycle, a signal on this pin stops being emitted and P0 is used now

for data transmission (Data Bus). In this way, by means of only one additional (and cheap) integrated circuit, data

multiplexing from the port is performed. This port at the same time used for data and address transmission.

Pin 31: EA By applying logic zero to this pin, P2 and P3 are used for data and address transmission with no regard to

whether there is internal memory or not. That means that even there is a program written to the microcontroller, it will not

be executed, the program written to external ROM will be used instead. Otherwise, by applying logic one to the EA pin, the

microcontroller will use both memories, first internal and afterwards external (if it exists), up to end of address space.

Pin 32-39: Port 0 Similar to port 2, if external memory is not used, these pins can be used as universal inputs or outputs.

Otherwise, P0 is configured as address output (A0-A7) when the ALE pin is at high level (1) and as data output (Data Bus),

when logic zero (0) is applied to the ALE pin.

Pin 40: VCC Power supply +5V


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2.3 Input/Output Ports (I/O Ports)

All 8051 microcontrollers have 4 I/O ports, each consisting of 8 bits which can be configured as inputs or outputs. This

means that the user has on disposal in total of 32 input/output lines connecting the microcontroller to peripheral devices.

A logic state on a pin determines whether it is configured as input or output: 0=output, 1=input. If a pin on the

microcontroller needs to be configured as output, then a logic zero (0) should be applied to the appropriate bit on I/O port.

In this way, a voltage level on the appropriate pin will be 0.

Similar to that, if a pin needs to be configured as input, then a logic one (1) should be applied to the appropriate port. In

this way, as a side effect a voltage level on the appropriate pin will be 5V (as it is case with any TTL input). This may

sound a bit confusing but everything becomes clear after studying a simplified electronic circuit connected to one I/O pin.


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Input/Output (I/O) pin

This is a simplified overview of what is connected to a

pin inside the microcontroller. It concerns all pins except

those included in P0 which do not have embedded pullup

resistor.

Output pin

A logic zero (0) is applied to a bit in the P register. By

turning output FE transistor on, the appropriate pin is

directly connected to ground.

Input pin

A logic one (1) is applied to a bit in the P register.

Output FE transistor is turned off. The appropriate pin

remains connected to voltage power supply through a

pull-up resistor of high resistance.

A logic state (voltage) on any pin can be changed or read at any moment. A logic zero (0) and logic one (1) are not equal.

A logic one (0) represents almost short circuit to ground. Such a pin is configured as output.

A logic one (1) is “loosely” connected to voltage power supply through resistors of high resistance. Since this voltage can

be easily “pulled down” by an external signal, such a pin is configured as input.

Port 0

It is specific to this port to have a double purpose. If external memory is used then the lower address byte (addresses

A0-A7) is applied on it. Otherwise, all bits on this port are configured as inputs or outputs.

Another characteristic is expressed when it is configured as output. Namely, unlike other ports consisting of pins with

embedded pull-up resistor ( connected by its end to 5 V power supply ), this resistor is left out here. This, apparently little

change has its consequences:

If any pin on this port is configured as input then it performs as if

it “floats”. Such input has unlimited input resistance and has no

voltage coming from “inside”.


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When the pin is configured as output, it performs as “open drain”,

meaning that by writing 0 to some port’s bit, the appropriate pin

will be connected to ground (0V). By writing 1, the external output

will keep on “floating”. In order to apply 1 (5V) on this output, an

external pull-up resistor must be embedded.

Only in case P0 is used

for addressing external

memory ( only in that

case), the microcontroller will provide internal

power supply source in order to establish logical

ones on pins. There is no need to add external

pullup resistors.

Port 1

This is a true I/O port, because there are no role assigning as it is the case with P0. Since it has embedded pull-up resistors

it is completely compatible with TTL circuits.

Port 2

Similar to P0, when using external memory, lines on this port occupy addresses intended for external memory chip. This

time it is the higher address byte with addresses A8-A15. When there is no additional memory, this port can be used as

universal input-output port similar by its features to the port 1.

Port 3

Even though all pins on this port can be used as universal I/O port, they also have an alternative function. Since each of

these functions use inputs, then the appropriate pins have to be configured like that. In other words, prior to using some of

reserve port functions, a logical one (1) must be written to the appropriate bit in the P3 register. From hardware’s

perspective , this port is also similar to P0, with the difference that its outputs have a pull-up resistor embedded.

Current limitations on pins

When configured as outputs ( logic zero (0) ), single port pins can "receive" current of 10mA. If all 8 bits on a port are

active, total current must be limited to 15mA (port P0: 26mA). If all ports (32 bits) are active, total maximal current must

be limited to 71mA. When configured as inputs (logic 1), embedded pull-up resistor provides very weak current, but strong

enough to activate up to 4 TTL inputs from LS series.

It may be seen from description of some ports, that even though all pins have more or less similar internal structure, it is

necessary to pay attention to which of them will be used for what and how.

For example: If they are used as outputs with high voltage level (5V), then port 0 should be avoided because its pins do

not have added resistor for connection to +5V. Only low logic level can be obtained therefore, if another port is used for

the same purpose, one should have in mind that pull-up resistors have a relatively high resistance. Consequentaly it can be

counted on only several hundreds microamperes of current coming out of a pin.

2.4 8051 Microcontroller Memory Organisation

The microcontroller memory is divided into Program Memory and Data Memory. Program Memory (ROM) is used for

permanent saving program being executed, while Data Memory (RAM) is used for temporarily storing and keeping

intermediate results and variables. Depending on the model in use ( still referring to the whole 8051 microcontroller family)

at most a few Kb of ROM and 128 or 256 bytes of RAM can be used. However…

All 8051 microcontrollers have 16-bit addressing bus and can address 64 kb memory. It is neither a mistake nor a big

ambition of engineers who were working on basic core development. It is a matter of very clever memory organization

which makes these controllers a real “ programmers’ tidbit“ .

Program Memory


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The oldest models of the 8051 microcontroller family did not have internal program memory . It was added from outside as

a separate chip. These models are recognizable by their label beginning with 803 ( for ex. 8031 or 8032). All later models

have a few Kbytes ROM embedded, Even though it is enough for writing most of the programs, there are situations when

additional memory is necessary. A typical example of it is the use of so called lookup tables. They are used in cases when

something is too complicated or when there is no time for solving equations describing some process. The example of it can

be totally exotic (an estimate of self-guided rockets’ meeting point) or totally common( measuring of temperature using

non-linear thermo element or asynchronous motor speed control). In those cases all needed estimates and approximates are

executed in advance and the final results are put in the tables ( similar to logarithmic tables ).

How does the microcontroller handle external memory depends on the pin EA logic state:

EA=0 In this case, internal program memory is completely ignored, only a program stored in external memory is to be

executed.

EA=1 In this case, a program from builtin ROM is to be executed first ( to the last location). Afterwards, the execution is

continued by reading additional memory.

in both cases, P0 and P2 are not available to the user because they are used for data nd address transmission. Besides, the


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pins ALE and PSEN are used too.

Data Memory

As already mentioned, Data Memory is used for temporarily storing and keeping data and intermediate results created and

used during microcontroller’s operating. Besides, this microcontroller family includes many other registers such as:

hardware counters and timers, input/output ports, serial data buffers etc. The previous versions have the total memory size

of 256 locations, while for later models this number is incremented by additional 128 available registers. In both cases,

these first 256 memory locations (addresses 0-FFh) are the base of the memory. Common to all types of the 8051

microcontrollers. Locations available to the user occupy memory space with addresses from 0 to 7Fh. First 128 registers

and this part of RAM is divided in several blocks.

The first block consists of 4 banks each including 8 registers designated as R0 to R7. Prior to access them, a bank

containing that register must be selected. Next memory block ( in the range of 20h to 2Fh) is bit- addressable, which means

that each bit being there has its own address from 0 to 7Fh. Since there are 16 such registers, this block contains in total of

128 bits with separate addresses (The 0th bit of the 20h byte has the bit address 0 and the 7th bit of th 2Fh byte has the bit

address 7Fh). The third group of registers occupy addresses 2Fh-7Fh ( in total of 80 locations) and does not have any

special purpose or feature.

Additional Memory Block of Data Memory

In order to satisfy the programmers’ permanent hunger for Data Memory, producers have embedded an additional memory

block of 128 locations into the latest versions of the 8051 microcontrollers. Naturally, it’s not so simple…The problem is

that electronics performing addressing has 1 byte (8 bits) on disposal and due to that it can reach only the first 256

locations. In order to keep already existing 8-bit architecture and compatibility with other existing models a little trick has

been used.

Using trick in this case means that additional memory block shares the same addresses with existing locations intended for

the SFRs (80h- FFh). In order to differentiate between these two physically separated memory spaces, different ways of

addressing are used. A direct addressing is used for all locations in the SFRs, while the locations from additional RAM are

accessible using indirect addressing.


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How to extend memory?

In case on-chip memory is not enough, it is possible to add two external memory chips with capacity of 64Kb each. I/O

ports P2 and P3 are used for their addressing and data transmission.


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From the users’ perspective, everything functions quite simple if properly connected because the most operations are

performed by the microcontroller itself. The 8051 microcontroller has two separate reading signals RD#(P3.7) and PSEN#.

The first one is activated byte from external data memory (RAM) should be read, while another one is activated to read

byte from external program memory (ROM). These both signals are active at logical zero (0) level. A typical example of

such memory extension using special chips for RAM and ROM, is shown on the previous picture. It is called Hardward

architecture.

Even though the additional memory is rarely used with the latest versions of the microcontrollers, it will be described here

in short what happens when memory chips are connected according to the previous scheme. It is important to know that the

whole process is performed automatically, i.e. with no intervention in the program.

When the program during execution encounters the instruction which resides in exter nal memory (ROM), the

microcontroller will activate its control output ALE and set the first 8 bits of address (A0-A7) on P0. In this way, IC

circuit 74HCT573 which "lets in" the first 8 bits to memory address pins is activated.

A signal on the pin ALE closes the IC circuit 74HCT573 and immediately afterwards 8 higher bits of address

(A8-A15) appear on the port. In this way, a desired location in addtional program memory is completely addressed.

The only thing left over is to read its content.

Pins on P0 are configured as inputs, the pin PSEN is activated and the microcon troller reads content from memory

chip. The same connections are used both for data and lower address byte.

Similar occurs when it is a needed to read some location from external Data Memory. Now, addressing is performed in the

same way, while reading or writing is performed via signals which appear on the control outputs RD or WR .

Addressing

While operating, processor processes data according to the program instructions. Each instruction consists of two parts.

One part describes what should be done and another part indicates what to use to do it. This later part can be data (binary


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number) or address where the data is stored. All 8051 microcontrollers use two ways of addressing depending on which

part of memory should be accessed:

Direct Addressing

On direct addressing, a value is obtained from a memory location while the address of that location is specified in

instruction. Only after that, the instruction can process data (howdepends on the type of instruction: addition, subtraction,

copy…). Obviously, a number being changed during operating a variable can reside at that specified address. For example:

Since the address is only one byte in size ( the greatest number is 255), this is how only the first 255 locations in RAM can

be accessed in this case the first half of the basic RAM is intended to be used freely, while another half is reserved for the

SFRs.

MOV A,33h; Means: move a number from address 33 hex. to accumulator

Indirect Addressing

On indirect addressing, a register which contains address of another register is specified in the instruction. A value used in

operating process resides in that another register. For example:

Only RAM locations available for use are accessed by indirect addressing (never in the SFRs). For all latest versions of the

microcontrollers with additional memory block ( those 128 locations in Data Memory), this is the only way of accessing

them. Simply, when during operating, the instruction including “@” sign is encountered and if the specified address is

higher than 128 ( 7F hex.), the processor knows that indirect addressing is used and jumps over memory space reserved for

the SFRs.

MOV A,@R0; Means: Store the value from the register whose address is in the R0 register

into accumulator

On indirect addressing, the registers R0, R1 or Stack Pointer are used for specifying 8-bit addresses. Since only 8 bits are

avilable, it is possible to access only registers of internal RAM in this way (128 locations in former or 256 locations in

latest versions of the microcontrollers). If memory extension in form of additional memory chip is used then the 16-bit

DPTR Register (consisting of the registers DPTRL and DPTRH) is used for specifying addresses. In this way it is possible

to access any location in the range of 64K.


SFRs are a kind of control table used for running and monitoring microcontroller’s operating. Each of these registers, even

each bit they include, has its name, address in the scope of RAM and clearly defined purpose ( for example: timer control,

interrupt, serial connection etc.). Even though there are 128 free memory locations intended for their storage, the basic

core, shared by all types of 8051 controllers, has only 21 such registers. Rest of locations are intensionally left free in order

to enable the producers to further improved models keeping at the same time compatibility with the previous versions. It

also enables the use of programs written a long time ago for the microcontrollers which are out of production now.


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A Register (Accumulator)

This is a general-purpose register which serves for storing intermediate results during operating. A number (an operand)

should be added to the accumulator prior to execute an instruction upon it. Once an arithmetical operation is preformed by

the ALU, the result is placed into the accumulator. If a data should be transferred from one register to another, it must go

through accumulator. For such universal purpose, this is the most commonly used register that none microcontroller can be

imagined without (more than a half 8051 microcontroller's instructions used use the accumulator in some way).

B Register

B register is used during multiply and divide operations which can be performed only upon numbers stored in the A and B

registers. All other instructions in the program can use this register as a spare accumulator (A).

During programming, each of registers is called by name so that their exact address is not so

important for the user. During compiling into machine code (series of hexadecimal numbers

recognized as instructions by the microcontroller), PC will automatically, instead of registers’

name, write necessary addresses into the microcontroller.

R Registers (R0-R7)

This is a common name for the total 8

generalpurpose registers (R0, R1, R2 ...R7). Even

they are not true SFRs, they deserve to be

discussed here because of their purpose. The bank

is active when the R registers it includes are in use.

Similar to the accumulator, they are used for

temporary storing variables and intermediate

results. Which of the banks will be active depends

on two bits included in the PSW Register. These

registers are stored in four banks in the scope of


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RAM.

The following example best illustrates the useful

purpose of these registers. Suppose that

mathematical operations on numbers previously

stored in the R registers should be performed:

(R1+R2) - (R3+R4). Obviously, a register for

temporary storing results of addition is needed. Everything is quite simple and the program is as follows :

MOV A,R3; Means: move number from R3 into accumulator

ADD A,R4; Means: add number from R4 to accumulator (result remains in accumulator)

MOV R5,A; Means: temporarily move the result from accumulator into R5

MOV A,R1; Means: move number from R1 into accumulator

ADD A,R2; Means: add number from R2 to accumulator

SUBB A,R5; Means: subtract number from R5 ( there are R3+R4 )

PSW Register (Program Status Word)

This is one of the most important SFRs. The Program Status Word (PSW) contains several status bits that reflect the

current state of the CPU. This register contains: Carry bit, Auxiliary Carry, two register bank select bits, Overflow flag,

parity bit, and user-definable status flag. The ALU automatically changes some of register’s bits, which is usually used in

regulation of the program performing.

P - Parity bit. If a number in accumulator is even then this bit will be automatically set (1), otherwise it will be cleared (0).

It is mainly used during data transmission and receiving via serial communication.

- Bit 1. This bit is intended for the future versions of the microcontrollers, so it is not supposed to be here.

OV Overflow occurs when the result of arithmetical operation is greater than 255 (deci mal), so that it can not be stored in

one register. In that case, this bit will be set (1). If there is no overflow, this bit will be cleared (0).

RS0, RS1 - Register bank select bits. These two bits are used to select one of the four register banks in RAM. By writing

zeroes and ones to these bits, a group of registers R0-R7 is stored in one of four banks in RAM.

RS1 RS2 Space in RAM

0 0 Bank0 00h-07h

0 1 Bank1 08h-0Fh

1 0 Bank2 10h-17h

1 1 Bank3 18h-1Fh

F0 - Flag 0. This is a general-purpose bit available to the user.

AC - Auxiliary Carry Flag is used for BCD operations only.

CY - Carry Flag is the (ninth) auxiliary bit used for all arithmetical operations and shift instructions.

DPTR Register (Data Pointer)

These registers are not true ones because they do not physically exist. They consist of two separate registers: DPH (Data

Pointer High) and (Data Pointer Low). Their 16 bits are used for external memory addressing. They may be handled as a

16-bit register or as two independet 8-bit registers.Besides, the DPTR Register is usually used for storing data and

intermediate results which have nothing to do with memory locations.


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SP Register (Stack Pointer)

A value of the Stack Pointer ensures that the Stack Pointer will point to valid RAM and permits Stack availability. By

starting each subprogram, the value in the Stack Pointer is incremented by 1. In the same manner, by ending subprogram,

this value is decremented by 1. After any reset, the value 7 is written to the Stack Pointer, which means that the space of

RAM reserved for the Stack starts from this location. If another value is written to this register then the entire Stack is

moved to a new location in the memory.

P0, P1, P2, P3 - Input/Output Registers

In case that external memory and serial communication system are not in use then, 4 ports with in total of 32 input-output

lines are available to the user for connection to peripheral environment. Each bit inside these ports coresponds to the

appropriate pin on the microcontroller. This means that logic state written to these ports appears as a voltage on the pin ( 0

or 5 V). Naturally, while reading, the opposite occurs – voltage on some input pins is reflected in the appropriate port bit.

The state of a port bit, besides being reflected in the pin, determines at the same time whether it will be configured as input

or output. If a bit is cleared (0), the pin will be configured as output. In the same manner, if a bit is set to 1 the pin will be

configured as input. After reset, as well as when turning the microcontroller on , all bits on these ports are set to one (1).

This means that the appropriate pins will be configured as inputs.

Conditionally said, I/O ports are directly connected to the microcontroller’s pins. This means that a logic state of these

registers can be checked by voltmeter and vice versa-voltage on the pins can be checked by testing their bits!


As explained in the previous chapter, the main oscillator of the microcontroller uses quartz crystal for its operating. As the

frequency of this oscillator is precisely defined and very stable, these pulses are the most suitable for time measuring (such

oscillators are used in quartz clocks as well). In order to measure time between two events it is only needed to count up


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pulses from this oscillator. That is exactly what the timer is doing. Namely, if the timer is properly programmed, the value

written to the timer register will be incremented or decremented after each coming pulse, i.e. once per each machine cycle

cycle. Taking into account that one instruction lasts 12 quartz oscillator periods (one machine cycle), by embedding quartz

with oscillator frequency of 12MHz, a number in the timer register will be changed million times per second, i.e. each

microsecond.

The 8051 microcontrollers have 2 timer counters called T0 and T1. As their names tell, their main purpose is to measure

time and count external events. Besides, they can be used for generating clock pulses used in serial communication, i.e.

Baud Rate.

Timer T0

As it is shown in the picture below, this timer consists of two registers – TH0 and TL0. The numbers these registers include

represent a lower and a higher byte of one 16-digit binary number.

This means that if the content of the timer 0 is equal to 0 (T0=0) then both registers it includes will include 0. If the same

timer contains for example number 1000 (decimal) then the register TH0 (higher byte) will contain number 3, while TL0

(lower byte) will contain decimal number 232.

Formula used to calculate values in registers is very simple:

TH0 × 256 + TL0 = T

Matching the previous example it would be as follows :

3 × 256 + 232 = 1000

Since the timers are virtually 16-bit registers, the greatest value that could be written to them is 65 535. In case of

exceeding this value, the timer will be automatically reset and afterwords that counting starts from 0. It is called overflow.

Two registers TMOD and TCON are closely connected to this timer and control how it operates.

TMOD Register (Timer Mode)

This register selects mode of the timers T0 and T1. As illustrated in the following picture, the lower 4 bits (bit0 - bit3) refer

to the timer 0, while the higher 4 bits (bit4 - bit7) refer to the timer 1. There are in total of 4 modes and each of them is


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described here in this book.

Bits of this register have the following purpose:

GATE1 starts and stops Timer 1 by means of a signal provided to the pin INT1 (P3.3):

1 - Timer 1 operates only if the bit INT1 is set

0 - Timer 1 operates regardless of the state of the bit INT 1

C/T1 selects which pulses are to be counted up by the timer/counter 1:

1 - Timer counts pulses provided to the pin T1 (P3.5)

0 - Timer counts pulses from internal oscillator

T1M1,T1M0 These two bits selects the Timer 1 operating mode.

T1M1 T1M0 Mode Description

0 0 0 13-bit timer

0 1 1 16-bit timer

1 0 28-bit

auto-reload

1 1 3 Split mode

GATE0 starts and stops Timer 1, using a signal provided to the pin INT0 (P3.2):

1 - Timer 0 operates only if the bit INT0 is set

0 - Timer 0 operates regardless of the state of the bit INT0

C/T0 selects which pulses are to be counted up by the timer/counter 0:

1 - Timer counts pulses provided to the pin T0(P3.4)

0 - Timer counts pulses from internal oscillator

T0M1,T0M0 These two bits select the Timer 0 operating mode.

T0M1 T0M0 Mode Description

0 0 0 13-bit timer

0 1 1 16-bit timer

1 0 28-bit

auto-reload

1 1 3 Split mode

Timer 0 in mode 0 (13-bit timer)

This is one of the rarities being kept only for compatibility with the previuos versions of the microcontrollers. When using

this mode, the higher byte TH0 and only the first 5 bits of the lower byte TL0 are in use. Being configured in this way, the

Timer 0 uses only 13 of all 16 bits. How does it operate? With each new pulse coming, the state of the lower register (that

one with 5 bits) is changed. After 32 pulses received it becomes full and automatically is reset, while the higher byte TH0 is

incremented by 1. This action will be repeated until registers count up 8192 pulses. After that, both registers are reset and

counting starts from 0.


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Timer 0 in mode 1 (16-bit timer)

All bits from the registers TH0 and TL0 are used in this mode. That is why for this mode is being more commonly used.

Counting is performed in the same way as in mode 0, with difference that the timer counts up to 65 536, i.e. as far as the

use of 16 bits allows.

Timer 0 in mode 2 (Auto-Reload Timer)

What does auto-reload mean? Simply, it means that such timer uses only one 8-bit register for counting, but it never counts

from 0 but from an arbitrary chosen value (0- 255) saved in another register.

The advantages of this way of counting are described in the following example: suppose that for any reason it is

continuously needed to count up 55 pulses at a time from the clock generator.

When using mode 1 or mode 0, It is needed to write number 200 to the timer registers and check constantly afterwards

whether overflow occured, i.e. whether the value 255 is reached by counting . When it has occurred, it is needed to rewrite

number 200 and repeat the whole procedure. The microcontroller performs the same procedure in mode 2 automatically.


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Namely, in this mode it is only register TL0 operating as a timer ( normally 8-bit), while the value from which counting

should start is saved in the TH0 register. Referring to the previous example, in order to register each 55th pulse, it is needed

to write the number 200 to the register and configure the timer to operate in mode 2.

Timer 0 in Mode 3 (Split Timer)

By configuring Timer 0 to operate in Mode 3, the 16-bit counter consisting of two registers TH0 and TL0 is split into two

independent 8-bit timers. In addition, all control bits which belonged to the initial Timer 1 (consisting of the registers TH1

and TL1), now control newly created Timer 1. This means that even though the initial Timer 1 still can be configured to

operate in any mode ( mode 1, 2 or 3 ), it is no longer able to stop, simply because there is no bit to do that. Therefore, in

this mode, it will uninterruptedly “operate in the background “.


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The only application of this mode is in case two independent 'quick' timers are used and the initial Timer 1 whose operating

is out of control is used as baud rate generator.

TCON - Timer Control Register

This is also one of the registers whose bits directly control timer operating.

Only 4 of all 8 bits this register has are used for timer control, while others are used for interrupt control which will be

discussed later.

TF1 This bit is automatically set with the Timer 1 overflow

TR1 This bit turns the Timer 1 on

1 - Timer 1 is turned on

0 - Timer 1 is turned off

TF0 This bit is automatically set with the Timer 0 overflow.

TR0 This bit turns the timer 0 on

1 - Timer 0 is turned on

0 - Timer 0 is turned off

How to start Timer 0 ?

Normally, first this timer and afterwards its mode should be selected. Bits which control that are resided in the register

TMOD:

This means that timer 0 operates in mode 1 and counts pulses from internal source whose frequency is equal to 1/12 the

quartz frequency.

In order to enable the timer, turn it on:


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Immediately upon the bit TR0 is set, the timer starts operating. Assuming that a quartz crystal with frequency of 12MHz is

embedded, a number it contains will be incremented every microsecond. By counting up to 65.536 microseconds, the both

registers that timer consists of will be set. The microcontroller automatically reset them and the timer keeps on repeating

counting from the beginning as far as the bit’s value is logic one (1).

How to 'read' a timer ?

Depending on the timer’s application, it is needed to read a number in the timer registers or to register a moment they have

been reset.

- Everything is extremely simple when it is needed to read a value of the timer which uses only one register for counting

(mode 2 or Mode 3) . It is sufficient to read its state at any moment and it is it!

- It is a bit complicated to read a timer’s value when it operates in mode 2. Assuming that the state of the lower byte is read

first (TL0) and the state of the higher byte (TH0) afterwards, the result is:

TH0 = 15 TL0 = 255

Everything seems to be in order at first sight, but the current state of register at the moment of reading was:

TH0 = 14 TL0 = 255

In case of negligence, this error in counting ( 255 pulses ) may occur for not so obvious but quite logical reason. Reading

the lower byte is correct ( 255 ), but at the same time the program counter “ was taking “ a new instruction for the TH0

state reading, an overflow occurred and both registers have changed their contents ( TH0: 14→15, TL0: 255→0). The

problem has simple solution: the state of the higher byte should be read first, then the state of the lower byte and once again

the state of the higher byte. If the number stored in the higher byte is not the same both times it has been read then this

sequence should be repeated ( this is a mini- loop consisting of only 3 instructions in a program).

There is another solution too. It is sufficient to simply turn timer off while reading ( the bit TR0 in the register TCON

should be 0), and turn it on after that.

Detecting Timer 0 Overflow

Usually, there is no need to continuously read timer registers’ contents. It is sufficient to register the moment they are reset,

i.e. when counting starts from 0. It is called overflow. When this has occurred, the bit TF0 from the register TCON will be

automatically set. The microcontroller is waiting for that moment in a way that program will constantly check the state of

this bit. Furthermore, an interrupt to stop the main program execution can be enabled. Assuming that it is needed to provide

a program pause ( time the program appeared to be stopped) in duration of for example 0.05 seconds ( 50 000 machine

cycles ):

First, it is needed to calculate a number that should be written to the timer registers:


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This number should be written to the timer registers TH0 and TL0:

Once the timer is started it will continue counting from the written number. Program instruction checks if the bit TF0 is set,

which happens at the moment of overflow, i.e. after exactly 50.000 machine cycles and 0.05 seconds respectively.

How to measure pulses?

Suppose it is needed to measure the

duration of an event, for example how

long some device has been turned on?

Look at the picture of the timer and pay

attention to the purpose of the bit

GATE0 ( which resides in the TMOD

register ). If this bit is cleared then the

state on the pin P3.2 does not affect the

timer operating. If GATE0 = 1 the

timer will operate as far as the pin P3.2

has logic one (1) value. If this pin is

supplied with 5V through some

external switch at the moment the

device is being turned on, the timer will

measure duration of its operating,

which actually was the aim.

How to count up pulses?

This time, the answer lies in the

register TCON, and bit C/T0

respectively. Similar to the previous

example, this bit brings into an external

signal. If the bit is cleared everything

occurs in the same way as in the previous examples and the timer counts pulses from oscillator of defined frequency, i.e.

measures the time that went by. If the bit is set, the timer input is provided with pulses from the pin P3.4 (T0). Since these

pulses do not have some definite time or order, it is not possible to measure time by counting them. For that reason, this

timer is turned into the counter. The highest frequency that could be measured by such a counter is 1/24 frequency of used

quartz-crystal.


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Timer 1

Referring to its characteristics, this timer is “ a twin brother “ to the Timer 0. This means that they have the same purpose,

their operating is controlled by the same registers TMOD and TCON and both of them can operate in one of 4 different

modes.

2.7 UART (Universal Asynchronous Receiver and Transmitter)

One of the features that makes this microcontroller so powerful is an integrated UART, better known as a serial port. It is a

duplex port, which means that it can transmit and receive data simultaneously. Without it, serial data sending and receiving

would be endlessly complicated part of the program where the pin state continuously is being changed and checked

according to strictly determined rhythm. Naturally, it does not happen here because the UART resolves it in a very elegant

manner. All the programmer needs to do is to simply select serial port mode and baud rate. When the programmer is such

configured, serial data sending is done by writing to the register SBUF while data receiving is done by reading the same

register. The microcontroller takes care of all issues necessary for not making any error during data exchange.

Serial port should be configured prior to being used. That determines how many bits one serial “word” contains, what the

baud rate is and what the pulse source for synchronization is. All bits controlling this are stored in the SFR Register SCON

(Serial Control).

SCON Register (Serial Port Control Register)

SM0 - bit selects mode

SM1 - bit selects mode

SM2 - bit is used in case that several microcontrollers share the same interface. In normal circumstances this bit must

be cleared in order to enable connection to function normally.

REN - bit enables data receiving via serial communication and must be set in order to enable it.

TB8 - Since all registers in microcontroller are 8-bit registers, this bit solves the problem of sending the 9th bit in

modes 2 and 3. Simply, bits content is sent as the 9th bit.

RB8 - bit has the same purpose as the bit TB8 but this time on the receiver side. This means that on receiving data in

9-bit format , the value of the last ( ninth) appears on its location.


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TI - bit is automatically set at the moment the last bit of one byte is sent when the USART operates as a transmitter.

In that way processor “knows” that the line is available for sending a new byte. Bit must be clear from within the

program!

RI - bit is automatically set once one byte has been received. Everything functions in the similar way as in the

previous case but on the receive side. This is line a “doorbell” which announces that a byte has been received via

serial communication. It should be read quickly prior to a new data takes its place. This bit must also be also cleared

from within the program!

As seen, serial port mode is selected by combining the bits SM0 and SM2 :

SM0 SM1 Mode Description Baud Rate

0 0 0 8-bit Shift Register 1/12 the quartz frequency

0 1 1 8-bit UART Determined by the timer 1

1 0 2 9-bit UART1/32 the quartz frequency

(1/64 the quartz frequency)

1 1 3 9-bit UART Determined by the timer 1

In mode 0, the data are transferred through the

RXD pin, while clock pulses appear on the TXD

pin. The bout rate is fixed at 1/12 the quartz

oscillator frequency. On transmit, the least

significant bit (LSB bit) is being sent/received first.

(received).

TRANSMIT - Data transmission in form of pulse

train automatically starts on the pin RXD at the

moment the data has been written to the SBUF

register.In fact, this process starts after any

instruction being performed on this register. Upon

all 8 bits have been sent, the bit TI in the SCON

register is automatically set.

RECEIVE - Starts data receiving through the pin RXD once two necessary conditions are met: bit REN=1 and RI=0 (both

bits reside in the SCON register). Upon 8 bits have been received, the bit RI (register SCON) is automatically set, which

indicates that one byte is received.


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Since, there are no START and STOP bits or any other bit except data from the SBUF register, this mode is mainly used on

shorter distance where the noise level is minimal and where operating rate is important. A typical example for this is I/O

port extension by adding cheap IC circuit ( shift registers 74HC595, 74HC597 and similar).

Mode 1

In Mode1 10 bits are transmitted through TXD or

received through RXD in the following manner: a

START bit (always 0), 8 data bits (LSB first) and a

STOP bit (always 1) last. The START bit is not

registered in this pulse train. Its purpose is to start

data receiving mechanism. On receive the STOP

bit is automatically written to the RB8 bit in the

SCON register.

TRANSMIT - A sequence for data transmission

via serial communication is automatically started

upon the data has been written to the SBUF

register. End of 1 byte transmission is indicated by

setting the TI bit in the SCON register.

RECEIVE - Receiving starts as soon as the START bit (logic zero (0)) appears on the pin RXD. The condition is that bit

REN=1and bit RI=0. Both of them are stored in the SCON register. The RI bit is automatically set upon receiving has been

completed.


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The Baud rate in this mode is determined by the timer 1 overflow time.

Mode 2

In mode 2, 11 bits are sent through TXD or

received through RXD: a START bit (always 0), 8

data bits (LSB first), additional 9th data bit and a

STOP bit (always 1) last. On transmit, the 9th data

bit is actually the TB8 bit from the SCON register.

This bit commonly has the purpose of parity bit.

Upon transmission, the 9th data bit is copied to the

RB8 bit in the same register ( SCON).The baud

rate is either 1/32 or 1/64 the quartz oscillator

frequency.

TRANSMIT - A sequence for data transmission

via serial communication is automatically started

upon the data has been written to the SBUF

register. End of 1 byte transmission is indicated by

setting the TI bit in the SCON register.

RECEIVE - Receiving starts as soon as the START bit (logic zero (0)) appears on the pin RXD. The condition is that bit

REN=1and bit RI=0. Both of them are stored in the SCON register. The RI bit is automatically set upon receiving has been

completed.

Mode 3

Mode 3 is the same as Mode 2 except the baud rate. In Mode 3 is variable and can be selected.

The parity bit is the bit P in the PSW register. The simplest way to check correctness of the received byte is to add this

parity bit to the transmit side as additional bit. Simply, immediately before transmit, the message is stored in the

accumulator and the bit P goes into the TB8 bit in order to be “a part of the message”. On the receive side is the opposite

: received byte is stored in the accumulator and the bit P is compared with the bit RB8 ( additional bit in the message). If

they are the same- everything is OK!

Baud Rate

Baud Rate is defined as a number of send/received bits per second. In case the UART is used, baud rate depends on:

selected mode, oscillator frequency and in some cases on the state of the bit SMOD stored in the SCON register. All

necessary formulas are specified in the table :


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Baud Rate BitSMOD

Mode 0 Fosc. / 12

Mode 1

1 Fosc.

16 12

(256-TH1)

BitSMOD

Mode 2Fosc. / 32

Fosc. / 64

1

0

Mode 3

1 Fosc.

16 12

(256-TH1)

Timer 1 as a baud rate generator

Timer 1 is usually used as a baud rate generator because it is easy to adjust various baud rate by the means of this timer.

The whole procedure is simple:

First, Timer 1 overflow interrupt should be disabled

Timer T1 should be set in auto-reload mode

Depending on necessary baud rate, in order to obtain some of the standard values one of the numbers from the table

should be selected. That number should be written to the TH1 register. That's all.

Baud RateFosc. (MHz)

Bit SMOD11.0592 12 14.7456 16 20

150 40 h 30 h 00 h 0

300 A0 h 98 h 80 h 75 h 52 h 0

600 D0 h CC h C0 h BB h A9 h 0

1200 E8 h E6 h E0 h DE h D5 h 0

2400 F4 h F3 h F0 h EF h EA h 0

4800 F3 h EF h EF h 1

4800 FA h F8 h F5 h 0

9600 FD h FC h 0

9600 F5 h 1

19200 FD h FC h 1

38400 FE h 1

76800 FF h 1

Multiprocessor Communication

As described in the previous text, modes 2 and 3 enable the additional 9th data bit to be part of message. It can be used for

checking data via parity bit. Another useful application of this bit is in communication between two microcontrollers, i.e.

multiprocessor communication. This feature is enabled by setting the SM2 bit in the SCON register. The consequence is the

following: when the STOP bit is ready, indicating end of message, the serial port interrupt will be requested only in case

the bit RB8 = 1 (the 9th bit).

The whole procedure will be performed as follows:

Suppose that there are several connected microcontrollers having to exchange data. That means that each of them must

have its address. The point is that each address sent via serial communication has the 9th bit set (1), while data has it

cleared (0). If the microcontroller A should send data to the microcontroller C then it at will place first send address of C

and the 9th bit set to 1. That will generate interrupt and all microcontrollers will check whether they are called.


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Of course, only one of them will recognize this address and immediately clear the bit SM2 in the SCON register. All

following data will be normally received by that microcontroller and ignored by other microcontrollers.

2.8 8051 Microcontroller Interrupts

There are five interrupt sources for the 8051, which means that they can recognize 5 different event that can interrupt

regular program execution. Each interrupt can be enabled or disabled by setting bits in the IE register. Also, as seen from

the picture below the whole interrupt system can be disabled by clearing bit EA from the same register.

Now, one detail should be explained which is not completely obvious but refers to external interrupts- INT0 and INT1.

Namely, if the bits IT0 and IT1 stored in the TCON register are set, program interrupt will occur on changing logic state

from 1 to 0, (only at the moment). If these bits are cleared, the same signal will generate interrupt request and it will be

continuously executed as far as the pins are held low.


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IE Register (Interrupt Enable)

EA - bit enables or disables all other interrupt sources (globally)

0 - (when cleared) any interrupt request is ignored (even if it is enabled)

1 - (when set to 1) enables all interrupts requests which are individually enabled

ES - bit enables or disables serial communication interrupt (UART)

0 - UART System can not generate interrupt

1 - UART System enables interrupt

ET1 - bit enables or disables Timer 1 interrupt

0 - Timer 1 can not generate interrupt

1 - Timer 1 enables interrupt

EX1 - bit enables or disables INT 0 pin external interrupt

0 - change of the pin INT0 logic state can not generate interrupt

1 - enables external interrupt at the moment of changing the pin INT0 state

ET0 - bit enables or disables timer 0 interrupt

0 - Timer 0 can not generate interrupt

1 - enables timer 0 interrupt

EX0 - bit enables or disables INT1 pin external interrupt

0 - change of the INT1 pin logic state can not cause interrupt

1 - enables external interrupt at the moment of changing the pin INT1 state

Interrupt Priorities

It is not possible to predict when an interrupt will be required. For that reason, if several interrupts are enabled. It can easily

occur that while one of them is in progress, another one is requested. In such situation, there is a priority list making the

microcontroller know whether to continue operating or meet a new interrupt request.

The priority list cosists of 3 levels:

Reset! The apsolute master of the situation. If an request for Reset omits, everything is stopped and the

microcontroller starts operating from the beginning.

1.

Interrupt priority 1 can be stopped by Reset only.2.


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Interrupt priority 0 can be stopped by both Reset and interrupt priority 1.3.

Which one of these existing interrupt sources have higher and which one has lower priority is defined in the IP Register (

Interrupt Priority Register). It is usually done at the beginning of the program. According to that, there are several

possibilities:

Once an interrupt service begins. It cannot be interrupted by another inter rupt at the same or lower priority level, but

only by a higher priority interrupt.

If two interrupt requests, at different priority levels, arrive at the same time then the higher priority interrupt is

serviced first.

If the both interrupt requests, at the same priority level, occur one after another , the one who came later has to wait

until routine being in progress ends.

If two interrupts of equal priority requests arrive at the same time then the interrupt to be serviced is selected

according to the following priority list :

External interrupt INT01.

Timer 0 interrupt2.

External Interrupt INT13.

Timer 1 interrupt4.

Serial Communication Interrupt5.

IP Register (Interrupt Priority)

The IP register bits specify the priority level of each interrupt (high or low priority).

PS - Serial Port Interrupt priority bit

Priority 0

Priority 1

PT1 - Timer 1 interrupt priority

Priority 0

Priority 1

PX1 - External Interrupt INT1 priority

Priority 0

Priority 1

PT0 - Timer 0 Interrupt Priority

Priority 0

Priority 1

PX0 - External Interrupt INT0 Priority

Priority 0

Priority 1

Handling Interrupt

Once some of interrupt requests arrives, everything occurs according to the following order:

Instruction in progress is ended1.

The address of the next instruction to execute is pushed on the stack2.

Depending on which interrupt is requested, one of 5 vectors (addresses) is written to the program counter in

accordance to the following table:

3.

Interrupt Source Vector (address)

IE0 3 h

TF0 B h

TF1 1B h

RI, TI 23 h

All addresses are in hexadecimal format4.


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The appropriate subroutines processing interrupts should be located at these addresses. Instead of them, there are

usually jump instructions indicating the location where the subroutines reside.

When interrupt routine is executed, the address of the next instruction to execute is poped from the stack to the

program counter and interrupted program continues operating from where it left off.

5.

From the moment an interrupt is enabled, the microcontroller is on alert all the time. When interrupt request arrives, the

program execution is interrupted, electronics recognizes the cause and the program “jumps” to the appropriate address

(see the table above ). Usually, there is a jump instruction already prepared subroutine prepared in advance. The

subroutine is executed which exactly the aim- to do something when something else has happened. After that, the

program continues operating from where it left off…

Reset

Reset occurs when the RS pin is supplied with a positive pulse in duration of at least 2 machine cycles ( 24 clock cycles of

crystal oscillator). After that, the microcontroller generates internal reset signal during which all SFRs, excluding SBUF

registers, Stack Pointer and ports are reset ( the state of the first two ports is indefinite while FF value is being written to the

ports configuring all pins as inputs). Depending on device purpose and environment it is in, on power-on reset it is usually

push button or circuit or both connected to the RS pin. One of the most simple circuit providing secure reset at the moment

of turning power on is shown on the picture.

Everything functions rather simply: upon the power is on,

electrical condenser is being charged for several milliseconds

through resistor connected to the ground and during this process

the pin voltage supply is on. When the condenser is charged,

power supply voltage is stable and the pin keeps being connected

to the ground providing normal operating in that way. If later on,

during the operation, manual reset button is pushed, the condenser

is being temporarily discharged and the microcontroller is being

reset. Upon the button release, the whole process is repeated…

Through the program- step by step...

The microcontrollers normally operate at very high speed. The use

of 12 Mhz quartz crystal enables 1.000.000 instructions per second

to be executed! In principle, there is no need for higher operating

rate. In case it is needed, it is easy to built-in crystal for high

frequency. The problem comes up when it is necessary to slow

down. For example, when during testing in real operating

environment, several instructions should be executed step by step

in order to check for logic state of I/O pins.

Interrupt system applied on the 8051 microcontrollers practically stops operating and enables instructions to be executed

one at a time by pushing button. Two interrupt features enable that:

Interrupt request is ignored if an interrupt of the same priority level is being in progress.

Upon interrupt routine has been executed, a new interrupt is not executed until at least one instruction from the main

program is executed.

In order to apply this in practice, the following steps should be done:

External interrupt sensitive to the signal level should be enabled (for example INT0).1.

Three following instructions should be entered into the program (start from address 03hex.):2.


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What is going on? Once the pin P3.2 is set to “0” (for example, by pushing button), the microcontroller will interrupt

program execution jump to the address 03hex, will be executed a mini-interrupt routine consisting of 3 instructions is

located at that address.

The first instruction is being executed until the push button is pressed ( logic one (1) on the pin P3.2). The second

instruction is being executed until the push button is released. Immediately after that, the instruction RETI is executed and

processor continues executing the main program. After each executed instruction, the interrupt INT0 is generated and the

whole procedure is repeated ( push button is still pressed). Button Press = One Instruction.

2.9 8051 Microcontroller Power Consumption Control

Conditionally said microcontroller is the most part of its “lifetime” is inactive for some external signal in order to takes its

role in a show. It can make a great problem in case batteries are used for power supply. In extremely cases, the only

solution is to put the whole electronics to sleep in order to reduce consumption to the minimum. A typical example of this

is remote TV controller: it can be out of use for months but when used again it takes less than a second to send a command

to TV receiver. While normally operating, the AT89S53 uses current of approximately 25mA, which shows that it is not

too sparing microcontroller. Anyway, it doesn’t have to be always like this, it can easily switch the operation mode in order

to reduce its total consumption to approximately 40uA. Actually, there are two power-saving modes of operation: Idle and

Power Down.


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Idle mode

Immediately upon instruction which sets the bit IDL in the PCON register, the microcontroller turns off the greatest power

consumer- CPU unit while peripheral units serial port, timers and interrupt system continue operating normally consuming

6.5mA. In Idle mode, the state of all registers and I/O ports is remains unchanged.

In order to terminate the Idle mode and make the microcontroller operate normally, it is necessary to enable and execute

any interrupt or reset.Then, the IDL bit is automatically cleared and the program continues executing from instruction

following that instruction which has set the IDL bit. It is recommended that three first following one which set NOP

instructions. They do not perform any operation but keep the microcontroller from undesired changes on the I/O ports.

Power Down mode

When the bit PD in the register PCON is set from within the program, the microcontroller is set to Powerdown mode. It and

turns off its internal oscillator reducing drastically consumption in that way. In powerdown mode the microcontroller can

operate using only 2V power supply while the total power consumption is less than 40uA. The only way to get the

microcontroller back to normal mode is reset.

During Power Down mode, the state of all SFR registers and I/O ports remains unchanged, and after the microcontroller is

put get into the normal mode, the content of the SFR register is lost, but the content of internal RAM is saved. Reset signal

must be long enough approximately 10mS in order to stabilize quartz oscillator operating.

PCON register


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Previous Chapter | Table of Contents | Next Chapter

The purpose of the Register PCON bits :

SMOD By setting this bit baud rate is doubled.

GF1 General-purpose bit (available for use).



PD By setting this bit the microcontroller is set into Power Down mode.

IDL By setting this bit the microcontroller is set into Idle mode.


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Chapter 3 : The 8051 Instruction Set

3.1 Types of instructions

3.2 Description of the 8051 instructions

Introduction

Writing program for the microcontroller mainly consists of giving instructions (commands) in that order in which they

should be executed later in order to carry out specific task. As electronics can not “understand” what for example

instruction “if the push button is pressed- turn the light on” means, then a certain number of more simpler and precisely

defined orders that decoder can recognise must be used. All commands are known as INSTRUCTION SET. All

microcontrollers compatibile with the 8051 have in total of 255 instructions, i.e. 255 different words available for

program writing.

At first sight, it is imposing number of odd signs that must be known by heart. However, It is not so complicated as it

looks like. Many instructions are considered to be “different”, even though they perform the same operation, so there are

only 111 truly different commands. For example: ADD A,R0, ADD A,R1, ... ADD A,R7 are instructions that perform the

same operation (additon of the accumulator and register) but since there are 8 such registers, each instruction is counted

separately! Taking into account that all instructions perform only 53 operations ( addition, subtraction, copy etc.) and

most of them are rarely used in practice, there are actually 20-30 shortened forms needed to be known, which is

acceptable.

3.1 Types of instructions

Depending on operation they perform, all instructions are divided in several groups:

Arithmetic Instructions

Branch Instructions

Data Transfer Instructions

Logical Instructions

Logical Instructions with bits

The first part of each instruction, called MNEMONIC refers to the operation an instruction performs (copying, addition,

logical operation etc.). Mnemonics commonly are shortened form of name of operation being executed. For example:

INC R1 - Means: Increment R1 (increment register R1)

LJMP LAB5 - Means: Long Jump LAB5 (long jump to address specified as LAB5)

JNZ LOOP - Means: Jump if Not Zero LOOP (if the number in the accumulator is not 0, jump to address specified as

LOOP)

Another part of instruction, called OPERAND is separated from mnemonic at least by one empty space and defines data

being processed by instructions. Some instructions have no operand, some have one, two or three. If there is more than

one operand in instruction, they are separated by comma. For example:

RET - (return from sub-routine)

JZ TEMP - (if the number in the accumulator is not 0, jump to address specified as TEMP)

ADD A,R3 - (add R3 and accumulator)

CJNE A,#20,LOOP - (compare accumulator with 20. If they are not equal, jump to address specified as LOOP)

Arithmetic instructions

These instructions perform several basic operations ( addition, subtraction, division, multiplication etc.) After execution,

the result is stored in the first operand. For example:

ADD A,R1 - The result of addition (A+R1) will be stored in the accumulator.

Arithmetical Instructions

Mnemonic Description Byte NumberOscillator

Period



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ADD A,Rn Add R Register to accumulator 1 1

ADD A,Rx Add directly addressed Rx Register to accumulator 2 2

ADD A,@Ri Add indirectly addressed Register to accumulator 1 1

ADD A,#X Add number X to accumulator 2 2

ADDC A,Rn Add R Register with Carry bit to accumulator 1 1

ADDC A,RxAdd directly addressed Rx Register with Carry bit to

accumulator2 2

ADDC A,@RiAdd indirectly addressed Register with Carry bit to

accumulator1 1

ADDC A,#X Add number X with Carry bit to accumulator 2 2

SUBB A,Rn Subtruct R Register with borrow from accumulator 1 1

SUBB A,RxSubtruct directly addressed Rx Register with borrow from

accumulator2 2

SUBB A,@RiSubtruct indirectly addressed Register with borrow from

accumulator1 1

SUBB A,#X Subtruct number X with borrow from accumulator 2 2

INC A Increment accumulator by 1 1 1

INC Rn Increment R Register by 1 1 1

INC Rx Increment directly addressed Rx Register by 1 2 2

INC @Ri Increment indirectly addressed Register by 1 1 1

DEC A Decrement accumulator by 1 1 1

DEC Rn Decrement R Register by 1 1 1

DEC Rx Decrement directly addressed Rx Register by 1 2 2

DEC @Ri Decrement indirectly addressed Register by 1 1 1

INC DPTR Increment Data Pointer by 1 1 3

MUL AB Multiply number in accumulator by B register 1 5

DIV AB Divide number in accumulator by B register 1 5

DA A Decimal adjustment of accumulator according to BCD code 1 1

Branch Instructions

There are two kinds of these instructions:

Unconditional jump instructions: after their execution a jump to a new location from where the program continues

execution is executed.

Conditional jump instructions: if some condition is met - a jump is executed. Otherwise, the program normally proceeds

with the next instruction.

Branch Instruction

Mnemonic Description Byte NumberOscillator

Period

ACALL adr11Call subroutine located at addreess within 2 K byte Program

Memory space2 3

LCALL adr16Call subroutine located at any address within 64 K byte

Program Memory space3 4

RET Return from subroutine 1 4

RETI Return from interrupt routine 1 4

AJMP adr11Jump to address located within 2 K byte Program Memory

space2 3

LJMP adr16Jump to any address located within 64 K byte Program

Memory space3 4


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SJMP relShort jump (from –128 to +127 locations in relation to first

next instruction).2 3

JC rel Jump if Carry bit is set. Short jump. 2 3

JNC rel Jump if Carry bit is cleared. Short jump. 2 3

JB bit,rel Jump if addressed bit is set. Short jump. 3 4

JBC bit,rel Jump if addressed bit is set and clear it. Short jump. 3 4

JMP

@A+DPTR

Indirect jump. Jump address is obtained by addition of

accumulator and DPTR Register1 3

JZ rel Jump if accumulator is 0. Short jump. 2 3

JNZ rel Jump if accumulator is not 0. Short jump. 2 3

CJNE A,Rx,relCompare accumulator and directly addressed Register Rx.

Jump if they are different. Short jump.3 4

CJNE A,#X,relCompare accumulator with number X. Jump if they are

different. Short jump.3 4

CJNE

Rn,#X,rel

Compare Register R with number X. Jump if they are

different. Short jump.3 4

CJNE

@Ri,#X,rel

Compare indirectly addressed register with number X. Jump

if they are different. Short jump.3 4

DJNZ Rn,relDecrement R Register by 1. Jump if the result is not 0. Short

jump.2 3

DJNZ Rx,relDecrement directly addressed Register Rx by 1. Jump if the

result is not 0. Short jump.3 4

NOP No operation 1 1

Data Transfer Instructions

These instructions move the content of one register to another one. The register which content is moved remains

unchanged. If they have the suffix “X” (MOVX), the data is exchanged with external memory.

Data Transfer Instruction

Mnemonic Description Byte Number Cycle Number

MOV A,Rn Move R register to accumulator 1 1

MOV A,Rx Move directly addressed Rx register to accumulator 2 2

MOV A,@Ri Move indirectly addressed register to accumulator 1 1

MOV A,#X Move number X to accumulator 2 2

MOV Rn,A Move accumulator to R register 1 1

MOV Rn,Rx Move directly addressed Rx register to R register 2 2

MOV Rn,#X Move number X to R register 2 2

MOV Rx,A Move accumulator to directly addressed Rx register 2 2

MOV Rx,Rn Move R register to directly addressed Rx register 2 2

MOV Rx,RyMove directly addressed register Ry to directly addressed Rx

register3 3

MOV Rx,@RiMove indirectly addressed register to directly addressed Rx

register2 2

MOV Rx,#X Move number X to directly addressed Rx register 3 3

MOV @Ri,A Move accumulator to indirectly addressed register 1 1

MOV @Ri,RxMove directly addressed Rx register to indirectly addressed

register2 2

MOV @Ri,#X Store number X in indirectly addressed register 2 2

MOV

DPTR,#XStore number X in Data Pointer 3 3


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MOVC

A,@A+DPTR

Move register from Program Memory to accumulator

(address= A+DPTR)1 3

MOVC

A,@A+PC

Move register from Program Memory to accumulator

(address= A+PC)1 3

MOVX A,@RiMove data from external memory to accumulator (8-bit

address)1 2

MOVX

A,@DPTR

Move data from external memory to accumulator (16- bit

address)1 2

MOVX @Ri,AMove accumulator to external memory register (8-bit

address)1 2

MOVX

@DPTR,A

Move accumulator to external memory register (16-bit

address)1 2

PUSH Rx Push directly addressed Rx register on Stack 2 2

POP Rx Pop data from Stack. Store it in directly addressed Rx register 2 2

XCH A,Rn Exchange accumulator with R register 1 1

XCH A,Rx Exchange accumulator with directly addressed Rx register 2 2

XCH A,@Ri Exchange accumulator with indirectly addressed register 1 1

XCHD A,@RiExchange 4 lower bits in accumulator with indirectly

addressed register1 1

Logical Instruction

These instructions perform logical operations between corresponding bits of two registers. After execution, the result is

stored in the first operand.

Logical Instructions


ANL A,Rn Logical AND between accumulator and R register 1 1

ANL A,RxLogical AND between accumulator and directly addressed

register Rx2 2

ANL A,@RiLogical AND between accumulator and indirectly addressed

register1 1

ANL A,#X Logical AND between accumulator and number X 2 2

ANL Rx,ALogical AND between accumulator and directly addressed

register Rx2 2

ANL Rx,#XLogical AND between directly addressed register Rx and

number X3 3

ORL A,Rn Logical OR between accumulator and R register 1 1

ORL A,RxLogical OR between accumulator and directly addressed

register Rx2 2

ORL A,@RiLogical OR between accumulator and indirectly addressed

register2 2

ORL Rx,ALogical OR between accumulator and directly addressed

register Rx2 2

ORL Rx,#XLogical OR between directly addressed register Rx and

number X3 3

XORL A,Rn Logical exclusive OR between accumulator and R register 1 1

XORL A,RxLogical exclusive OR between accumulator and directly

addressed register Rx2 2

XORL A,@RiLogical exclusive OR between accumulator and indirectly

addressed register1 1

XORL A,#X Logical exclusive OR between accumulator and number X 2 2


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XORL Rx,ALogical exclusive OR between accumulator and directly

addressed register Rx2 2

XORL Rx,#XLogical exclusive OR between accumulator and directly

addressed register Rx and number X3 3

CLR A Clear accumulator 1 1

CPL A Complement accumulator (1=0, 0=1) 1 1

SWAP A Swap nibbles in accumulator (left and right half of one byte) 1 1

RL A Rotate bits in accumulator left by 1 place 1 1

RLC A Rotate bits in accumulator left by 1 place through Carry 1 1

RR A Rotate bits in accumulator right by 1 place 1 1

RRC A Rotate bits In accumulator right by 1 place through Carry 1 1

Logical Operations on Bits

Similar to logical instructions, these instructions perform logical operations. The difference is that these operations are

performed on single bits.

Logical operations on bits


CLR C Clear Carry bit 1 1

CLR bit Clear directly addressed bit 2 2

SETB C Set Carry bit 1 1

SETB bit Set directly addressed bit 2 2

CPL C Complement Carry bit 1 1

CPL bit Complement directly addressed bit 2 2

ANL C,bit Logical AND between Carry bit and directly addressed bit 2 2

ANL C,/bitLogical AND between Carry bit and inverted directly

addressed bit2 2

ORL C,bit Logical OR between Carry bit and directly addressed bit 2 2

ORL C,/bitLogical OR between Carry bit and inverted directly addressed

bit2 2

MOV C,bit Move directly addressed bit to Carry bit 2 2

MOV bit,C Move Carry bit to directly addressed bit 2 2

3.2 Descriptiion of all 8051 instructiions

The operands listed below are written in shortened forms having the following meaning :

A - accumulator

Rn - Rn is one of R registers (R0-R7) in the currently active bank in RAM.

Rx - Rx is any register in RAM with 8-bit address. It can be a general-purpose register or SFR Register (I/O port, control

register etc.)

@Ri - Ri is R0 or R1 register in the currently active bank. It contains register.

address - the instruction is referring to.

#X - X is any 8-bit number (0-255).

#X16 - X is any 16-bit number (0-65535).

adr16 - 16-bit address is specified

adr11 - 11-bit address is specified

rel - The address of a close memory location is specified (-128 do +127 rela tive to the current one). Basing on that

address,

Asembler computes the value which is added or subtructed to the number which currently stored in the program counter.

bit - Bit address is specified.

C - Carry bit in the status register (register PSW)

ACALL adr11 - Call subroutine


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adr11: - Subroutine address

Description: Instruction unconditionally calls a subroutine located at the specified address. Therefore, the current

address and the address of called subroutine must be within the same 2K byte block of the program memory, starting

from the first byte of the instruction following ACALL.

Syntax: ACALL [subroutine name]

Bytes : 2 (Instruction Code, Address of the subroutine called)

STATUS register flags: No flags are affected.

EXAMPLE:

Before execution: PC=0123h

After execution: PC=0345h

ADD A,Rn - Add register Rn and accumulator

A: accumulator

Rn: Any R register (R0-R7)

Description: Instruction adds the number in the accumulator and the number in register Rn (R0-R7). After addition, the

result is stored in the accumulator.

Syntax: ADD A,Rn

Byte: 1 (Instruction Code)

STATUS register flags: C, OV i ACy

EXAMPLE:


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Before execution: A=2Eh (46 dec.) R4=12h (18 dec.)

After execution: A=40h (64 dec.) R4=12h

ADD A,@Ri - Add indirectly addressed register and accumulator

A: accumulator

Ri: Register R0 or R1

Description: Instruction adds number in the accumulator and number in Rx. The register Rx address is in the Ri register

(R0 or R1). After addition, the result is stored in the accumulator.

Syntax: ADD A,@Ri


STATUS register flags: C, OV and AC

EXAMPLE:

Register address: SUM = 4Fh R0=4Fh

Before execution: A= 16h (22 dec.) SUM= 33h (51 dec.)

After execution : A= 49h (73 dec.)

ADD A,Rx - Add directly addressed register Rx and accumulator

A: accumulator

Rx: Arbitrary register with address 0 - 255 (0 - FFh)

Description: Instruction adds the accumulator and Rx register. As it is direct addressing, Rx can be some of SFRs or

general-purpose register with address 0-7 Fh. The result is stored in the accumulator.

Syntax: ADD A, Register name

Bytes: 2 (Instruction Code, Rx Address)

STATUS register flags: C, OV and AC

EXAMPLE:

Before execution: SUM= 33h (51 dec.) A= 16h (22 dec.)

After execution: SUM= 33h (73 dec.) A= 49h (73 dec.)

ADDC A,Rn - Add register Rn, accumulator and Carry bit

A: accumulator


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Rn: any R register (R0-R7)

Description: Instruction adds the accumulator, Carry bit and value in Rn register (R0-R7). After addition, the result is

stored in the accumulator.

Syntax: ADDC A,Rn


STATUS register flags: C, OV i AC

EXAMPLE:

Before execution: A= C3h (195 dec.) R0= AAh (170 dec.) C=1

After execution: A= 6Eh (110 dec.) AC=0, C=1, OV=1

ADD A,#X - Add accumulator and number X

A: accumulator

X: Constant within 0 - 255 (0-FFh)

Description: Instruction adds the accumulator and number X (0-255). After addition, the result is stored in the

accumulator.

Syntax: ADD A,#X

Bytes: 2 (Instruction Code, Constant X)


EXAMPLE:

Before execution: A= 16h (22 dec.)

After execution: A= 49h (73 dec.)

ADDC A,Rx - Add accumulator, directly addressed register Rx and Carry bit

A: accumulator


Description: Instruction adds value in the accumulator and Rx register including the Carry bit as well. As it is direct

addressing, Rx can be some of SFRs or general purpose register with address 0-7Fh (0-127dec.). The result is stored in

the accumulator.

Syntax: ADDC A, register address

Bytes: 2 (Instruction Code, Address Rx)


EXAMPLE:


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Before execution: A= C3h (195 dec.) TEMP = AAh (170 dec.) C=1


ADDC A,@Ri - Add Carry bit, accumulator and indirectly addressed register Rx

A: accumulator


Description: Instruction adds value in the accumulator and number in the Rx register. The Carry bit is also added.

Register Rx address is in the Ri register (R0 or R1). After addition, the result is stored in the accumulator.

Syntax: ADDC A,@Ri



EXAMPLE:

Register address: SUM = 4Fh R0=4Fh

Before execution: A= C3h (195 dec.) SUM = AAh (170 dec.) C=1


ADDC A,#X - Add accumulator, number X and Carry bit

A: accumulator


Description: Instruction adds number in the accumulator and number X (0- 255). The Carry bit is also added. After

addition, the result is stored in the accumulator.

Syntax: ADDC A,#X



EXAMPLE:

Before execution: A= C3h (195 dec.) C=1


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After execution: A= 6Dh (109 dec.) AC=0, C=1, OV=1

AJMP adr11 - Jump to address

adr11: Jump address

Description : Program continues execution upon a jump to the specified address has been executed. Similar to the

ACALL instruction , this jump must be executed within the same 2K byte block of program memory (starting from the

first byte of the instruction following AJMP).

Syntax: AJMP address (label)

Bytes: 2 (Instruction Code, Jump Address)


EXAMPLE:

Before execution: PC=0345h SP=07h

After execution: PC=0123h SP=09h

ANL A,Rn - Logical-AND operation between accumulator and Register Rn

A: accumulator


Description: Instruction performs the logical-AND operation between the accumulator and Rn register. The result of this

logical operation is stored in the accumulator.

Syntax: ANL A,Rn



EXAMPLE:

Before execution: A= C3h (11000011 Bin.)

R5= 55h (01010101 Bin.)


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After execution: A= 41h (01000001 Bin.)

ANL A,Rx - Logical-AND operation between accumulator and directly addressed register

A: accumulator


Description: Instruction performs logical-AND operation between the accumulator and Rx register. As it is direct

addressing, Rx can be some of SFRs or general purpose register with address 0-7Fh (o-127 dec.). The result is stored in

the accumulator.

Syntax: ANL A,Rx

Byte: 2 (Instruction Code, Constant X)


EXAMPLE:


MASK= 55h (01010101 Bin.)


ANL A,@Ri - Logical-AND operation between accumulator and indirectly addressed register

A: accumulator


Description: Instruction performs logical-AND operation between the accumulator and Rx register. As it is indirect

addressing, register Rx address is in the Ri register (R0 or R1). The result is stored in the accumulator.

Syntax: ANL A,@Ri



EXAMPLE:

Register address SUM = 4Fh R0=4Fh


R0= 55h (01010101 Bin.)


ANL A,#X - Logical-AND operation between accumulator and number X


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A: accumulator

X: Constant in the range of 0 - 255 (0-FFh)

Description: Instruction performs logical-AND operation between the accumulator and number X. The result is stored in

the accumulator.

Syntax: ANL A,#X



EXAMPLE:



ANL Rx,A - Logical-AND operation between directly addressed register Rx and accumulator


A: accumulator

Description: Instruction performs logical-AND operation between the Rx register and accumulator. As it is direct

addressing, Rx can be some of SFRs or generalpurpose register with address 0-7Fh (0-127 dec.). The result of this logical

operation is stored in the register Rx.

Syntax: ANL register address ,A



EXAMPLE:


MASK= 55h (01010101 Bin.)

After execution: MASK= 41h (01000001 Bin.)

ANL Rx,#X - Logical-AND operation between number X and directly addressed register Rx



Description: Instruction performs logical-AND operation between the register Rx and number X. As it is direct

addressing, Rx can be some of SFRs or general-purpose register with address 0-7Fh (0-127 dec.). The result of this


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logical operation is stored in the Rx register.

Syntax: ANL register address ,#X

Bytes: 3 (Instruction Code, Address Rx, Constant X)


EXAMPLE:

Before execution: X= C3h (11000011 Bin.) MASK= 55h (01010101 Bin.) After execution: MASK= 41h (01000001

Bin.)

ANL C,bit - Logical-AND operation between bit and Carry bit

C: Carry

bit: Any bit in RAM

Description: Instruction performs logical-AND operation between the addressed bit and Carry bit.

bit C C AND bit

0 0 0

0 1 0

1 0 0

1 1 1

Syntax: ANL C, Bit Address

Bytes: 2 (Instruction Code, Bit Address)

STATUS register flags: C

EXAMPLE:

Before execution: ACC= 43h (01000011 Bin.)

C=1

After execution: ACC= 43h (01000011 Bin.)

C=0

ANL C,/bit - Logical-AND opertaion between complement of bit and Carry bit

C: Carry

bit: Any bit in RAM

Description: Instruction performs logical-AND operation between inverted addressed bit and Carry bit. The result is


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stored in the Carry bit.

bit bit C C AND bit

0 1 0 0

0 1 1 1

1 0 0 0

1 0 1 0

Syntax: ANL C,/[bit address]



EXAMPLE:

Before execution: ACC= 43h (01000011 Bin.)

C=1

After execution: ACC= 43h (01000011 Bin.)

C=1

CJNE A,Rx,rel - Compare directly addessed byte with accumulator and jump if they are not equal

A: accumulator


adr. Jump Address

Description: Instruction first compares the number in the accumulator with the number in Rx register. If they are equal,

the program continues execution. Otherwise, a jump to the indicated address in the program will be executed. This is a

short jump instruction, which means that the address of a new location must be relatively near the current position in the

program (-128 to +127 locations relative to the first following instruction).

Syntax: CJNE A,Rx,[jump address ]

Bytes: 3 (Instruction Code, Address Rx, Jump value)


EXAMPLE:


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Before execution: PC=0145h A=27h

After execution: if MAX≠27: PC=0123h

If MAX=27: PC=0146h

CJNE A,#X,rel - Compare number X with accumulator and jump if they are not equal

A: accumulator


Description: Instruction first compares the number in the accumulator with number X. If they are equal, the program

continues execution. Otherwise, a jump to the specified address in the program will be executed. This is a short jump

instruction, which means that the address of a new location must be relatively near the current position in the program

(-128 to +127 locations relative to the first following instruction)

Syntax: CJNE A,X,[jump address ]

Bytes: 3 (Instruction Code, Constant X, Jump value)


EXAMPLE:


After execution: If A≠33: PC=0423h

If A=33: PC=0446h


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CJNE Rn,#X,rel - Compare number X with register Rn and jump if they are not equal



adr: Jump address

Description: Instruction first compares number in the Rx register with number X. If they are equal, the program

continues execution. Otherwise, a jump to the specified address in the program will be executed. This is a short jump

instruction, which means that the address of a new location must be relatively near the current position in the program (

-128 to + 127 locations relative to the first following instruction).

Syntax: CJNE Rn,X,[jump address ]



EXAMPLE:


After execution : If R5≠44h: PC=0323h

If R5=44h: PC=0346h

CJNE @Ri,#X,rel - Compare indirectly addressed register with number X and jump if they are not equal



Description: Register Rx address is stored in the Ri register (R0 or R1). This instruction first compares the number in Rx

register with number X. If they are equal, the program continues execution. Otherwise, a jump to the indicated address in

the program will be executed.This is a short jump instruction, which means that the address of a new location must be

relatively near the current position in the program (-128 to +127 locations relative to the next instruction).

Syntax: CJNE @Ri,X,[jump address ]



EXAMPLE:


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Before execution: Register Address SUM=F3h

PC=0345h R0=F3h

After execution : If SUM≠44h: PC=0323h

If SUM=44h: PC=0346h

CLR A - Clear accumulator

A: accumulator

Description: Instruction clears the accumulator.

Syntax: CLR A



EXAMPLE:

After execution : A=0

CLR C - Clear Carry Bit

C: Carry Bit

Description: Instruction writes 0 to the Carry bit.

Syntax: CLR C



EXAMPLE:


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After execution: C=0

CLR bit - Clear Directly Addressed Bit

bit: Any bit in RAM

Description: Instruction clears the specified bit.

Syntax: CLR [bit address]



EXAMPLE:

Before execution: P0.3=1 (input pin)

After execution: P0.3=0 (output pin)

CPL A - Complement number in accumulator

A: accumulator

Description: Instruction complements all bits in the accumulator (1==>0, 0==>1)

Syntax: CPL A

Bytes: 1 (Instruction Code)


EXAMPLE:

Before execution: A= (00110110)

After execution A= (11001001)

CPL bit - Complement bit


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bit: Any bit in RAM

Description: Instruction coplements the specified bit (0==>1, 1==>0)

Syntax: CPL [bit address]



EXAMPLE:

Before execution: P0.3=1 (input pin)

After execution: P0.3=0 (output pin)

CPL C - Complement Carry Bit

C: Carry bit

Description: Instruction complements the Carry bit (0==>1, 1==>0)

Syntax: CPL C



EXAMPLE:

Before execution: C=1


DA A - Conversion into BCD format

A: accumulator

Description: This instruction corrects the value after binary addition in order to fit BCD format. Prior to addition, both

numbers have to be in BCD format, which means that it is not just a simple coversion of hexadecimal numbers into BCD

numbers.The result of this operation in form of two 4-digit BCD numbers is stored in the Accumaulator.

Syntax: DA A



EXAMPLE:


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Before execution: A=56h (01010110) 56 BCD

B=67h (01100111) 67BCD

After execution: A=BDh (10111101)

After BCD conversion: A=23h (00100011), C=1 (Overflow)

(C+23=123) = 56+67

DEC A - Decrement value in accumulator by 1

A: accumulator

Description: Instruction decrements value in the accumulator by 1. If there is a 0 in the accumulator, the result of the

operation is FFh. (255 dec.)

Syntax: DEC A



EXAMPLE:

Before execution: A=E4h

After execution: A=E3h

DEC Rn - Decrement value in register Rn by 1


Description: Instruction decrements value in the Rn register by 1. If there is a 0 in the register, the result of the operation

will be FFh. (255 dec.)

Syntax: DEC Rn



EXAMPLE:


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Before execution: R3=B0h

After execution: R3=AFh

DEC Rx - Decrement value in register Rx by 1


Description: Instruction decrements value of the register Rx by 1. As it is direct addressing, Rx must be within the first

255 locations of RAM. If there is a 0 in the register, the result will be FFh.

Syntax: DEC [register address]

Byte: 2 (Instruction Code, Address Rx)


EXAMPLE:

Before execution: CNT=0

After execution: CNT=FFh

DIV AB - Divide value in accumulator by value in register B

A: accumulator

B: Register B

Description: Instruction divides value in the accumulator by the value in the B register. After division the integer part of

result is stored in the accumulator while the register contains the remainder. In case of dividing by 1, the flag OV is set

and the result of division is unpredictable. The 8-bit quotient is stored in the accumulator and the 8-bit remainder is stored

in the B register.

Syntax: DIV AB


STATUS register flags: C, OV

EXAMPLE:

Before execution: A=FBh (251dec.) B=12h (18 dec.)

After execution: A=0Dh (13dec.) B=11h (17dec.)

13·18 + 17 =251

DEC @Ri - Decrement value in indirectly Addressed Register by 1


Description: This instruction decrements value in the Rx register by 1. Rx register address is in the Ri register (R0 or

R1). If there is 0 in the register, the result will be FFh.


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Syntax: DEC @Ri



EXAMPLE:

Register Address CNT = 4Fh R0=4Fh

Before execution: CNT=35h

After execution: CNT= 34h

DJNZ Rx,rel - Decrement Rx register by 1 and jump if the result is not 0


adr: Jump address

Description: This instruction first decrements value in the register. If the result is 0, the program continues execution.

Otherwise, a jump to the specified address in the program will be executed. As it is direct addressing, Rx must be within

the first 255 locations in RAM. This is a short jump instruction, which means that the address of a new location must be

relatively near the current position in the program ( -128 to +127 locations relative to the first following instruction).

Syntax: DJNZ Rx,[jump address]

Bytes: 3 (Instruction Code, Address Rx, Jump Value)


EXAMPLE:


After execution: If CNT≠0: PC=0423h

If CNT=0: PC=0446h

DJNZ Rn,rel - Decrement Rn Register by 1 and jump if the result is not 0


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adr: jump address

Description: This instruction first decrements value in the Rn register. If the result is 0, the program continues execution.

Otherwise, a jump to the specified address in the program will be executed. This is a short jump instruction, which means

that the address of a new location must be realtively near the current position in the program (- 128 to +127 locations

relative to the first following instruction).

Syntax: DJNZ Rn, [jump address]

Bytes: 2 (Instruction Code, Jump Value)


EXAMPLE:


After execution: If R1≠0: PC=0423h

If R1=0: PC=0446h

INC Rn - Increment value in Rn register by 1


Description: This instruction increments value in the Rn register by 1. If the register includes the number 255, the result

of the operation will be 0.

Syntax: INC Rn



EXAMPLE:

Before execution: R4=18h

After execution: R4=19h


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INC A - Increment Value in accumulator by 1

A: accumulator

Description: This instruction increments value in the accumulator by 1. If the accumulator includes the number 255, the

result of the operation will be 0.

Syntax: INC A



EXAMPLE:

Before execution: A=E4h

After execution: A=E5h

INC @Ri - Increment Value in indirectly addressed register by 1


Description: This instruction increments value in the Rx register by 1. Register Rx address is in the Ri Register ( R0 or

R1). If the register includes the number 255, the result of the operation will be 0.

Syntax: INC @Ri



EXAMPLE:

Register Address CNT = 4Fh

Before execution: CNT=35h R1=4Fh

After execution: CNT=36h

INC Rx - Increment Number in Rx register by 1


Description: This instruction increments value of the Rx register by 1.If the register includes the number 255, the result

of the operation will be 0. As it is direct addressing, Rx must be within the first 255 RAM locations.

Syntax: INC Rx


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EXAMPLE:

Before execution: CNT=33h

After execution: CNT=34h

JB bit,rel - Jump if Bit is Set

adr: Jump address

bit: Any bit in RAM

Description: If the bit is set, a jump to the specified address will be executed. Otherwise, if the value of bit is 0, the

program proceeds with the next instruction. This is a short jump instruction, which means that the address of a new

location must be relatively near the current position in the program (-128 to + 127 locations relative to the first following

instruction).

Syntax: JB bit, [jump address]

Bytes: 3 (Instruction Code, Bit Address, Jump value)


EXAMPLE:


After execution: If P0.5=0: PC=0324h

If P0.5=1: PC=0345h

INC DPTR - Increment Value of Data Pointer by 1

DPTR: Data Pointer

Description: This instruction increments value of the 16-bit data pointer by 1. This is a single 16-bit register on which


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this operation can be performed.

Syntax: INC DPTR



EXAMPLE:

Before execution: DPTR = 13FF (DPH = 13h DPL = FFh )

After execution: DPTR = 1400 (DPH = 14h DPL = 0)

JC rel - Jump if Carry Bit is Set

adr: Jump address

Description: This instruction first check if the Carry bit is set. If it is set, a jump to the indicated address is executed.

Otherwise, the program proceeds with the next instruction. This is a short jump instruction, which means that the address

of a new location must be relatively near the current position in the program (-129 to + 127 locations relative to the first

following instruction).

Syntax: JC [jump address]



EXAMPLE:

Before instruction: PC=0323h

After instruction: If C=0: PC=0324h

If C=1: PC=0345h

JBC bit,rel - If bit is set, clear it and jump to a new address

bit: Any bit in RAM

adr: Jump Addrress


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Description: This instruction first checks if the bit is set. If it is set, a jump to the specified address is executed and

afterwards the bit is cleared. Otherwise, the program proceeds with the first next instruction. This is a short jump

instruction, which means that the address of a new location must be relatively near the current position in the program

(-129 to + 127 locations relative to the first following instruction).

Syntax: JBC bit, [jump address]

Bytes: 3 (Instruction Code, Bit Address, Jump Value)


EXAMPLE:


After execution: If TEST0.4=1: PC=0345h, TEST0.4=0

If TEST0.4=0: PC=0324h, TEST0,4=0

JNB bit,rel - Jump if the bit is not set

adr: Jump address

bit: Any bit in RAM

Description: If the bit is cleared, a jump to the specified address will be executed. Otherwise, if the bit value is 1, the

program proceeds with the first next instruction. This is a short jump instruction, which means that the address of a new

location must be relatively near the current position in the program (-129 to + 127 locations relative to the first following

instruction).

Syntax: JNB bit,[jump address]

Bytes: 3 (Instruction Code, Bit Address, Jump Value)


EXAMPLE:


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After execution: If P0.5=1: PC=0324h

If P0.5=0: PC=0345h

JMP @A+DPTR - Indirect jump

A: accumulator

DPTR: Data Pointer

Description: This instruction causes a jump to address which is calculated by adding value in the accumulator and 16-bit

number in the DPTR Register. It is used with complex program branching where the accumulator affects jump address,

for example when reading table. Neither accumulator nor DPTR register are affected.

Syntax: JMP @A+DPTR



EXAMPLE:

Before execution: PC=223 DPTR=1400h

After execution: PC = 1402h if A=2

PC = 1404h if A=4

PC = 1406h if A=6

Note:

As instructions AJMP LABELS occupy two locations each, the values in the accumulator indicating them must be


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mutually different from each other by 2.

JNZ rel - Jump if value in accumulator is not 0

adr: Jump Address

Description: This instruction checks if value in the accumulator is 0. If it is not 0, a jump to the specified address will be

executed. Otherwise, the program proceeds with the first next instruction. This is a short jump instruction, which means

that the address of a new location must be relatively near the current position in the program (-129 to + 127 locations

relative to the first following instruction).

Syntax: JNZ [jump address]



EXAMPLE:


After execution: If A=0: PC=324h

If A≠0: PC=283h

JNC rel - Jump if Carry Bit is cleared

adr: Jump Address

Description: This instruction first checks if the bit is set. If it is not set, a jump to the specified address will be executed.

Otherwise, the program proceeds with the first next instruction.This is a short jump instruction, which means that the

address of a new location must be relatively near the current position in the program (-129 to + 127 locations relative to

the first following instruction).

Syntax: JNC [jump address]



EXAMPLE:


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After Sexecution: If C=0: PC=360h

If C=1: PC=324h

LCALL adr16 - Apsolute 'long' subroutine call

adr16: Subroutine Address

Description: This instruction unconditionally calls a subroutine located at the specified address. The current address and

the start of the called subroutine can be located anywhere within the memory space of 64K.

Syntax: LCALL [subroutine name]

Bytes: 3 (Instruction Code, Address (15-8), Address (7-0))


EXAMPLE:



JZ rel - Jump if the value in accumulator is 0

adr: Jump Address

Description: The instruction checks if the value in the accumulator is 0. If it is 0, a jump to the specified address will be

executed. Otherwise, the program proceeds with the next instruction.This is a short jump instruction, which means that

the address of a new location must be relatively near the current position in the program (-129 to + 127 locations relative

to the first following instruction).

Syntax: JZ [jump address]



EXAMPLE:


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After execution: If A0: PC=324h

If A=0: PC=283h

MOV A,Rn - Move Rn register to accumulator


A: accumulator

Description: The instruction moves the Rn register to the accumulator. The Rn register is not affected.

Syntax: MOV A,Rn



EXAMPLE:


After execution: R3=58h A=58h

LJMP adr16 - 'Long' jump

adr16: Jump address

Description: Instruction causes a jump to the specified 16-bit address.

Syntax: LJMP [jump address]

Bytes: 3 (Instruction Code, Address (15-8), Address (7-0))


EXAMPLE:


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MOV A,@Ri - Move indirectly addressed register Rx to accumulator


A: accumulator

Description: Instruction moves the Rx register to the accumulator. Rx register address is stored in the Ri register (R0 or

R1). After instruction execution, the result is stored in the accumulator. The Rx Register is not affected.

Syntax: MOV A,@Ri



EXAMPLE:

Register Address SUM=F2h R0=F2h

Before execution: SUM=58h

After execution : A=58h SUM=58h

MOV A,Rx - Move Rx register to accumulator

Rx: arbitrary register with address 0 - 255 (0 - FFh)

A: accumulator

Description: Instruction moves the Rx register to the accumulator. As it is direct addressing, Rx can be some of SFRs or

general-purpose register with address 0-7Fh. (0-127 dec.). After instruction execution, Rx is not affected.

Syntax: MOV A,Rx

Byte: 2 (Instruction Code, Address Rx)


EXAMPLE:


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Before execution: Rx=68h

After execution : Rx=68h A=68h

MOV Rn,A - Move accumulator to Rn register


A: accumulator

Desription: Instruction moves the accumulator to Rn register. The accumulator is not affected.

Syntax: MOV Rn,A



EXAMPLE:

Before execution: A=58h

After execution: R3=58h A=58h

MOV A,#X - Move number X to accumulator

A: accumulator


Desription: Instruction writes number X to the accumulator.

Syntax: MOV A,#X



EXAMPLE:

After execution: A=28h

MOV Rn,#X - Move number X to Rn register


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Rn: Any R register (R0-R7) X: Constant in the range of 0 - 255 (0-FFh)

Description: Instruction writes number X to the Rn register .

Syntax: MOV Rn,#X



EXAMPLE:

After execution : R5=32h

MOV Rn,Rx - Move Rx register to Rn register

Rn: Any R registar (R0-R7)


Description: Instruction moves the Rx register to Rn register. As it is direct addressing, Rx can be some of SFRs or


Syntax: MOV Rn,Rx



EXAMPLE:

Before execution: SUM=58h

After execution: SUM=58h R3=58h

MOV Rx,Rn - Move Rn register to Rx register



Description: Instruction moves the Rn register to Rx register. As it is direct addressing, Rx can be some of SFRs or


Syntax: MOV Rx,Rn



EXAMPLE:


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After execution: R3=18h CIF=18h

MOV Rx,A - Move accumulator to Rx register


A: accumulator

Description: Instruction moves the accumulator to Rx register. As it is direct addressing, Rx can be some of SFRs or


Syntax: MOV Rx,A



EXAMPLE:

Before execution: A=98h

After execution: A=98h REG=98h

MOV Rx,@Ri - Move number from indirectly addressed register to Rx register



Description: Instruction moves the Ry register to Rx register. Ry register address is stored in the Ri register (R0 or R1).

The Ry register is not affected .

Syntax: MOV Rx,@Ri



EXAMPLE:

Register Address SUM=F3


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Before execution: SUM=58h R1=F3

After execution: SUM=58h TEMP=58h

MOV Rx,Ry - Move Ry register to Rx register


Ry: Arbitrary register with address 0 - 255 (0 - FFh)

Description: Instruction moves the Ry register to Rx register. As it is direct addressing, Rx and Ry can be some of SFRs

or general-purpose registers with address 0-7Fh. (0-127 dec.). The Ry register is not affected.

Syntax: MOV Rx,Ry

Bytes: 3 (Instruction Code, Address Ry, Address Rx)


EXAMPLE:

Before execution: TEMP=58h

After execution: TEMP=58h SUM=58h

MOV @Ri,A - Move accumulator to indirectly addressed register

A: accumulator


Description: Instruction moves the accumulator to the Rx register. The Rx register address is stored in the Ri register (R0

or R1). After instruction execution, the accumulator is not affected.

Syntax: MOV @Ri,A



EXAMPLE:

Register Address SUMA=F2h

Before execution: R0=F2h A=58h

After execution: SUMA=58h A=58h

MOV Rx,#X - Move number X to Rx register



Description: Instruction moves number X to the Rx register. As it is direct addressing, Rx can be some of SFRs or

general-purpose register with address 0-7Fh. (0-127 dec.).


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Syntax: MOV Rx,#X



EXAMPLE:

After execution: TEMP=22h

MOV @Ri,#X - Move number X to indirectly addressed register



Description: Instruction moves number X to the idirectly addressed register Rx. The Register Rx address is stored in the

Ri register ( R0 or R1).

Syntax: MOV @Ri,#X



EXAMPLE:

Register address TEMP=E2h

Before execution: R1=E2h

After execution: TEMP=44h

MOV @Ri,Rx - Move Rx register to indirectly addressed register



Description: Instruction moves the Rx register to Ry register. The register Ry address is stored in the Ri register ( R0 or

R1). After instruction execution, the Rx register is not affected.

Syntax: MOV @Ri,Rx



EXAMPLE:


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Register address TEMP=E2h

Before execution: SUM=58h R1=E2h

After execution: SUM=58h TEMP=58h

MOV bit,C - Move Carry bit to specified bit

C: Carry bit

bit: Any bit in RAM

Description: Instruction moves the value of the Carry bit to the specified bit. After this operation, the Carry bit is not

affected.

Syntax: MOV bit,C

Bytes: 2 (Instruction Code, Address bit)


EXAMPLE:

After execution: If C=0 P1.2=0

If C=1 P1.2=1

MOV C,bit - Move indicated bit to Carry bit

C: Carry bit

bit: Any bit in RAM

Description: Instruction moves value of the specified bit to the Carry bit. After this operation, the bit is not affected.

Syntax: MOV C,bit

Bytes: 2 (Instruction Code, Bit address)


EXAMPLE:

After execution: If P1.4=0 C=0

If P1.4=1 C=1


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MOVC A,@A+DPTR - Move relatively addressed byte from program memory to accumulator

A: accumulator

DPTR: Data Pointer

Description: Instruction first adds the 16-bit DPTR Register and the accumulator. The result of addition is afterwards

used as address in the program memory indicating from which the 8-bit content is moved to the accumulator.

Syntax: MOVC A,@A+DPTR


STATUS register flags: No flags affected.

EXAMPLE:

Before execution :

DPTR=1000:

A=0

A=1

A=2

A=3

After execution:

A=66h

A=77h

A=88h

A=99h

Note: DB (Define Byte) is a directive in assembler used to define constant.

MOV DPTR,#X16 - Write 16-bit number to Data Pointer

X: constant in the range of 0 - 65535 (0-FFFFh)

DPTR: Data Pointer

Description: Instruction writes 16-bit number into the DPTR register. The 8 high bits of this number are stored in the

DPH register while the 8 low bits are stored in the DPL register.

Syntax: MOV DPTR,#X

Bytes: 3 (Instruction Code, Constant (15-8), Constant (7-0))


EXAMPLE:


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After execution: DPH=12h DPL=34h

MOVX A,@Ri - Move from external memory (8-bit address) to accumulator


A: accumulator

Description: Instruction reads the content of the Rx register in external RAM and moves it to the accumulator. The

register Rx address is stored in the Ri register (R0 or R1).

Syntax: MOVX A,@Ri



EXAMPLE:

Register Address SUMA=12h

Before execution: SUMA=58h R0=12h


Note:

SUMA Register is stored in external RAM in size of 256 bytes.

MOVC A,@A+PC - Move relatively addressed byte from program memory to accumulator

A: accumulator

PC: Program Counter

Description: Instruction first adds the 16-bit PC register with the content of the accumulator (the current address in the

program is stored in the PC register). The result of addition is afterwards used as address in the program memory from

which the 8-bit content is moved to the accumulator.

Syntax: MOVC A,@A+PC



EXAMPLE:


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After the subroutine "Tabela" has been executed, one of four values is stored in the accumulator:

Before execution:

A=0

A=1

A=2

A=3

After execution:

A=66h

A=77h

A=88h

A=99h

Note: DB (Define Byte) is directiv in assembler used to define constant.

MOVX @Ri,A - Write the content of accumulator into byte of external memory (8-bit address)


A: accumulator

Description: Instruction reads the content of the accumulator and moves it to the Rx register which is stored in external

RAM. The Rx register address is located in the Ri register.

Syntax: MOVX @Ri,A



EXAMPLE:

Register address SUM=34h

Before execution: A=58 R1=34h

After execution: SUM=58h

NOTE:

Register SUM is located in external RAM in size of 256 byte.


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MOVX A,@DPTR - Write the content of accumulator into byte of external memory (8-bit address)

A: accumulator

DPRTR: Data Pointer

Description: Instruction reads the content of the Rx register in external memory and moves it to the accumulator. The

16-bit address of the Rx register is stored in the DPTR register (DPH and DPL).

Syntax: MOVX A,@DPTR



EXAMPLE:

Register Address SUM=1234h

Before execution: DPTR=1234h SUM=58


Note:

Register SUM is located in external RAM in size of up to 64K.

MUL AB - Multiply value in accumulator with value in B register

A: accumulator

B: Register B

Description: Instruction multiplies the value in the accumulator with the value in the B register . The low-order byte of

the 16-bit result is stored in the accumulator , and the high byte is left in the B register. If the result is greater than 255,

the overflow flag is set. The Carry bit (C flag) is not affected.

Syntax: MUL AB



EXAMPLE:

Before execution: A=80 (50h) B=160 (A0h)

After execution: A=0 B=32h

A·B=80·160=12800 (3200h)

MOVX @DPTR,A - Write value in accumulator to byte of external memory (16-bit address)

A: accumulator

DPTR: Data Pointer

Description: Instruction reads value in the accumulator and moves it to the Rx register which is stored in external RAM.


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16-bit address of the Rx register is stored in the DPTR register (DPH and DPL).

Syntax: MOVX @DPTR,A



EXAMPLE:

Register address SUM=1234h

Before execution: A=58 DPTR=1234h

After execution: SUM=58h

Note:

Register SUM is located in RAM in size of up to 64K.

ORL A,Rn - Logical-OR operation between accumulator and Rn register


A: accumulator

Description: Instruction performs logical-OR operation between the accumulator and Rn register. The result of this


Syntax: ORL A,Rn



EXAMPLE:


R5= 55h (01010101 Bin.)

After execution : A= D7h (11010111 Bin.)

NOP - No operation

Description: Instruction doesn’t perform any operation and is used when additional time delays are needed.

Syntax: NOP



EXAMPLE:


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Sequence like this one provides on the P2.3 a negative pulse which lasts exactly 5 machine cycles. If a 12 MHz quartz

crystal is used then 1 cycle lasts 1uS, which means that this output will be a low-going output pulse for 5 uS.

ORL A,@Ri - Logical-OR operation between accumulator and indirectly addressed register


A: accumulator

Description: Instruction performs logical-OR operation between the accumulator and Rx register. As it is indirect

addressing, register Rx address is stored in the Ri register ( R0 or R1). The result of this logical operation is stored in the

accumulator.

Syntax: ANL A,@Ri



EXAMPLE:

Register Address TEMP=FAh

Before execution: R1=FAh

TEMP= C2h (11000010 Bin.)

A= 54h (01010100 Bin.)

After execution: A= D6h (11010110 Bin.)

ORL A,Rx - logical-OR operation between accumulator and Rx register


A: accumulator

Description: Instruction performs logical-OR operation between the accumulator and Rx register. As it is direct



Syntax: ORL A,Rx




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EXAMPLE:


LOG= 54h (01010100 Bin.)


ORL Rx,A - Logical-OR operation between directly addressed register Rx and accumulator


A: accumulator

Description: Instruction performs logical-OR operation between the Rx register and accumulator. As it is direct

addressing, the Rx register can be some of SFRs or general- purpose register with address 0-7Fh (0-127 dec.) . The result

of this logical operation is stored in the Rx register.

Syntax: ORL [register address], A



EXAMPLE:

Before execution: TEMP= C2h (11000010 Bin.)

A= 54h (01010100 Bin.)


ORL A,#X - Logical-OR operation between accumulator and number X


A: accumulator

Description: Instruction performs logical-OR operation between the accumulator and number X. The result of this


Syntax: ORL A, #X



EXAMPLE:


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After execution: A= C3h (11000011 Bin.)

ORL C,bit - Logical-OR operation between bit and Carry bit

C: Carry bit

bit: Any bit in RAM

Description: Instruction performs logical-OR operation (logical OR) between the addressed bit and Carry bit. The result

is stored in the Carry bit.

Syntax: ORL C,bit



EXAMPLE:

Before execution: ACC= C6h (11001010 Bin.)

C=0


ORL Rx,#X - Logical-OR operation between directly addressed register Rx and number X



Description: Instruction performs logical-OR operation between the Rx registers and number X. As it is direct


logical operation is stored in the Rx register.

Syntax: ORL [register address],#X



EXAMPLE:


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POP Rx - Pop data from Stack


Description: Instruction first reads data from the location the Stack Pointer is currently ponting to. Afterwards, the data is

copied to the register Rx and the value of the Stack Pointer is decremented by 1. As it is direct addressing, Rx can be

some of SFRs or general-purpose register with address 0-7Fh. (0-127 dec.)

Syntax: POP Rx



EXAMPLE:

Before execution: Address Value

030h 20h

031h 23h

SP==> 032h 01h

DPTR=0123h (DPH=01, DPL=23h)

After execution: Address Value

SP==> 030h 20h

031h 23h

032h 01h

ORL C,/bit - Logical-OR operation between complement bit and Carry bit

C: Carry bit

bit: Any bit in RAM

Description: Instruction performs logical-AND operation (logical OR) between addressed inverted bit and Carry bit. The

result is stored in the Carry bit.

bit bit C C AND bit

0 1 0 0

0 1 1 1

1 0 0 0

1 0 1 0

Syntax: ORL C,/bit



EXAMPLE:


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Before execution: ACC= C6h (11001010 Bin.)

C=0


RET - Return from subroutine

Description: This instruction ends every subroutine. After execution, the program proceeds with the instruction currently

following an ACALL or LCALL.

Syntax: RET



EXAMPLE:

PUSH Rx - Push data onto Stack


Description: Address currently pointed to by the Stack Pointer is first incremented by 1 and afterwards the data from the

register Rx are copied to it. As it is direct addressing, Rx can be some of SFRs or general-purpose register with address


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0-7Fh. (0-127 dec.)

Syntax: PUSH Rx



EXAMPLE:

Before execution: Address Value

SP==> 030h 20h

DPTR=0123h (DPH=01, DPL=23h)

After execution: Address Value

030h 20h

031h 23h

SP==> 032h 01h

RL A - Rotate accumulator one bit left

A: accumulator

Description: Eight bits in the accumulator are rotated one bit left, so that the bit 7 is rotated into the bit 0 position.

Syntax: RL A



EXAMPLE:


After execution: A=85h (10000101 Bin.)

RETI - Return from interrupt


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Description: This instruction ends every interrupt routine and informs processor that interrupt routine is no longer in

progress. After instruction execution, the execution of the interrupted program continues from where it left off. The PSW

is not autotomatically restored to its pre-interrupt status.

Syntax: RETI



RR A - Rotate accumulator one bit right

A: accumulator

Description: All eight bits in the accumulator are rotaded one bit right so that the bit 0 is rotated into the bit 7 position.

Syntax: RR A



EXAMPLE:



RLC A - Rotate accumulator one bit left through Carry bit

A: accumulator

Description: All eight bits in the accumulator and Carry bit are rotated one bit left. After this operation, the bit 7 is

rotated into the Carry bit position and the Carry bit is rotated into the bit 0 position.

Syntax: RLC A



EXAMPLE:


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C=0


C=1

SETB C - Set Carry bit

C: Carry bit

Description: Instruction sets the Carry bit.

Syntax: SETB C



EXAMPLE:


RRC A - Rotate accumulator one bit right through Carry bit

A: accumulator

Description: All eight bits in the accumulator and Carry bit are rotated one bit right. After this operation, the Carry bit is

rotated into the bit 7 position and the bit 0 is rotated into the Carry position.

Syntax: RRC A



EXAMPLE:


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C=0


C=0

SJMP rel - Short Jump

adr: Jump Address

Description: Instruction enables jump to the new address that address should be in the range of -128 to +127 locations

relative to the first following instruction.

Syntax: SJMP [jump address]



EXAMPLE:

Before execution: PC=323

After execution: PC=345

SETB bit - Set bit

bit: Any bit in RAM


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Description: Instruction sets the specified bit. The register including that bit must belong to the group of so called bit

addressable registers.

Syntax: SETB [bit address]



EXAMPLE:

Before execution: P0.1 = 34h (00110100)

pin 1 is configured as output

After execution: P0.1 = 35h (00110101)

pin 1 is configured as input

SUBB A,Rx - Subtract Rx from accumulator


A: accumulator

Description: Instruction performs subtract operation: A-Rx including the Carry bit as well which acts as borrow. If the

higher bit is subtracted from the lower bit then the Carry bit is set. As it is direct addressing, Rx can be some of SFRs or

general-purpose register with address 0-7Fh. (0-127 dec.). The result is stored in the accumulator.

Syntax: SUBB A,Rx


STATUS register flags: C, OV, AC

EXAMPLE:

Before execution: A=C9h, DIF=53h, C=0

After execution: A=76h, C=0

SUBB A,Rn - Subtruct Rn from accumulator


A: accumulator

Description: Instruction performs subtract operation: A-Rn including the Carry as well which acts as borrow. If the

higher bit is subtracted from the lower bit then the Carry bit is set. The result is stored in the accumulator.

Syntax: SUBB A,Rn



EXAMPLE:


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Before execution: A=C9h, R4=54h, C=1

After execution: A=74h, C=0

Note:

The result is different (C9 - 54=75!) because the Carry bit has been set (C=1)before instruction execution.

SUBB A,#X - Subtract number X from accumulator

A: accumulator


Description: Instruction performs subtract operation: A-X including the Carry bit as well which acts as borrow. If the

higher bit is subtracted from the lower bit then the Carry bit is set. The result is stored in the accumulator.

Syntax: SUBB A,#X



EXAMPLE:

Before execution: A=C9h, C=0

After execution: A=A7h, C=0

SUBB A,@Ri - Subtract indirectly addressed register from accumulator


A: accumulator

Description: Instruction performs subtract operation: A-Rx including the Carry bit as well which acts as borrow. If the

higher bit is subtracted from the lower bit then the Carry bit is set. As it is indirect addressing, register Rx address is

located in the Ri register (R0 or R1) . The result of the operation is stored in the accumulator.

Syntax: SUBB A,@Ri



EXAMPLE:


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Register Address MIN=F4

Before execution: A=C9h, R1=F4h, MIN=04, C=0

After execution: A=C5h, C=0

XCH A,Rn - Exchange registers Rn with accumulator


A: accumulator

Description: Instruction causes the accumulator and Rn registers to exchange data. The content of the accumulator is set

in the register Rn. At the same time, the content of the Rn register is set in the accumulator.

Syntax: XCH A,Rn



EXAMPLE:

Before execution: A=C6h, R3=29h

After execution: R3=C6h, A=29h

SWAP A - Swap nibbles within accumulator

A: accumulator

Description: A word “nibble” designates a group of 4 adjacent bits within one register (bit0-bit3 and bit4-bit7).This

instruction interchanges the high and low nibbles of the accumulator.

Syntax: SWAP A



EXAMPLE:

Before execution: A=E1h (11100001)bin.

After execution: A=1Eh (00011110)bin.


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XCH A,@Ri - Exchange accumulator with indirectly addressed register Rx


A: accumulator

Description: Instruction sets the contents of accumulator into register Rx. At the same time, the content of register Rx is

set into the accumulator. As it is indirect addressing, register Rx address is located in the register Ri (R0 or R1).

Syntax: XCH A,@Ri



EXAMPLE:

Register Address SUM=E3

Before execution: R0=E3, SUM=29h, A=98h

After execution: A=29h, SUM=98h

XCH A,Rx - Exchange the content of registers Rx with the content of accumulator


A: accumulator

Description: Instruction sets the contents of the accumulator into the register Rx. At the same time, the content of the Rx

register is set into the accumulator. As it is direct addressing, the register Rx can be some of SFRs or general-purpose

register with address 0-7Fh (0-127 dec.).

Syntax: XCH A,Rx



EXAMPLE:

Before execution: A=FFh, SUM=29h

After execution: SUM=FFh A=29h


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XRL A,Rn - Exclusive-OR operation between register Rn and accumulator


A: accumulator

Description: Instruction performs exclusive-OR operation between the accumulator and Rn register. The result of this


Syntax: XRL A,Rn



EXAMPLE:


R3= 55h (01010101 Bin.)


XCHD A,@Ri - Exchange the content of low nibbles accumulator with indirectly addressed register Rx


A: accumulator

Description: This instruction interchanges the low nibbles (bits 0-3) of the accumulator with the low nibbles of indirectly

addressed register Rx. High nibbles of the accumulator and Rx register are not affected. This instruction is mainly used in

operating with BCD values. As it is indirect addressing, the regiter Rx address is stored in the register Ri (R0 or R1).

Syntax: XCHD A,@Ri



EXAMPLE:

Register Address SUM=E3

Before execution: R0=E3 SUM=29h A=A8h,

After execution: A=A9h, SUM=28h


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XRL A,@Ri - Exclusive-OR operation between accumulator and indirectly addressed register


A: accumulator

Description: Instruction performs exclusive-OR operation between the accumulator and Rx register. As it is indirect

addressing, register Rx address is stored in the Ri register (R0 or R1). The result of this logical operation is stored in the

accumulator.

Syntax: XRL A,@Ri



EXAMPLE:

Register Address TEMP=FAh, R1=FAh


A= 54h (01010100 Bin.)


XRL A,Rx - Exclusive-OR operation between accumulator and register Rx


A: accumulator

Description: Instruction performs exclusive-OR operation between the accumulator and Rx Register. As it is direct

addressing, the Rx register can be some of SFRs or general-purpose register with address 0-7Fh (0-127 dec.). The result

of this logical operation is stored in the accumulator.

Syntax: XRL A,Rx



EXAMPLE:


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LOG= 54h (01010100 Bin.)


XRL Rx,A - Exclusive-OR operation between directly addressed register Rx and accumulator


A: accumulator

Description: Instruction performs exclusive-OR operation between the Rx Register and accumulator. As it is direct



Syntax: XRL Rx,A



EXAMPLE:


A= 54h (01010100 Bin.)


XRL A,#X - Exclusive-OR between accumulator and number X


A: accumulator

Description: Instruction performs exclusive-OR operation between the accumulator and number X. The result of this


Syntax: XRL A,#X



EXAMPLE:


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X= 11h (00010001 Bin.)


XRL Rx,# - Exclusive-OR operation between directly addressed register Rx and number X



Description: Instruction performs exclusive-OR operation between the Rx Register and number X. As it is direct



Syntax: XRL Rx,#X



EXAMPLE:


X=12h (00010010 Bin.)


Architecture and Programming of 8051 Microcontrollers | Chapter 4 :... http://www.mikroe.com/en/books/8051book/ch4/

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Chapter 4 : AT89S8253 Microcontroller

4.1 AT89S8253 Microcontroller ID

4.2 Pin Description

4.3 AT89S8253 Microcontroller Memory Organisation


4.5 Watchdog Timer (WDT)

4.6 Interrupts


4.8 UART (Universal Asynchronous Receiver Transmitter)

4.9 SPI System (Serial Peripheral Interface)

4.10 Power Consumption Control

Introduction

Today, after more than 20 years of continuous improvement, the 8051 microcontroller is being manufactured across the

world by many companies and under different trademarks. Of course, the latest models are by far more advanced than the

original one. Many of these models are labeled as “8051 compatible”, “8051 compliant”or “8051 family” in order to

emphasize their “noble heritage”. The tags should imply that microcontrollers have similar architecture and are

programmed in a similar way, using the same instruction set. In practice, if you know how to handle one of them, you will

be able to handle any other belonging to 8051 family, which encompasses several hundreds of different models!

This book covers one model named AT89S8253, manufactured by Atmel. Why this particular one? Because it is

widespred, cheap and uses Flash memory for program storage. This last feature makes it ideal for experimentation due to

the fact that program can be loaded and erased a number of times. Besides, thanks to the built-in SPI System (Serial

Programing Interface), program can be loaded to the microcontroller even if the chip has already been embedded in the

final device.

4.1 AT89S8253 Microcontroller ID

Compatible with 8051 family.

12Kb of Flash Memory for program storage.

Program is loaded via SPI System (Serial Peripheral Interface).

Program may be loaded / erased up to 1000 times.

2Kb of EEPROM Memory

Power Supply Voltage: 4 - 6V.

Operating clock frequency: 0 - 24MHz.

256 bytes of internal RAM for storing variables.

32 input/output lines.

Three 16-bit Timers / Counters.

9 interrupt sources.

2 additional power saving modes (Low-power Idle and Power-down Mode).

Programmable UART serial communication.

Programmable Watchdog Timer.

Three-level Program Memory Lock



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Packages in which AT89S53 appears on the market.


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4.2 Pin Description

VCC Power supply voltage (4-6V)

GND Ground ( Negative supply pole)

Port 0 (P0.0-P0.7) If configured as output, each of these pins can be connected up to 8 TTL inputs. If configured as

input, the pins can be used as high-impedance inputs as their potential is undefined relative to the ground, i.e. these inputs

are floating. If additional (external) memory is used, these pins are used for alternate transfer of data and addresses

(A0-A7) for accessing this additional memory chip. Signal on ALE pin determines what and when will be transferred on

the port.

Port 1 (P1.0-P1.7) If configured as output, each of these pins can be connected up to 4 TTL inputs. When configured as

input, these pins act as standard TTL inputs, that is, each of them is internally connected to the positive supply voltage via

relatively high impedance resistor. The voltage on these inputs is 5V. Also, this Port 1 pins have alternate functions as

shown in the table below :

Port Pin Alternate Function

P1.0 T2 (Timer 2 input)

P1.1 T2EX (Timer 2 control input)

P1.4 SS (SPI system control input)

P1.5 MOSI (SPI system I/O)

P1.6 MISO (SPI system I/O)

P1.7 SCK (SPI system clock signal)


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Port 2 (P2.0-P2.7) If configured as input or output, this port is identical to Port 1. If external memory is used, Port 2

stores the higer address byte (A8-A15) for addressing additional memory chip.

Port 3 (P3.0-P3.7) Similar to P1, Port 3 pins can be used as universal input or output, but also have additional functions

which will be explained later in the chapter.

Port Pin Alternate Function

P3.0 RXD (serial input)

P3.1 TXD (serial output)

P3.2 INT0 (external interrupt 0)

P3.3 INT1 (external interrupt 1)

P3.4 T0 (Timer 0 external input)

P3.5 T1 (Timer 1 external input)

P3.6 WR (External data memory write signal)

P3.7 RD (External data memory read signal)

RST Logic one (1) on this pin resets the microcontroller.

ALE/PROG In normal operation, this pin emits a pulse sequence with a frequency equal to 1/6 of the main oscillator

frequency. If additional memory is used, signal from this pin controls the additional register for temporary storage of the

lower address byte (A0-A7). During writing program to the microcontroller, this pin also serves as a control input.

PSEN This pin signal is used for reading from external program memory (ROM).

EA/VPP When this pin is connected to the ground, the microcontroller takes program instructions from external program

memory. In case that internal program memory is used (common case), this pin should be connected to the positive supply

voltage (VCC). During loading program to internal Flash mamory, this pin is supplied with +12V.

XTAL 1 This is internal oscillator input. It is used for synchronizing the microcontroller with another circuit or when the

external oscillator which generates clock pulses is for some reason used.

XTAL 2 This pin is connected to internal oscillator output. In case that external oscillator is used, this pin is out of use.

4.3 AT89S8253 Microcontroller Memory Organisation

ROM (Program Memory)

Program memory with a capacity of 12Kb is designed in FLASH technology, which enables a great number of writing

to/erasing up programs. It is programmed via embedded SPI module (Serial Peripheral Interface). Although, it is possible to

add external ROM memory chip, 12Kb is more than enough.

RAM (Random Access Memory)

This memory consists of 3 blocks with 128 registers each, and structure that falls into the 8051 Standard:

128 general-purpose registers

128 memory locations reserved for SFRs. Even though only some of them are trully used, the free ones shouldn’t be

used for storing variables.

128 additional registers free to be used (have no special purpose). They have the same addresses as SFRs, but are

accessed by indirect


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EEPROM Memory

EEPROM is a special type of memory, having features of both RAM and ROM. The data are being written to and erased

during operation, but saved after the power is turned off. This microcontroller has total of 2K of EEPROM (2048

locations).

Memory Extension

Taking into account that AT89S8253 microcontroller is based on the 8051 core, all mentioned before for this model’s

ROM and RAM memory extension remains in force. Meaning that both memories can be added as external chips with the

capacity up to 64Kb. Addressing is also the same as in the 8051 Standard.

Types of addressing

Similar to all the microcontrollers compatible with the 8051, there are two ways of addressing:

Direct (for example: MOV A,30h)

Indirect (for example: MOV A,@R0)


The AT89S8253 microcontroller has total of 40 Special Function Registers. For the sake of the compatibility with eather

8051 models, the basic group of registers (22 of them ) kept their functions and addresses, while the rest were added to

manage new functions of the microcontroller.


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As shown in the table above, each of these registers has its name and specific address in RAM. Unoccupied locations are

intended for future expansions and new models of the microcontroller and shouldn’t be used. This chapter covers “general”

SFRs’ function.

Specialized registers as the ones controlling timers or SPI will be described in the following chapters.

ACC (Accumulator)

Accumulator is designated as ACC or A and belongs to the basic register group of the 8051 core. There are no changes on

bits of this register.

B register

B register also belongs to the basic register group of the 8051 core and there are no changes on its bits. Instructions of

multiplication and division (MUL and DIV instructions) can be applied only to operands located in registers A and B.

PSW register (Program Status Word)

PSW register belongs to the basic register group of the 8051 core. There are no changes on bits of this register.

SP registar (Stack Pointer)

SP Register belongs to the basic register group of the 8051 core. There are no changes on bits of this register.


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Registers P0, P1, P2, P3

Every bit of these registers corresponds to one of the ports pins having the same name. These registers are therefore used

for comminicating to peripheral environment by copying data from registers to corresponding pins and vice versa. The

registers belongs to the basic register group of the 8051 core and there are no changes on their bits.

R registers (R0 - R7)

They belong to the basic register group of the 8051 core. There are no changes on their bits.

AUXR register (Auxiliary register)

The AUXR register contains only two active bits:

DISALE


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0 - Pulse sequence with the frequency equal to 1/6 of the quartz oscillator frequency appears on ALE.

1 - Pin ALE is active only during execution of MOVX or MOVC instructions.

Intel_Pwd_Exit

0 - When the microcontroller is in Power Down mode, the program proceeds with execution after the falling

edge signal appears (1-0).

1 - When the microcontroller is in Power Down mode, the program proceeds with execution after the raising

edge signal appears (0-1).

CLKREG register (Clock Register) X2

0 - The oscillator frequency (at XTAL1 pin) is divided by 2 before it is used as clock (machine cycle lasts for 6 such

periods).

1 - The oscillator signal is directly used as clock generator. In this way, for the same microcontroller’s operating rate,

a quartz crystal for two times lower frequency may be used (for example 6MHz instead of 12MHz).

Data Pointers

Data Pointers are not true registers because they don’t physically exist. They consist of two separate registers: DPH (Data

Pointer High) and DPL (Data Pointer Low). Data Pointer’s 16 bits are used for addressing external memory and internal

EEPROM memory. DPS bit located in the EECON register is in command which registers are to be used as data pointers:

DPS=0 -> Data pointer consists of DP0L and DP0H registers and is designated as DPTR0.

DPS=1 -> Data pointer consists of registers DP1L and DP1H and is designated as DPTR1.


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Handling EEPROM memory

2 Kb of on-chip EEPROM memory enable this microcontroller to be used in devices which have to permanently store

runtime data. All data stored in this memory will be saved even after the power supply is off and the producer warrants at

least 100 000 writing cycles. It is easy for use since there are only a few control bits enabling it.

EEPROM write and read is controlled by EECON special function register. Since the process of programming of EEPROM

is relatively slow (writing to one register takes approximately 4mS), a small hardware trick is done in order to enhance it.

When bit EELD in EECON is set, the data is not directly written to EEPROM registers but loaded in a small buffer

(temporary memory) with the capacity of 32 bytes. When this bit is cleared, the first data following it will be normally

written ( takes 4 mS). Also, all registers currently loaded in the buffer will be written simultaneously. In this way, all 32

bytes require only 4mS to be written instead of 128mS required in single byte writing.

EEPROM memory is treated as external memory and that’s why a special instruction for handling additional memory chip

(MOVX) is used. The EEMEN bit in the EECON register controlls whether the data will be written/read from an true

additional chip or on-chip EEPROM memory.

EECON register

This register’s bits controls the operation of EEPROM memory:


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WRTINH

This bit can be read only. When the power supply level is too low for programming of EEPROM, hardware automatically

clears the bit, which means that programming can not be executed or that ongoing programming will be aborted.

RDY/BSY

This bit can be read only.

0 - Programming is in progress (takes approximately 4mS).

1 - Programming is completed (data is written to EEPROM).

DPS

0 - Address for programming or reading from EEPROM is stored in the DP0H and DP0L registers.

1 - Address for programming or reading from EEPROM is stored in the DP1H and DP1L registers.

EEMEN

0 - Instruction MOVX is used for accessing external memory chip.

1 - Instruction MOVX is used for accessing internal EEPROM memory. If register addess is greater than 2K, the

microcontroller will access external memory.

EEMWE

When set, EEMWE bit enables writing data to EEPROM. The MOVX instruction is used for data writing. After EEPROM

write is completed, the bit needs to be cleared from within the program.

EELD

When set, EELD bit enables writing up to 32 bytes simultaneously. When bit is set, the MOVX instruction will not initiate

programming of EEPROM, it will just load data to the data buffer of EEPROM memory. Before writing the last data, the

bit is cleared and upon the last MOVX, the entire buffer is automatically programmed to EEPROM for 4mS.

4.5 Watchdog Timer (WDT)

Watch-dog timer uses pulses from the quartz oscillator. After Reset and during Power Down Mode, this timer is disabled

and has no effect on the program execution. When enabled, a well known battle against the time starts and timer is always

“loose”. If program works properly. In this case, the program will always manage reset watchdog timer on time. Otherwise,

if watch-dog timer manages to count a full cycle, it indicates that the program doesn’t work properly for some reason. Then

WDT comes into force and resets the microcontroller. Obviously, the point is to set instruction in the main program loop

which will unceasingly reset the watch-dog timer. In practice, several bits of WDTCON register control this simple and

efficient mechanism.

Three bits (PS2, PS1 and PS0) which are in control of prescaler, determine the most important feature of Watch-dog timer-

so called nominal time, i.e. time needed to count a full cycle.

The values in the table are valid in case that quartz crystal with frequency of 12MHz is used.

Prescaler BitsNominal Time

PS2 PS1 PS0

0 0 0 16ms

0 0 1 32ms

0 1 0 64ms

0 1 1 128ms

1 0 0 256ms

1 0 1 512ms


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1 1 0 1024ms

1 1 1 2048ms

WDTCON (Watchdog Control Register)

PS2,PS1,PS0

These bits are in control of prescaler and define so called nominal time of the Watchdog timer. If the program doesn’t clear

WSWRST bit during that time, this timer will reset the microcontroller. (When all three bits are cleared to 0, the watch-dog

timer has a nominal period of 16K machine cycles. When all three bits are set to 1, the nominal period is 2048K machine

cycles).

WDIDLE

This bit enables/disables the Watch-dog Timer in Idle mode:

0 - Watch-dog timer continues to count in Idle mode (power consumption control).

1 - InstructionWatch-dog timer is halted while the microcontroller is in Idle mode.

DISRTO

This bit enables/disables reset of external electronic circuits (out of the microcontroller) connected to the RST pin:

0 - Watch-dog controls state of the reset pin. This means that, at the moment of reset, this pin is driven high and acts

for a short time as output. In that way, the micro controller as well as all other circuits connected to the RST pin are

reset.

1 - Reset which generates Watch-dog doesn’t affect state of the reset pin. Watch-dog resets only the microcontroller

while the reset pin continues acting as input.

HWDT

This bit selects hardware or software mode for the Watch-dog Timer:

0 - Watch-dog is in so called software mode, meaning that it can be enabled or disabled by simply setting or clearing

WDTEN bit.

1 - Watch-dog is in so called hardware mode. In order to activate watch-dog in this mode, the sequence 1E/E1(hex)

should be written to the WDTRST register. After being set in this way, WDT cannot be disabled except by reset. In

order to pre vent the hardware WCDT from resetting the entire device, the same sequence 1E/E1hex must be written

to the same WDTRS before the timer nominal time is ran out.

WSWRST

When set, this bit resets watch-dog timer in software mode (bit HWDT=0). In order to enable the microcontroller to work

normally, this bit must be regularly cleared from within the program. After being set by software, this bit is cleared by

hardware during the next machine cycle.

If watch-dog is in hardware mode, this bit has no effect, and if set by software, it will not be cleared by hardware.

WDTEN

This bit enables/disables watch-dog timer in software mode (bit HWDT=0):

0 - Watch-dog halts

1 - Watch-dog starts counting

When watch-dog is in hardware mode (bit HWDT=1), this bit is read-only and reflects the status of the Watch-dog timer

(whether it is turned on or off).

Bit WDTEN doesn’t reset watch-dog timer, it only enables/disables it. This means that the status

of the counter is “frozen” while WDTEN=0.


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4.6 Interrupts

The AT89S8253 has a total of six interrupt sources, meaning that it can recognize up to 6 different events that can interrupt

regular program execution. Each of these interrupts can be individually enabled or disabled by setting bits of the IE register

while the whole interrupt system can be disabled by clearing the EA bit in the same register.

This microcontroller has embedded Timer T2 and SPI (they are not part of the “8051 Standard”). Since both of them can

interrupt program execution, it was necessary to make minimal changes in registers that control interrupt system. A new

interrupt vector (address 2B) is added, i.e. address in program memory from where the program continues its execution in

case the Timer T2 causes interrupt. All these changes are only “added” to the positions of previously unused bits. This

means that all programs already written for some of the former models of the microcontrollers can , with no changes, be

executed in this one too. It is one of the main reason for popularity of all microcontrollers based on “noble gene” of the

8051.

IE register (Interrupt Enable)

EA Bit enables or disables all interrupt sources (globally):

0 - disables all interrupts (even it is enabled)

1 - allows those interrupts which are individually enabled

ET2 Bit enables or disables Timer T2 interrupt:

0 - Timer T2 can not cause interrupt

1 - Enables Timera T2 interrupt

ES Bit enables or disables serial communication (UART and SPI) interrupts:

0 - UART and SPI systems can not cause interrupts

1 - Enables UART and SPI interrupts



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1 - Enables Timer T1 interrupt

EX1 Bit enables or disables external interrupt through the pin INT0:

0 - Changing of logical state on the pin INT0 can not cause interrupt

1 - Enables external interrupt at the moment of changing state on the pin INT0



1 - Enables Timer T0 interrupt

EX0 Bit enables or disables external interrupt through the pin INT1:

0 - Changing of logical state on the pin INT1 can not cause interrupt

1 - Enables external interrupt at the moment of changing state on the pin INT1

Interrupt Priorities

If multiple interrupts are enabled, it is possible to have interrupt requests during execution of another interrupt routine. In

such situations, the microcontroller needs to resolve whether to proceed with the current interrupt routine or to meet a new

interrupt request, which is based on priority levels. The former models of the microcontrollers differentiate between two

priority levels defined in the IP register.

The AT89S8253 has additional SFR register IPH which assigns 1 of 4 priorities to each interrupt (excluding reset). The

new list of priorities is as follows:

Reset. If there is a request for reset, all processes are stopped and the microcontroller behaves as if the power has just

been turned on.

1.

The highest priority interrupt (3). It can be stopped only by reset.2.

Lower priority interrupt (2, 1 or 0). It can be stopped by any interrupt with higher priority level.3.

It is usually defined at the beginning of the program which one of these existing inter rupt sources have higher and which

one has lower priority level. According to this, the following occurs:

If two interrupt requests, at different priority levels, arrive at the same time then the higher priority interrupt is

always serviced first.

If the both interrupt requests, at the same priority level, occur one after another , that one who came later has to wait

until routine being in progress ends.

If two interrupts of equal priority requests arrive at the same time then the interrupt to be serviced is selected

according to the following priority list :

external interrupt INT01.

Timer T0 interrupt2.

external interrupt INT13.

Timer T1 interrupt4.

Serial communication Interrupt5.

Timer T2 Interrupt6.

IP register (Interrupt Priority)

This register bits determine the priority of interrupt sources.

PT2 Timer T2 interrupt priority

0 - Priority 0

1 - Priority 1

PS Serial port interrupt priority


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0 - Priority 0

1 - Priority 1

PT1 Timer T1interrupt priority

0 - Priority 0

1 - Priority 1

PX1 External interrupt INT1 priority

0 - Priority 0

1 - Priority 1

PT0 Timer T0 interrupt priority

0 - Priority 0

1 - Priority 1

PX0 External interrupt INT0 priority

0 - Priority 0

1 - Priority 1

IPH register (Interrupt Priority High)

PT2H Timer T2 interrupt priority

PSH Serial port interrupt priority

PT1H Timer T1interrupt priority

PX1H External interrupt INT1 priority

PT0H Timer T0 interrupt priority

PX0H External interrupt INT0 Priority

Bits of this register can be combined with the appropriate bits of the IP register. The new priority list with 4 levels (5

including reset ) is based on it.

IP bit IPH bit Interrupts

0 0 Priority 0 (lowest)

0 1 Priority 1 (low)

1 0 Priority 2 (high)

1 1 Priority 3 (highest)

Handling interrupt

Upon receiving an interrupt requests, the microcontroller recognizes the source and following scenario takes place:

Ongoing instruction executed first.1.

Address of the instruction that would be executed next if there was no interrupt request is pushed on the stack.2.

Depending on the interrupt in question, the program proceeds with execution at one of five possible addresses

according to the table below:

3.

Interrupt Source Jump Address

IE0 3h

TF0 Bh


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IE1 13h

TF1 1Bh

RI, TI, SPIF 23h

TF2, EXF2 2Bh

All addresses are in hex format

These addresses should hold the appropriate subroutines for handling interrupt. Instead , there are usually instructions

pointing to the locations where the appropriate subroutines reside (jump instructions).

4. When interrupt routine is executed, address of the next instruction to be executed is poped from the stack to the program

counter and the program proceeds from where it left off.


Timers T0 and T1

The AT89S8253 has three timers/counters marked as T0, T1 and T2. Both timers T0 and T1 completely fall under the 8051

Standard. There are no changes in their operating.

Timer T2

Timer2 is the third 16-bit timer/counter installed only in newer models of 8051 family. Unlike timers T0 and T1, this timer

comprises total of 4 registers. The first two, TH2 and TL2, are connected serially in order to form a bigger one, 16-bit

counting register. Other two registers, RCAP2H and RCAP2L, are also connected serially and have the function to capture

the contents of the counting register, i.e. the register in which counting is being executed is temporarily copied to them and

vice versa .

The main adventage of this organization lies in the fact that all reading and swapping take place concurrently, using one

instruction and with no need for programming acrobatics. Besides, T2 like older T0 and T1, has several different operating

modes, which will be described later in this chapter.

T2CON (Timer/Counter 2 Control Register)


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This register contains bits controlling the operation of T2.

TF2 - This bit is automatically set on counter overflow. In order to register next overflow, this bit needs to be cleared from

within a program. If bits RCLK and TCLK are set, overflow has no effect on TF2.

EXF2 - This bit is automatically set whenever pulse on pin T2EX causes transfer from counting register to capture register

or vice versa. If enabled, it gen erates interrupt, unless bit DCEN in T2CON register is set. EXF2 has to be cleared from

within a program.

RCLK - This bit defines which timer determines receive rate of serial connection:

1 - Receive rate of serial connection is determined by T2

0 - Receive rate of serial connection is determined by T1

TCLK - This bit defines which timer determines transmit rate of serial connection:

1 - Transmit rate of serial connection is determined by T2

0 - Transmit rate of serial connection is determined by T1

EXEN2 - This bit involves pin T2EX in the operation of timer:

1 - Signal on pin T2EX has affect on the operation of timer

0 - Ignore logic state on pin T2EX

TR2 - This bit starts/stops timer T2

1 - Start Timer T2

0 - Stop Timer T2

C/T2 - Bit defines which pulses will be counted by counter/timer T2:

1 - 16-bit register (T2H and T2L) counts pulses on pin C/T2 (counter)

0 - 16-bit register (T2H and T2L) counts pulses from oscillator (timer)

CP/RL2 - Bit defines transfer direction:

1 - If enabled, (bit EXEN=1) pulse on pin T2EX will cause transfer from counter to capture register.

0 - Under same condition, signal on pin T2EX will cause transfer in the opposite direction – from capture to counter

register.

Timer T2 in Capture mode

If bit CP/RL2 of register T2CON is set, timer T2 will operate according to the scheme presented below. This is the so

called Capture mode in which value of the counter (comprises registers RCAP2H and RCAP2L) can be “captured” and

copied to the capture register. The transfer has no effect on the counting process. How does the timer like this operate?


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16-bit register (TH2+TL2) holds the number from which the counting starts.1.

If set, bit TR2 of register TCON starts the timer. Each incoming pulse increments the value by 1. When both registers

are full (decimal value of 65536), the first next pulse causes overflow, then reset occurs, and counting starts from

zero.

2.

Settings:

Timer T2 in Auto-reload mode

In order to set the timer T2 to Auto-reload mode, the bit CP/RL2 needs to be cleared. Then, the Timer will be able to count

up or down from the specified value, depending on the bit DCEN in register T2MOD:

T2OE - Enables Timer to act as independent clock generator.


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DCEN - When set, it enables counting in either direction- "up" and "down".

As it is presented in the scheme above, unlike Capture mode, the value of the capture register (RCAP2H, RCAP2L) is in

this case copied to the counter (TH2, TL2) upon overflow.

Settings of Auto Reload mode are shown in the following table:

Everything previously stated on Timer T2 remains in effect only if register T2MOD hasn't been changed, i.e. if bit DCEN =

0. Otherwise, if this bit is set, timer (or counter) is enabled to count in either direction. Direction depends on logical state

on the pin T2EX:

T2EX = 0 T2 counts down

T2EX = 1 T2 counts up


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If counting up, situation is similar to the previously described mode with one exception concerning the bit EXF2 role.

If counting down, overflow occurs when values in the counting and capture registers match. At that moment, bit TF2 and

all bits of registers T2H and T2L are set while the counting goes on “from the top” : 65535, 65534,65533...

In eather case, bit EXF2 is assigned a new role. Namely, upon overflow, this bit only inverts the signal and can not be used

for generating interrupt anymore. Instead, this bit serves as supplementary bit (17th bit) of the counting register, making the

counter virtually 17-bit register.

Timer T2 as clock generator in serial communication

If bits RCLK or TCLK of the register TCON are set, timer T2 turnes into clock generator (so called Baud Rate generator)

which determines the transfer rate of serial communication. This mode is very similar to Auto-Reload mode with the rate of

serial connection calculated according to the following formula:

Naturally, there are several specific details that should be taken into account:

Previous equation works only if the internal oscillator is used for counting ( in this mode, clock is divided by 2,

instead of 12)

1.

Overflow has no effect on bit TF2 and does not generate interrupt.2.

Whether the bit EXEN2 is set or not, logic state on T2EX has no effect on the counter. It means that pin T2EX can

be used as an external interrupt source in this mode.

3.

When working in this mode, timer should be turned off (TR2 = 0) ahead of writing or reading the contents of

registers TH2 and TL2. Otherwise, an error in serial communication may occur.

4.

Timer T2 as independent clock generator

In previous examples, pin P1.0 (marked as T2 in figures), serves as an alternative clock generator for this timer -i.e. it acts

as input. It can be also used as output for generating sequence of pulses. Using the 16MHz quartz crystal, frequency of

generated pulses ranges from 61Hz to 4MHz with pulse-to-pause ratio of 50%.

To configure the pin as output, bit C/T2 (in register T2CON) needs to be cleared, and bit T2OE (in register T2MOD) needs

to be set. After that, bit TR2 starts the timer and the pin generates rectangular waves with frequency calculated according to

the formula:


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4.8 UART (Universal Asynchronous Receiver Transmitter)

Universal Asynchronous Receiver Transmitter UART preserved all the features of standard 8051 microcontrollers. It means

that it can operate in 1 of 4 different modes, which is determined by bits SM0 and SM1 in register SCON.

Multiprocessor Communication

If using multiprocessor communication (bit SM2 in the SCON register is set), it is possible to automatically recognize

microcontroller’s addresses. It enables easier program writing because the mutually connected microcontrollers don’t need

to examine every serial address. How does it work?

Two new special Function Registers, SADDR and SADEN, enable it. Microcontroller’s address (an arbitrary number) is

written to SADDR register, while so called address mask is written to register SADEN. The address mask is a binary

number used to define which bits in the SADDR are to be used and which bits are to be ignored.

Since the bit SM2 is set, the microcontroller will recognize serial received 9-bit data as an address. Internal electronics

immediately performs operation “logical AND” on these two registers and compares the result with received address. In

that way, the processor recognizes whether upcoming data refer to it or not. Since some of the bits in address can be

ignored (all corresponding bits with 0 in SADEN register), the data received via serial communication can be transferred to

one, some or all microcontrollers which are mutually connected.

The most simple example is a “mini-network” comprising only 3 microcontrollers:

Microcontroller A is the master and communicate with devices “B” and “C”.

Microcontroller B: SADDR = 1100 0000

SADEN = 1111 1101

Address = 1100 00X0

Microcontroller C: SADDR = 1100 0000

SADEN = 1111 1110

Address = 1100 000X


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Although both microcontrollers B and C are assigned the same address (1100 0000), the mask in register SADEN is used to

differentiate between these two slaves. It enables to communicate with both registers independently or at the same time:

If transmit address is 1100 0010, the data will be exchanged with slave device B.

If transmit address is 1100 0001 the data will be exchanged with slave device C.

If transmit address is 1100 0000 the data will be exchanged with both slave devices.

4.9 SPI System (Serial Peripheral Interface)

In addition to UART system, the AT89S8253 has also another system for serial communication which doesn’t fall into the

8051 Standard. It is SPI system which provides a high-speed synchronous data transfer between the microcontroller and

one or more peripheral devices or between multiple microcontrollers. In such connection, one microcontroller is always

considered as the main one- master device. It defines rate, transfer direction (whether the data are transferred or received)

and data format. The other is slave device which is in subordinated position, meaning that it cannot start data transfer and

has to adjust to conditions imposed by the master device.

The data are transferred via full duplex connection using 3 conductors connected to pins MISO (P1.6), MOSI (P1.5) and

SCK (P1.7). The forth pin-control pin SS- is not in use on the master device and may be used as general-purpose

input/output while on the slave device it must have voltage level 0. When pin SS on the slave device is set, its SPI system is

deactivated and the MOSI pin can be used as a general-purpose input.


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As it is shown in the scheme, pins MISO and MOSI perform differently on the master and slave device (as inputs or

outputs), which is determined by the MSTR bit in register SPCR.

Connection is much easier when being familiar with abbraviations:

MISO - master in, slave out; MOSI - master out, slave in; SCK - serial clock; SS - slave select;

Like many other circuits inside the microcontroller, SPI system can also operate in several modes.

Normal SPI mode (buffer out of use)

After writting data to the SPI data register SPDR, it is automatically transferred to 8- bit shift register. SPI clock generator

gets start and data in serial form appears on the pin MOSI. An initial delay may occur for synchronization with the main

oscillator.

After shifting one byte, the SPI clock generator stops, bit SPIF(flag) is set, received byte is transferred to register SPDR and

if enabled, an interrupt is generated.

Any attempt to write byte to register SPDR while a transmission is in progress, will set the WCOL bit, which indicates that

error has occured. However, the transmission will be completed normally, meaning that error concerns new byte which will


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be ignored (byte will not be transferred).

Enhanced SPI mode (buffer in use)

Enhanced mode is similar to normal mode except that during transmission data goes through one more register. It doesn’t

have any sense at first though, but connection is really enhanced in that way! Look at the figure below...

After writting data to the SPI data register SPDR, it is transferred to capture register (buffer), which automatically set bit

WCOL signifying that the buffer is full and any further writes will cause overflow. Control electronics (hardware) cleares

this bit upon the data from buffer is transferred to the shift register and its transmission in serial format begins. If this is the

first byte in series, the data is immediately transferred to the shift register (still empty) and bit WCOL is immediately

cleared (buffer is empty).

While this byte transmitting, the next byte may be written to register SPDR (it will be immediately copied to buffer). In

order to know that sending data is in progress, it is sufficient to check if the bit LDEN (Load Enable) is set in register

SPSR. When this bit is set and bit WCOL is cleared means that data transfer is in progress and that buffer is free so the next

byte can be written to register SPDR.

How to select the right mode? If some individual byte is sent occasionally then there is no need to complicate- it is

sufficient to set up the normal mode. If it is needed to send a great amount of data, it is better to use enhanced mode which

offers obvious adventages: clock generator is not turned off as far as buffer is regularly kept full and as far as processor

“see” set bit WCOL. In this mode, there is no wasting time for the sake of synchronization and data is easily transferred in

format of long composition of bytes- as quick as lighting and with no holdups.

SPI system is under control of 3 SFRs: SPDR, SPSR and SPCR.

SPDR (SPI Data Register)

This is the register for storing data to be transferred via SPI (in serial format). It is also used for storing received data.

SPSR (SPI Status Register)

SPIF Interrupt flag. Upon data transfer, this bit is automatically set and an interrupt is generated if bits SPIE=1 and ES=1.

The SPIF bit is cleared by reading SPSR followed by reading/writing SPDR register.

WCOL In normal mode (ENH=0), the bit is set if SPDR register is written during data transfer. It means that writing is


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premature and has no effect (It is called Write Collision). This bit is cleared in the same manner as the bit SPIF.

In enhanced mode (ENH=1), bit is set when buffer is full. It signifies that a new data “is waiting” for transmission to the

shift register.

In enhanced mode, a new data can be written to buffer when this bit is set (if at the same time the bit WCOL=0).

DISSO When set, this bit causes the pin MISO to be floating, which make it possible that more than one slave

microcontrollers can share the same interface with a single master. Normally, only the first byte in sequence could be the

slave address, and all microcontrollers receive it. Afterwards, only one selected microcontroller should clear its DISSO bit.

ENH

0 SPI system is in normal mode (with no buffer)

1 SPI system is in enhanced mode

SPCR (SPI Control Register)

SPIE When this bit is set, SPI system can generate interrupt

SPE This bit “turns on” SPI system. When this bit is set, pins SS, MOSI, MISO and SCK are connected to

microcontroller’s pins P1.4, P1.5, P1.6 and P1.7.

DORD Bit determines which bytes in serial connection goes first:

0 - When sending data MSB bit goes first

1 - When sending data LSB bit goes first

MSTR Bit determines whether microcontroller will operate as master or slave:

0 - SPI system operates as slave

1 - SPI system operates as master

CPOL Bit controls state on the pin SCK when the SPI system is on wait :

0 - During waiting, pin SCK is cleared

1 - During waiting, pin SCK is set

CPHA This bit along with the CPOL bit controls relation between clock and data in serial format (see table below).

SPR1,SPR0 When SPI system operates as master, these two bits determine boud rate, i.e. the frequency of clock signal of

master device. When operates as slave , bits have no effect and SPI system is adjusted to the rate imposed by the master

device.

SPR1 SPR0 SCK

0 0 Fosc/4

0 1 Fosc/16

1 0 Fosc/64

1 1 Fosc/128

Data format in case CPHA=0


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* not defined. It is usually Msb byte previously received.

Data format in case CPHA=1

* not defined. It is usually Lsb byte previously received.

Two things are important to bear in mind when configuring SPI system:

master should be configured first, afterwards slave.

When writing bits to register SPCR, bit SPE which “turns on” SPI should be set last (when all other parameters are

already defined).

4.10 Power Consumption Control

Similar to all models belonging to the 8051 series, this microcontroller can operate in 1 of 3 modes: normal (consumption

ca. 25 mA), Idle (consumption ca. 6.5 mA) and Power Down (consumption ca. 40 uA). Mode is selected by bits in register

PCON (Power Control) with minimal supplements relative to the basic model:

PCON register

The purpose of bits in register PCON:


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SMOD1 When set, this bit makes boud rate is two times faster.

SMOD0 Bit determines the purpose of the seventh bit in SCON register:

0 Seventh bit in register SCON has SM0’s role (selects mode)

1 Same bit has FE’s role (detecting errors). It is rarely used.

POF Bit is automatically set when, after turning on, the voltage level reaches maximum (must be higher than 3V). It is

used for finding out/detecting causes for reset (turning on or restart after switching from Power Down mode).

GF1 General purpose bit (available for use)

GF0 General purpose bit (available for use)

PD When set, this bit sets up the microcontroller in Power Down mode

IDL When set, this bit sets up the microcontroller in Idle mode

Common mistakes (When things go wrong...)

If something unexpected happens, during the operation of the microcontrollers, what most bothers is the fact that it’s never

up to the microcontroller. Although it’s not always obvious, the microcontroller will always obediently and consistently

follow program instructions. For that reason, in order to avoid misunderstanding, special attention should be payed to

several “critical points” when writing program. The first of them is RAM memory.

Even it is designed to meet needs of the majority of users, even it has all that is needed, a memory space intended for RAM

is still only one single totality. It means that there are no phisically separated registers R0-R7, general purpose registers,

stack etc. Instead, they all are differently designated parts of the same “memory shelf” (look at the figure)

If this “detail”, with a bit of negligence, is overlooked, there is a danger that program (or device) suddenly starts performs

totally unpredictable. In order to prevent from such situations, attention should be payed to the following:

If only registers R0-R7 are in use, it is not possible that something unpredictable happens and memory locations at

addresses from 08h are available for use. If registers from some other bank are in use (other registers with the same

designations), be careful when using locations whose addresses are less than 20h because the contents of “R” registers can

be erased.

If bit-variables are not used in the program, RAM locations at addresses 20h-2Fh are available for use. If there are such

variables in the program, this space should be carefully used in order to avoid their accidental changes.

By default (if nothing has been changed), the data pushed on stack use RAM locations starting from address 08h. If the

banks of registers 1, 2 or 3 are in use, their contents will be almost for sure unintentionally erased. For that reason, at the

beginning ot the program, it is good to set value of Stack Pointer to be greater than 20h (more better-greater than 2Fh).

SFRs are in command to control and run microcontroller’s operation. Each of them has its specific purpose and so it should

be. It means that they cannot be used as general purpose registers even in case that some of addesses in SFRs is not

acctually in use.

In instruction set recognized by the microcontroller, there are such instructions which can be used for manipulating

individual bits in the scope of register. Beside registers at addresses 20h - 7Fh, such direct access to individual bits is

possible in some SFRs (not in all of them). Such SFRs are recognizable by their addresses divisible by 8.

If memory expanding is used (external RAM or ROM), complete ports P0 and P2 become unavailable regardless of how

many pins are actually in use for communication with additional memory.

Register DPTR is a 16-bit register comprised of registers DPH and DPL which are 8-bit each. In practice, they should be

treated like that. For example, if DPTR register should be pushed to the Stack, DPL should be pushed first, then DPH.

When serial communication is used, it is register SCON which controls this process. Since the Timer 1 is mostly used for

boud rate generating than registers TCON and TMOD should be configured too.


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When some of interrupts is enabled, one should be careful because there is danger that program starts executing in a strange

way. What is all this about? When interrupt request arrives, the microcontroller will execute ongoing instruction, push

address of the first following location on the stack (in order to know from where to continue) and jump to the address

defined for interrupt requested. When subroutine has been executed, processor will pop address from the stack and will

continue executing from where it left off. But...

The microcontroller “remembers” only return address. During subroutine execution, contents of many registers can be

changed. When program continues execution after returning from subroutine, changed contents of registers will be treated

as correct ones, if their previous values haven’t been saved, which can cause total chaos. The worst thing is that problem

like this can be manifested anytime: at the moment or several days of work later (depending on the moment interrupt

occurs). Obviously, the only solution is to memorize the state of all important registers in the beginning of interrupt routine

and to turn these true values back to the program before returning from subroutine. The following registers are concerned:

PSW

DPTR (DPH, DPL)

ACC

B

Registers R0 - R7

Procedure of memorizing the state of registers is commonly performed by placing them on the

Stack (by PUSH instruction). But instruction such as “PUSH R0” can not be applied because the

processor “doesn’t know” to which register it refers (there are 4 banks with registers R0-R7). So,

for memorizing values of R register, instead of their names, their addresses should be used ( by PUSH 00h instruction).

When some of instructions for indirect addressing are in use, one should pay attention to not use them for accessing SFRs

because processor ignores their addresses and accesses free RAM locations which have the same addresses as SFRs.

When UART system for serial connection is in use, bits RI and TI of register SCON generate the same interrupt routine so

one should first find out the cause of interrupt (byte is transferred, received or both). One should pay attention that

processor only sets these bits! If one forget to clear these bits at the end of routine, the program gets stuck and repeats the

same interrupt all the time.

Bit-addressable Registers List

Accumulator (Address: E0)

ACC

After reset 0 0 0 0 0 0 0 0

Bit name - - - - - - - -

Bit address E7 E6 E5 E4 E3 E2 E1 E0

B register (Address: F0)

B

After reset 0 0 0 0 0 0 0 0

Bit name - - - - - - - -

Bit address F7 F6 F5 F4 F3 F2 F1 F0

Interrupt Priority register (Address: B8)

IP

After reset X X 0 0 0 0 0 0

Bit name - - PT2 PS PT1 PX1 PT0 PX0

Bit address BF BE BD BC BB BA B9 B8

Interrupt Enable register (Address: A8)

IE

After reset 0 X 0 0 0 0 0 0

Bit name EA - ET2 ES ET1 EX1 ET0 EX0

Bit address AF AE AD AC AB AA A9 A8

Port 0 (Address: 80)


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P0

After reset 1 1 1 1 1 1 1 1

Bit name - - - - - - - -

Bit address 87 86 85 84 83 82 81 80

Port 1 (Address: 90)

P1

After reset 1 1 1 1 1 1 1 1

Bit name - - - - - - - -

Bit address 97 96 95 94 93 92 91 90

Port 2 (Address: A0)

P2

After reset 1 1 1 1 1 1 1 1

Bit name - - - - - - - -

Bit address A7 A6 A5 A4 A3 A2 A1 A0

Port 3 (Address: B0)

P3

After reset 1 1 1 1 1 1 1 1

Bit name - - - - - - - -

Bit address B7 B6 B5 B4 B3 B2 B1 B0

Program Status Word (Address: D0)

PSW

After reset 0 0 0 0 0 0 0 0

Bit name CY AC F0 RS1 RS0 OV - P

Bit address D7 D6 D5 D4 D3 D2 D1 D0

Serial Port Control register (Address: 98)

SCON

After reset 0 0 0 0 0 0 0 0

Bit name SM0 SM1 SM2 REN TB8 RB8 TI RI

Bit address 9F 9E 9D 9C 9B 9A 99 98

Timer Control register (Address: 88)

TCON

After reset 0 0 0 0 0 0 0 0

Bit name TF1 TR1 TF0 TR0 IF1 IT1 IF0 IT0

Bit address 8F 8E 8D 8C 8B 8A 89 88

Timer/Counter 2 Control register (Address: C8)

T2CON

After reset 0 0 0 0 0 0 0 0

Bit name TF2 EXF2 RCLK TCLK EXEN2 TR2 C/T2 CP/RL2

Bit address CF CE CD CC CB CA C9 C8

Non Bit-addressable Registers List

Auxiliary register (Address: 8E)

AUXRAfter reset X X X X X X X 0

Bit name - - - - - - Intel_Pwd_Exit DISALE

Clock register (Address: 8F)


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CLKREGAfter reset X X X X X X X 0

Bit name - - - - - - - X2

Data Pointer 0 High (Address: 83)

DP0HAfter reset 0 0 0 0 0 0 0 0

Bit name - - - - - - - -

Data Pointer 0 Low (Address: 82)

DP0LAfter reset 0 0 0 0 0 0 0 0

Bit name - - - - - - - -

Data Pointer 1 High Byte (Address: 85)

DP1HAfter reset 0 0 0 0 0 0 0 0

Bit name - - - - - - - -

Data Pointer 1 Low Byte (Address: 84)

DP1LAfter reset 0 0 0 0 0 0 0 0

Bit name - - - - - - - -

EEPROM Control (Address: 96)

EECONAfter reset X X 0 0 0 0 1 1

Bit name - - EELD EEMWE EEMEN DPS RDY/BSY WRTINH

Interrupt Priority High Byte (Address: B7)

IPHAfter reset X X 0 0 0 0 1 1

Bit name - - PT2H PSH PT1H PX1H PT0H PX0H

Power Control (Address: 87)

PCONAfter reset 0 X X X 0 0 0 0

Bit name SMOD - - - GF1 GF0 PD IDL

Slave Address (Address: A9)

SADDRAfter reset 0 0 0 0 0 0 0 0

Bit name - - - - - - - -

Slave Address Enable (Address: B9)

SADENAfter reset 0 0 0 0 0 0 0 0

Bit name - - - - - - - -

Serial buffer (Address: 99)

SBUFAfter reset X X X X X X X X

Bit name - - - - - - - -

Stack Pointer (Address: 81)


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SPAfter reset 0 0 0 0 0 1 1 1

Bit name - - - - - - - -

SPI Control register (Address: D5)

SPCRAfter reset 0 0 0 0 0 1 0 0

Bit name SPIE SPE DORD MSTR CPOL CPHA SPR1 SPR0

SPI Data register (Address: 86)

SPDRAfter reset - - - - - - - -

Bit name - - - - - - - -

SPI Status register (Address: AA)

SPSRAfter reset 0 0 0 - - - 0 0

Bit name SPIF WCOL LDEN - - - DISSO ENH

Timer 2 Reload Capture High (Address: CB)

RCAP2HAfter reset 0 0 0 0 0 0 0 0

Bit name - - - - - - - -

Timer 2 Reload Capture Low (Address: CA)

RCAP2LAfter reset 0 0 0 0 0 0 0 0

Bit name - - - - - - - -

Timer 0 Low (Address: 8A)

TL0After reset 0 0 0 0 0 0 0 0

Bit name - - - - - - - -

Timer 1 Low (Address: 8B)


Bit name - - - - - - - -

Timer 2 Low (Address: CC)


Bit name - - - - - - - -

Timer 0 High Byte (Address: 8C)

TH0After reset 0 0 0 0 0 0 0 0

Bit name - - - - - - - -

Timer 1 High Byte (Address: 8D)


Bit name - - - - - - - -

Timer 2 High Byte (Address: CD)


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Bit name - - - - - - - -

Timer Mode (Address: 89)

TMODAfter reset 0 0 0 0 0 0 0 0

Bit name GATE1 C/T1 T1M1 T1M0 GATE0 C/T0 T0M1 T0M0

Timer 2 Mode Control (Address: C9)

T2MODAfter reset X X X X X X 0 0

Bit name - - - - - - T2OE DCEN

Watchdog Timer Control (Address: A7)

WDTCONAfter reset 0 0 0 0 0 0 0 0

Bit name PS2 PS1 PS0 WDIDLE DISRTO HWDT WSWRST WDTEN

Watchdog Timer Reset (Address: A6)

WDTCONAfter reset - - - - - - - -

Bit name - - - - - - - -

Voltage characteristics of the AT89S8253 microcontrollers

Symbol Parameter Condition Min. Max.

VIL Input Low-voltage All pins except EA -0.5 V 0.2Vcc - 0.1V

VIL1Input Low-voltage on

EA pin-0.5 V 0.2Vcc - 0.3V

VIH Input High-voltageAll pins except

XTAL1 and RST0.2 Vcc + 0.9V Vcc + 0.5 V

VIH1Input High-voltage on

pins XTAL1 and RST0.7 Vcc Vcc + 0.5 V

VOL Output High-voltageIol = 10mA, Vcc =

4.0V, Ta = 85°C0.4 V

VOH1

Output High-voltage

when Pull-up resistors

are enabled (Port P0 in

External BUS mode,

ports P1,2,3, pins ALE

and PSEN)

Ioh = -40mA, Ta =

85°C

Ioh = -25mA, Ta =

85°C

Ioh = -10mA, Ta =

85°C

2.4 V

0.75 Vcc

0.9 Vcc

IILLogical 0 input current

(ports P1,2,3)

Vin = 0.45V, Vcc =

5.5V, Ta = -40°C- 50 µA

IILIInput leakage current

(port P0, pin EA)0.45V < Vin < Vcc ± 10 µA

RRSTReset pull-down

resistor50 KΩ 150 KΩ

CIO I/O pin Capacitance f = 1Mhz, Ta = 25°C 10 pF

ICC

Power-supply current

Normal mode: f =

12Mhz, Vcc = 5.5V Ta

= -40°C

Idlle mode f = 12Mhz,

Vcc = 5.5V Ta =

-40°C

25 mA

6.5 mA

Power-down mode

Vcc = 5.5V Ta =

-40°C

Vcc = 4V Ta = -40°C

100 µA

40 µA


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Chapter 5 : Programming language Assembler

5.1 Elements of Assembler

Introduction

The moment has come that hardware-oriented to the core make compromise if they want to stay “in the game”.

Namely, unlike other circuits which need to be connected to other components and power supply in order to be of

any use, the microcontrollers require program too. Luckily, their evolution still did not progress so far, so all of

them (for the time being) “understand” only one machine language. It is a good news. The bad one is that even

primitive, only microcontrollers and some experts can understand this language of zeros and ones. In order to

bridge this gap between machine and humans, the first high-level programming language-Assembler was created.

The main problem- to remember the codes which electronics recognizes as commandswas solved, but a new one-

equally complicated to both us and “them”(microcontrollers) arose. The conflict was resolved at common pleasure

by means of the program for PC called assembler (not original at all) and a simple device called programmer.

By means of this program, the computer receives commands in form of abbreviations in environment familiar to us

and unerringly returns them afterwards into so called “executable file”. This is the crucial moment when the

program is compiled into machine code and this file ( named HEX file too) represents a series of binary numbers

not understandable to us but completely clear to electronic circuits.The program written in assembly language

cannot be executed in practice unless this file is programmed to the microcontroller’s memory. This is the moment

when the last link on a chain-programmer- comes on the scene. It is nothing special- a small device connected to a

PC using a port and contains socket for placing chip in. Press the button or click on a mouse and that’s it!

5.1 Elements of Assembler

Even simple, assembler is basically like any other language, which means that it has its words, rules and syntax. Its

basic elements are:

Labels

Orders

Directives

Comments



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Syntax of Assembly language

When writing program in assembler it is necessary to observe specific rules in order to enable the process of

compiling into executable “HEX-code” run with no errors. These obligatory rules in writing program are called

syntax and there are only several of them:

Every line in a program may consists of maximum 255 characters.

Every line in a program that should be compiled, must start with a symbol, an assembly control, a label,

mnemonics or directive.

All that follows mark “;” in a program line denotes comment and will not be translated.

All elements of one program line (labels, instructions etc.) must be separated at least by one whitespace. For

the sake of better clearness, pushbutton TAB on a keyboard is commonly used instead of whitespace, so it is

easy to recognize columns with labels, directives etc. in a program.

Numbers

If octal numeric system, already considered as obsolite one is neglected, it is allowed in assembler to use numbers

in one of three numeric systems:

Decimal Numbers


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If there are no particular indications, the assembler interpretes all numbers as decimal ones. All ten digits are in use

(0,1,2,3,4,5,6,7,8,9). Since at most 2 bytes are used for their memorizing to the microcontroller, the greatest

number that can be written in this system is 65535. If it has to be emphasised that some number is in decimal

format, that number is followed by the letter “D”. For example 1234D.

Hexadecimal Numbers

This is a common way of writing numbers in programming. Instead of 10, there are 16 digits in use (0, 1, 2, 3, 4, 5,

6, 7, 8, 9, A, B, C, D, E, F). In Assembler, the greatest number that can be written in this system is FFFF (

corresponds to decimal number 65535). In order to distinguish them from decimal numbers, there is the letter

“h”(in upper-or lowercase) following hexadecimal numbers. For example 54h.

Binary Numbers

Binary numbers are often used when the value of each individual bit in some register is important, since each digit

of binary number represents one bit. There are only two digits in use (0 and 1). The greatest number in this numeric

system recognizable by assembler as correct one is 1111111111111111. In order to distinguish them from other

numbers, there is the letter “b” (in upper-or lowercase) following binary numbers. For example 01100101B.

Operators

Instead of writing symbols which have specific value, some assembler-used commands allow the use of logical and

mathematical expessions. For example:

IF (VERSION>1) LCALL Table_2 USING VERSION+1ENDIF ...

As it can be seen, the assembler is able to compute some values on its own and place them in a programming code.

In that case, it distinguishes between the following mathematical and logical operations:

Name Operation Example Result

+ Addition 10+5 15

- Subtraction 25-17 8

* Multiplication 7*4 28

/Division (with no

remainder)7/4 1

MOD Remainder of division 7 MOD 4 3

SHRShift register bits to the

right1000B SHR 2 0010B

SHLShift register bits to the

left1010B SHL 2 101000B

NOTNegation (first

complement of number)NOT 1 1111111111111110B

AND Logical AND 1101B AND 0101B 0101B

OR Logical OR 1101B OR 0101B 1101B

XOR Exclusive OR 1101B XOR 0101B 1000B

LOW 8 low significant bits LOW(0AADDH) 0DDH

HIGH 8 high significant bits HIGH(0AADDH) 0AAH

EQ, = Equal 7 EQ 4 or 7=4 0 (false)

NE,<> Not equal 7 NE 4 or 7<>4 0FFFFH (true)

GT, > Greater than 7 GT 4 or 7>4 0FFFFH (true)

GE, >= Greater or equal 7 GE 4 or 7>=4 0FFFFH (true)


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LT, < Less than 7 LT 4 or 7<4 0 (false)

LE,<= Less or equal 7 LE 4 or 7<=4 0 (false)

Symbols

In assembly language, every register, constant, address or subroutine can be assigned a specific symbol, which

considerably facilitates writing program. Hence, for example, if input pin P0.3 is connected to a pushbutton for

manually interrupting of some process (pushbutton STOP), writing program will be simpler if the bit P0.3 is

assigned the same name: “pushbutton_STOP”. Of course, like in any other language, there are specific rules as

well:

While writing symbols, it is allowed to use all letters from alphabet (A-Z, a-z), dec imal numbers (0-9) and

two special characters ("?" and "_"). Assembler does not dif ferentiate between upper case and lower case.

For example, following symbols will be treated as identical:

Serial_Port_BufferSERIAL_PORT_BUFFER

In order to be different from a constant (number), every symbol must start with a letter or one of two special

characters (? or _).

Symbol may consist of maximum 255 characters, but only first 32 of them are taken into account. In the

following example, the first two symbols will be interpreted as duplicate (error), while the third and forth

symbols will be accepted as different ones:

START_ADDRESS_OF_TABLE_AND_CONSTANTS_1START_ADDRESS_OF_TABLE_AND_CONSTANTS_2TABLE_OF_CONSTANTS_1_START_ADDRESSTABLE_OF_CONSTANTC_2_START_ADDRESS

Some symbols cannot be used because they are already part of instructions or assembly directives.

Consequently, for example, a register or subroutine cannot be assigned name “A” or “DPTR” because the

registers having the same name exist already.

The list of symbols not allowed to be used:

A AB ACALL ADD

ADDC AJMP AND ANL

AR0 AR1 AR2 AR3

AR4 AR5 AR6 AR7

BIT BSEG C CALL

CJNE CLR CODE CPL

CSEG DA DATA DB

DBIT DEC DIV DJNZ

DPTR DS DSEG DW

END EQ EQU GE

GT HIGH IDATA INC

ISEG JB JBC JC

JMP JNB JNC JNZ

JZ LCALL LE LJMP

LOW LT MOD MOV

MOVC MOVX MUL NE

NOP NOT OR ORG

ORL PC POP PUSH

R0 R1 R2 R3

R4 R5 R6 R7


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RET RETI RL RLC

RR RRC SET SETB

SHL SHR SJMP SUBB

SWAP USING XCH XCHD

XDATA XOR XRL XSEG

Labels

Labels are a special type of symbols used to denote address in subroutine and are recognizable by being always

written at the beginning of a program line. Without them it is not allowed to call subroutine or execute some of

jump or branch instructions. They are easy for use:

Symbol (label) with some easily recognizable name should be written at the beginning of a program line

from where subroutine starts or where jump should be executed.

In instruction which calls this subroutine or a jump, instead of address in form of 16-bit number, it is

sufficient to enter the name of label.

During program compiling into machine code, the assembler will automatically replace such symbols with correct

addresses.

Directives

Unlike instructions being translated into machine code and written to on-chip program memory, directives are

commands of assembler itself and have no effect on the operation of the microcontroller. Some of them are

obligatory part of every program while some of them are used only to facilitate or enhance the operation.

Directives are written to the column reserved for instructions. There is the rule allowing only one directive per

program line.

EQU directive

By means of this directive, a numeric value is replaced by a symbol. For example:

MAXIMUM EQU 99

After this directive, every appearance of the label “MAXIMUM” in the program, the assembler will interprete as

number 99 (MAXIMUM = 99). It is only once possible to define symbols in this way so the EQU directive is

mostly used at the beginning of the program.

SET directive

Similar to the EQU directive, by means of the SET directive, a numeric value is replaced by a symbol. Significant

difference is that with this directive it can be done for unlimited number of times:

SPEED SET 45SPEED SET 46SPEED SET 57

BIT directive

By means of this directive, bit address is replaced by a symbol (bit address must be in the range of 0-255). For

example:

TRANSMIT BIT PSW.7 ;Transmit bit (the seventh bit in PSW register) ;is assigned the name "TRANSMIT"OUTPUT BIT 6 ;Bit at address 06 is assigned the name "OUTPUT"RELAY BIT 81 ;Bit at address 81 (Port 0)is assigned the name ;"RELAY"

CODE directive

By means of this directive, an address in program memory is designated as a symbol. Since the maximal capacity

of program memory is 64K, the address must be in the range of 0-65535. For example:


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RESET CODE 0 ;Memory location 00h called "RESET"TABLE CODE 1024 ;Memory location 1024h called "TABLE"

DATA directive

By means of this directive, an address within internal RAM is designated as a symbol (address must be in the range

of 0-255). In other words, any selected register may change its name or be assigned a new one. For example:

TEMP12 DATA 32 ;Register at address 32 is named ;as "TEMP12"STATUS_R DATA D0h ;PSW register is assigned the name ;"STATUS_R"

IDATA directive

By means of this directive, indirectly addressed register (its addrress is located in the specified register) changes its

name or is assigned a new one. For example:

TEMP22 IDATA 32 ;Register whose address is in register ;at address 32 is named as "TEMP22"TEMP33 IDATA T_ADR ;Register whose address is in ;register T_ADR is named as "TEMP33"

XDATA directive

This directive is used to name registers within external (additional) RAM memory. Address of such defined

ragister cannot be greater than 65535. For example:

TABLE_1 XDATA 2048 ;Register stored in external ;memory at address 2048 is named ;as "TABLE_1"

ORG directive

This directive is used to define location in program memory where the program following directive is to be placed.

For example:

BEGINNING ORG 100 ... ... ORG 1000hTABLE ... ...

Program begins at location 100. The table with data will start at location 1024 (1000h).

USING directive

This directive is used to define which register bank (registers R0-R7) will be used in the following program.

USING 0 ;Bank 0 is used (registers R0-R7 at RAM-addresses 0-7)USING 1 ;Bank 1 is used (registers R0-R7 at RAM-addresses 8-15)USING 2 ,Bank 2 is used (registers R0-R7 at RAM-addresses 16-23)USING 3 ;Bank 3 is used (registers R0-R7 at RAM-addresses 24-31)

END directive

This directive must be at the end of every program. Once it encounters this directive, the assembler will stop

interpreting program into machine code. For example:

...END ;End of program

Directive selecting memory segments

There are 5 such directives used for selecting one of five available memory segments in the microcontroller :

CSEG ;Indicates that next segment refers to program memory.

BSEG ;Selects bit-adressable part of RAM.

DSEG ;Indicates that next segment refers to internal RAM (part accessed by direct addressing).

ISEG ;Indicates that next segment refers to the “upper” part of internal RAM (part accessed by


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;indirect addressing using registers R0 and R1).

XSEG ;Selects external RAM memory

When assembler gets started, the segment CSEG is activated by default and remains active until a new directive is

specified. Each of these memory segments has its internal address counter which is cleared every time the

assembler is started. Its value can be changed by indicating value after the mark AT (it may be a number, an

arithmetical operation or a symbol). For example:

DSEG ;Next segment refers to registers with direct access.BSEG AT 32 ;Selects bit-addressable part of memory with address counter ;moved by 32 bit locations relative to the beginning of that ;memory segment.

Dollar symbol "$" denotes current value of address counter in the segment which is currently active. The following

two examples illustrate how this can be used in practice:

Example 1:

JNB FLEG,$ ;Program will constantly execute this ;instruction (jump instruction),until ;flag is cleared.

Example 2:

MESSAGE DB ‘ALARM turn off engine’LENGTH EQU $-MESSAGE-1

Using previous two program lines, exact number of characters in the message “ALARM turn off engine” which is

defined at address labelled as “MESSAGE”, can be computed.

DS directive

This directive reserves space in memory expressed in bytes. It is used if the segment ISEG, DSEG or XSEG is

currently active. For example:

Example 1:

DSEG ;Selects part of RAM with direct addressingDS 32 ;Actual value of address counter is incremented by 32SP_BUFF DS 16 ;Reserves space for serial port buffer ;(16 bytes)IO_BUFF DS 8 ;Reserves space for I/O buffer in size of 8 ;bytes

Example 2:

ORG 100 ;Starts at address 100DS 8 ;8 bytes are reservedLAB ......... ;Program continues execution (address of this ;location is 108)

DBIT directive

This directive reserves space within bit-addressable part of RAM (size is expressed in bits). It can be used only if

the BSEG segment is active. For example:

BSEG ;Bit-addressable part of RAM is selectedIO_MAP DBIT 32 ;First 32 bits occupy space provided ;for I/O buffer

DB directive

This directive is used for writing indicated value to program memory. If several values are indicated one after

another then they are separated by commas. If ASCII array should be indicated it is enclosed with single quotation

marks.This directive can be used only if the segment CSEG is active. For example:

CSEGDB 22,33,’Alarm’,44


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When written before this directive, the label will point to the first value in the array ( in this example number 22).

DW directive

This directive has the same purpose as DB directive, but it is followed by two-byte value (the high byte is written

first, the low byte afterwards).

IF, ENDIF and ELSE directives

These directives are used to create so called conditional blocks in a program. Each of these blocks starts with

directive IF and ends with directive ENDIF or ELSE. State or symbol (in parentheses) following the directive IF

represents a condition which determines the part of the program to be compiled into machine code:

If the statement is correct or if symbol is equal to one, program will include all instructions up to directive

ELSE or ENDIF.

If the statement is not correct or if the symbol value is equal to zero, all upcoming instructions are neglected

(are not interpreted) and program continues with com mands following directives ELSE or ENDIF.

Example 1:

IF (VERSION>3) LCALL Table_2 LCALL AdditionENDIF ...

If the program is of later date than version 3 (statement is correct), subroutines “Table 2” and “Addition” will be

executed. If the statement in parentheses is not correct (VERSION<3), two instructions calling subroutines are

neglected and are not compiled.

Example 2:

If the symbol value “Model” is equal to one, first two instructions after directive IF will be compiled into machine

code and program will afterwards continue with instructions following directive ENDIF (all instructions between

ELSE and ENDIF are neglected). Otherwise, if Model=0, instructions between IF and ELSE are neglected and

assembler compiles only instructions following directive ELSE.

IF (Model) MOV R0,#BUFFER MOV A,@R0ELSE MOV R0,#EXT_BUFFER MOVX A,@R0ENDIF ...

Control directives

These directives are recognizable by having the dollar symbol $ as the first letter. These commands are used to

define which files are to be used by assembler during compiling. It is also used to determine where executable file

is to be stored as well as the final appearance of the compiled program. Although, there are many directives

belonging to this category, only few of them is really important:

$INCLUDE directive

The name of this directive tells enough about its purpose. During compiling, it enables assembler to use data stored

in another file. For example:

$INCLUDE(TABLE.ASM)

$MOD8253 directive

$MOD8253 is the file where names and addresses of all SFRs of 8253 microcontrollers are stored. By using this

file and directive having the same name, assembler can execute program compiling only on the base of the

registers’ names. In case where those would not be used it is necessary in introductory part of the program to


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define an accurate name and address for every SFRs that will be used in the program.


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Chapter 6 : Examples

6.1 Basic connecting of the microcontroller

6.2 Additional components

6.3 Examples

Introduction

The purpose of this chapter is to inform you about basic issues on microcontrollers that one should know

in order to use them successfully in practice. That is why you will not find here some ultra interesting

program or device scheme with amazing solutions. Instead of that, examples described in this chapter are

more proof that program writing is neither privilege nor talent issue but ability of simple putting puzzle

pieces together using directives. Device development mainly comes to the method “test-correct-repeat”.

Of course, the more you are into it, the issues become more complicated as the puzzle pieces are put

together by both children and first-class architects...

6.1 Basic connecting of the microcontroller

As seen on the above figure, in order to enable microcontroller to operate properly it is necessary to

provide :

Power supply

Table of Contents Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6

Chapter 7


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Reset signal

Clock signal

Obviously, all this is about very simple circuits, but it does not have to be always like that. If device is

used for handling expensive machines or for maintaining vital functions, everything becomes more and

more complicated! This kind of solution is quite enough for the time being...

Power supply

Although this circuit can operate with different power supply voltage, why to test “Marphy’s low”?!

Voltage of 5V is so common that it imposes itself. The circuit, shown on the figure, uses cheap voltage

stabilisator LM7805 and provides high-quality voltage level and guite enough current to enable

microcontroller and “peripheral electronics” to operate ( sufficient current in this case amounts to 1A)!

Reset signal

In order to operate properly, the microcontroller must “see” logic 0 (0V) on reset pin RS (It explains

connection pin-resistor 10K-ground). Pushbutton which connects reset pin RS to power supply VCC is

not necessary but it is almost always built in because it enables microcontroller safe return to normal

operating conditions when the things go wrong. By activating this pin, 5V is brought to it, the

microcontroller is reset and program starts execution from the beginning.

Clock signal

Although the microcontroller has built in oscillator, it cannot operate without two external condensators

and quartz crystal which stabilize its frequency (microcontroller’s operating speed).

Naturally, there are some exceptions too:

if this solution cannot be applied for some reason, there

are always alternative ones. One of them is to bring

clock signal from special source through invertor. See

the figure on the left.

6.2 Additional components

Regardless of the fact that microcontrollers are the

product of modern technology, they are not so useful

without being connected to additional components.

Simply, the appearance of voltage on its pin means

nothing if it does not perform certain operations (turn

on/off, shift, display and similar).

Switches and Pushbuttons

There is nothing simpler than this! This is the simplest way of controlling appearance of some voltage on

microcontroller’s input pin. There is also no need for additional explanation of how these components

operate.

Nevertheless, it is not so simple in

practice... This is about something

commonly unnoticeable when using

these components in everyday life. It is

about contact bounce- a common


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problem with m e c h a n i c a l switches.

If contact switching does not happen so

quickly, several consecutive bounces can

be noticed prior to maintain stable state.

The reasons for this are: vibrations,

slight rough spots and dirt. Anyway,

whole this process does not last long (a few micro- or miliseconds), but long enough to be registered by

the microcontroller. Concerning pulse counter, error occurs in almost 100% of cases!

The simplest solution is to connect simple RC circuit

which will “suppress” each quick voltage change. Since

the bouncing time is not defined, the values of elements

are not strictly determined. In the most cases, the values

shown on figure are sufficient.

If complete safety is needed, radical measures should be

taken! The circuit, shown on the figure (RS flip-flop),

changes logic state on its output with the first pulse

triggered by contact bounce. Even though this is more expensive solution (SPDT switch), the problem is

definitely resolved! Besides, since the condensator is not used, very short pulses can be also registered in

this way.

In addition to these hardware solutions, a simple

software solution is commonly applied too: when a

program tests the state of some input pin and finds

changes, the check should be done one more time after

certain time delay. If the change is confirmed it means

that switch (or pushbutton) has changed its position. The

advantages of such solution are obvious: it is free of

charge, effects of disturbances are eliminated too and it

can be adjusted to the worst-quality contacts. Disadvantage is the same as in case of using RC filter-pulses

shorter than program delay cannot be registered.

Optocouplers

Optocoupler is a device commonly used

to galvanically separate microcontroller’s

electronics from potentionally dangerous

currents and voltages in environment.

Optocouplers usually have one, two or

four light sources (LE diodes) on their

input while on their output, opposite to

diodes, there are the same number of

elements sensitive to light

(phototransistors, photo-thyristors or

photo-triacs). The point is that there is no

electrical contact between input and

output, but the signal is transferred by

light. For this isolation to make sense, electrical power supply of diodes and photo-sensitive elements

must be independent. Being connected in this way, the microcontroller and expensive additional

electronics are completely protected from high voltage and disturbances which in practice are the most

common cause of destroying, damaging or unstable operating of electronic devices. Most frequently used

optocouplers are those with phototransistors on their output. In case the model of optocouplers with

internal base-to-pin 6 connection is on disposal (there are optocouplers without it), the base can be left


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unconnected. Optional connection, decreasing effects of disturbances by eliminating very short pulses, is

on the figure marked with a broken line .

Relays

Relays are elements connected to ouput pins of the

microcontroller and used to turn on/off all that being out of

board which has sensitive components: motors, transformators,

heaters, bulbs, high-voltage components, antenna systems etc.

There are various types of relays but all have the same operating

principle: when a current flows through the coil, it makes or

brakes machanical connection between one or more pairs of

contacts. As it is case with optocouplers, there is no galvanically

connection (electrical contact) between input and output circuits.

Relays usually demand both higher voltage and current to start

operating but there are also miniature versions which can be

activated with a low current directly obtained from the

microcontroller’s pin.

Below figure presents one solution specific to the 8051 microcontrollers. In this very case, darlington

transistor is used to activate relays because of its high current gain. This is not in accordance with “rules”,

but it is necessary in case of logic one activation since the current is then very low (pin acts as input)!

In order to be prevented from appearance of high voltage of self-induction caused by a sudden stop of

current flow through the coil, an inverted polarized diode is connected in parallel to the coil. The purpose


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of this diode is to “cut off” the voltage peak.

Light-emitting diode (LED)

Light-emitting diodes are elements for light signalization in electronics. They are manufactured in

different shapes, colors and sizes. For their low price, low consumption and simple use, they have almost

completely pushed aside other light sources- bulbs at first place. They perform similar to common diodes

with the difference that they emit light when current flows through them.

It is important to know that each diode will be

immediately destroyed unless its current is limited. This

means that a conductor must be connected in parallel to

a diode. In order to correctly determine value of this

conductor, it is necessary to know diode’s voltage drop

in forward direction, which depends on what material a

diode is made of and what colour it is. Values typical for

the most frequently used diodes are shown in table

below: As seen, there are three main types of LEDs.

Standard ones get ful brightness at current of 20mA.

Low Current diodes get ful brightness at ten times lower

current while Super Bright diodes produce more

intensive light than Standard ones.

Color TypeTypical current Id

(mA)

Maximal current If

(mA)

Voltage drop Ud

(V)

Infrared - 30 50 1.4

Red Standard 20 30 1.7

Red Super Bright 20 30 1.85

Red Low Current 2 30 1.7

Orange - 10 30 2.0

Green Low Current 2 20 2.1

Yellow - 20 30 2.1

Blue - 20 30 4.5

White - 25 35 4.4

Since the 8051 microcontrollers can provide only low

input current and since their pins are configured as

outputs when voltage level on them is equal to 0, direct

connectining to LEDs is carried out as it is shown on

figure (Low current LED, cathode is connected to output

pin).

LED displays

Basically, LED displays are nothing else but several

LEDs moulded in the same plastic case. Diodes are

arranged so that different marks-commonly digits: 0, 1,

2,...9 are displayed by activating them. There are many types of displays composed of several dozens of

built in diodes which can display different symbols.

The most commonly used are so called 7-segment displays. They are composed

of 8 LEDs, 7 segments are arranged as a rectangle for symbol displaying and


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there is additional segment for decimal point displaying. In order to simplify

connecting, anodes and catodes of all diodes are connected to the common pin

so that there are common cathode displays and common anode displays.

Segments are marked with the latters Ato G as shown on the figure on the left.

When connecting, each diode is treated independently, which means that each

must have its own conductor for current limitation.

When connecting displays to the microcontroller, the greatest problem is a great deal of valuable I/O pins

which they “occupy”, especially if it is needed to display several-digit numbers. Problem is more than

obvious if for example it is needed to display two 6-digit numbers (a simple calculation shows that 96

output pins are needed)!The solution on this problem is called MULTIPLEXING. This is how optical

illusion based on the same operating principle as filmcamera occurs. The principle is that only one digit is

active but by quick changing one gets impression that all digits of a number are active at the same time.

Referring to the previous example it would mean that firstly one byte representing units is applied on a

microcontroller’s port and only transistor T1 is activated at the same time. After a while, the transistor T1

is turned off, a byte representing tens is applied on a port and transistor T2 is activated. This process is

being cyclicly repeated at high speed for all digits and corresponding transistors.

When displaying any digit, a defeating fact that microcontroller is nevertheless only a machine made to

understand only language of units and zeros is fully expressed. Namely, it “does not know” what units,

tens or hundreds are, nor it knows how ten digits we are used to look like. Therefore, each number

intended to be shown on display must be prepared in the following way:

In special subroutine, a several digit number must be first separated in units, tens etc. Afterwards, each of

these digits must be stored in specific byte. In order to make these digits familiar to us, “masking” is


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carried out. Basically, it is a simple subroutine by which binary format of each number is replaced by

different combination of bits. For example, the digit 8 (0000 1000) is replaced by binary digit 0111 111 in

order to activate all LEDs which represent digit 8 on display. The only diode, inactive in this case is

reserved for decimal point. If a microcontroller’s port is connected to display in a way that bit 0 activates

segment “a”, bit 1 activates segment “b”, bit 2 segment “c” etc., the table below shows “mask” for each

digit.

Digits to display Display Segments

dp a b c d e f g

0 1 0 0 0 0 0 0 1

1 1 0 0 1 1 1 1 1

2 1 0 0 1 0 0 1 0

3 1 0 0 0 0 1 1 0

4 1 1 0 0 1 1 0 0

5 1 0 1 0 0 1 0 0

6 1 0 1 0 0 0 0 0

7 1 0 0 0 1 1 1 1

8 1 0 0 0 0 0 0 0

9 1 0 0 0 0 1 0 0

Beside digits 0 to 9, some latters of alphabet : A, C, E, J, F, U, H, L, b, c, d, o, r, t can be displayed by


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appropriate masking.

If common chatode displays are used all units in the table should be replaced by zeros and vice versa. In

that case NPN transistors should be also used as drivers.

Liquid Crystal Displays (LCD)

These components are “specialized” for being used with the microcontrollers, which means that they

cannot be activated by standard IC circuits. They are used for writing different messages on a miniature

LCD.

Amodel described here is for its low

price and great possibilities most

frequently used in practice. It is based on

the HD44780 microcontroller (Hitachi)

and can display messages in two lines

with 16 characters each . It displays all

letters of alphabet, greek letters,

punctuation marks, mathematical

symbols etc. In addition, it is possible to

display symbols that user makes up on its

own. Automatic shifting message on display (shift left and right), appearance of the pointer, backlight etc.

are considered as useful characteristics.

Pins Functions

There are pins along one side of the small printed board used for connection to the microcontroller. There

are total of 14 pins marked with numbers (16 in case the background light is built in). Their function is

described in the table bellow:

Function Pin Number Name Logic State Description

Ground 1 Vss - 0V

Power supply 2 Vdd - +5V

Contrast 3 Vee - 0 - Vdd

Control of operating

4 RS0

1

D0 – D7 are interpreted as commands

D0 – D7 are interpreted as data

5 R/W0

1

Write data (from controller to LCD)

Read data (from LCD to controller)

6 E

0

1

From 1 to 0

Access to LCD disabled

Normal operating

Data/commands are transferred to LCD

Data / commands

7 D0 0/1 Bit 0 LSB

8 D1 0/1 Bit 1

9 D2 0/1 Bit 2

10 D3 0/1 Bit 3

11 D4 0/1 Bit 4

12 D5 0/1 Bit 5

13 D6 0/1 Bit 6

14 D7 0/1 Bit 7 MSB

LCD screen

LCD screen consists of two lines with 16


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characters each. Each character consists

of 5x8 or 5x11 dot matrix. This book

covers 5x8 character display because it is

commonly used.

Contrast on display depends on the

power supply voltage and whether

messages are displayed in one or two

lines. For that reason, variable voltage

0-Vdd is applied on pin marked as Vee.

Trimmer potentiometer is usually used

for that purpose. Some versions of

displays have built in backlight (blue or green diodes). When used during operating, a resistor for current

limitation should be used (like with any LE diode).

If there are no characters on display or all of them are dimmed upon the display

is on, the first thing that should be done is to check the potentiometer for

contrast regulation. Is it properly adjusted? Same applies in case the operation

mode is changed (writing in one or two lines).

LCD Memory

There are three memory blocks inside the display:

DDRAM Display Data RAM

CGRAM Character Generator RAM

CGROM Character Generator ROM

DDRAM Memory

DDRAM memory is used for storing characters that should be displayed. The size of this memory is

sufficient for storing 80 characters. One part of these locations is directly connected to the characters on


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display.

All functions quite simply: it is sufficient to configure display so that addresses are automatically

incremented (shift right). Afterwards it sets starting value for the message that should be displayed (for

example 00 hex).

After that, all characters sent through lines D0-D7 will be displayed as a message we are used to- from left

to right. In this case, displaying starts from the first character in the first line on display since the address

is 00 hex. If more than 16 characters are sent, they all will be also memorized but not visible. In order to

display them, a shift command should be used. Virtually, everything looks as if LCD display is a

“window” which moves left-right over memory locations with characters. In reality, that is how the affect

of message “moving”on the screen is obtained (from left to right or vice versa).

If cursor is on, it will appear at location which is currently addressed. In other words, characters will

appear at cursor’s position while the cursor is automatically moved to the next addressed location.

Since this is a sort of RAM memory, data can be written to and read from it. Disadvantage is that the

contents will be lost forever upon the power is off.

CGROM Memory

A “map” with all characters that can be displayed are written by default. Each character has corresponding

location.


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Addresses of CGROM memory locations match standard ASCII values of characters. It means that if in a

program being currently executed by the microcontroller is written “send letter P to port”, the binary value

0101 0000 will appear on the port. This value is ASCII equivalent to the letter P. When this binary


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number is sent to LCD, a symbol stored on 0101 0000 location in CGROM will be displayed. In other

words, the letter “P” will be displayed . This applies to all alphabet letters (upper- and lowercase), but not

to numbers!

If one carefully looks at the “map” with characters in this memory, it can be seen that addresses of all

digits are “shifted” by 48 in comparison to the values of these digits (address of the digit 0 is 48, of digit 1

is 49, of digit 2 is 50 etc.). For that reason and in order to display digits correctly, each of them needs to be

added a decimal number 48 prior to being sent to LCD.

Since the time the first computer was made, it recognizes numbers but not

letters. It means that on sending any character from keyboard to PC, from PC to

printer or from microcontroller to other computer, through connection line are

actually sent binary numbers instead of characters . A table that links all standard symbols and their

number equivalents is called ASCII code.

CGRAM memory

Beside being able to display all standard characters, the LCD can display symbols that user defines on its

own. It enables displaying cyrilic fonts as well as many other symbols which fit to the frame of 5x8 dots

size. RAM memory (CGRAM) in size of 64 bytes enables the above.

The size of registers of this memory is a standard one (8 bits), but only 5 lower bits are in use. Logic one

(1) in every register represents a dimmed dot, while 8 locations considered jointly represent one character.

It is best illustrated on the figure below:


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Symbols are usually defined at the beginnig of a program by simple writing zeros and units to registers of

CGRAM memory so that they form desirable shapes. In order to display them it is sufficient to specify

their address. Pay attention to the first coloumn in CGROM map of characters- these are not addresses of

RAM memory but symbols which are discussed here.In this example, “display 0” means - display “č”,

“display 1” means - display “ž” etc.

LCD Basic Commands

All data transferred to LCD through outputs D0-D7 will be interpreted as commands or as data, which

depends on logic state on pin RS:


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RS = 1 - Bits D0 - D7 are addresses of characters that should be displayed. Built in processor addresses

built in “map of characters” and displays corresponding symbols. Displaying position is determined by

DDRAM address. This address is either previously defined or the address of previously transferred

character is automatically incremented.

RS = 0 - Bits D0 - D7 are commands which determine display mode. List of commands which LCD

“recognizes”are given in the table below:

Command RS RW D7 D6 D5 D4 D3 D2 D1 D0Execution

Time

Clear display 0 0 0 0 0 0 0 0 0 1 1.64mS

Cursor home 0 0 0 0 0 0 0 0 1 x 1.64mS

Entry mode set 0 0 0 0 0 0 0 1 I/D S 40uS

Display on/off control 0 0 0 0 0 0 1 D U B 40uS

Cursor/Display Shift 0 0 0 0 0 1 D/C R/L x x 40uS

Function set 0 0 0 0 1 DL N F x x 40uS

Set CGRAM address 0 0 0 1 CGRAM address 40uS

Set DDRAM address 0 0 1 DDRAM address 40uS

Read “BUSY” flag (BF) 0 1 BF DDRAM address -

Write to CGRAM or DDRAM 1 0 D7 D6 D5 D4 D3 D2 D1 D0 40uS

Read from CGRAM or

DDRAM1 1 D7 D6 D5 D4 D3 D2 D1 D0 40uS

I/D 1 = Increment (by 1) R/L 1 = Shift right 0 = Decrement (by 1) 0 = Shift left S 1 = Display shift on DL 1 = 8-bit interface 0 = Display shift off 0 = 4-bit interface D 1 = Display on N 1 = Display in two lines 0 = Display off 0 = Display in one line U 1 = Cursor on F 1 = Character format 5x10 dots 0 = Cursor off 0 = Character format 5x7 dots

B 1 = Cursor blink on D/C 1 = Display shift 0 = Cursor blink off 0 = Cursor shift

What is Busy flag ?

Comparing to the microcontroller, LCD is an extremly slow component. Because of that It was necessary

to provide a signal which will indicate that display is ready to receive a new data or a command following

the previous one has been executed. That signal is called busy flag and can be read from line D7. When

the bit BF is cleared (BF=0), display is ready to receive.

LCD Connection

Depending on how many lines are used for connection to the microcontroller, there are 8-bit and 4-bit

LCD modes. The appropriate mode is determined at the beginning of the process in a phase called

“initialization”. In the first case, the data are transferred through outputs D0-D7 as it has been already

explained. In case of 4-bit LED mode, for the sake of saving valuable I/O pins of the microcontroller,

there are only 4 higher bits (D4-D7) used for communication, while other may be left unconnected.

Consequently, each data is sent to LCD in two steps: four higher bits are sent first (that normally would be

sent through lines D4-D7), four lower bits are sent afterwards. With the help of initialization, LCD will

correctly connect and interprete each data received. Besides, with regards to the fact that data are rarely

read from LCD (data mainly are transferred from microcontroller to LCD) one more I/O pin may be saved


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by simpleconnecting R/W pin to the Ground. Such saving has its price. Even though message displaying

will be normally performed, it will not be possible to read from busy flag since it is not possible to read

from display.

Luckily, solution is simple. It is sufficient to give LCD enough time to perform its task upon sending

every character or command. Since execution of the slowest command is approximately 1.64mS, it will be

quite enough to wait for approximately 2mS.

LCD Initialization

Once the power supply is turned on, LCD is automatically cleared. This process lasts for approximately

15mS. After that, display is ready to operate. The mode of operating is set by default. This means that:

Display is cleared1.

Mode

DL = 1 Communication through 8-bit interface

N = 0 Messages are displayed in one line

F = 0 Character font 5 x 8 dots

2.

Display/Cursor on/off

D = 0 Display off

U = 0 Cursor off

B = 0 Cursor blink off

3.

Character entry

ID = 1 Addresses on display are automatically incremented by 1

4.


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S = 0 Display shift off

Automatic reset is mainly performed without any problems. Mainly but not always! If for any reason

power supply voltage does not reach ful value in the course of 10mS, display will start perform

completely unpredictably. If voltage supply unit can not meet this condition or if it is needed to provide

completely safe operating, the process of initialization by which a new reset enabling display to operate

normally must be applied.

Algorithm according to the initialization is being performed depends on whether connection to the

microcontroller is through 4- or 8-bit interface. All left over to be done after that is to give basic

commands and of course- to display messages...

Refer to the Figure below for the procedure on 8-bit initialization:

It is not a mistake!

In algorithm on figure, the same value is being transmitted three times in a row.


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In case of 4-bit initialization, the procedure is as follows:


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6.3 Examples

The scheme below is used in the several following examples:

Nothing special... Beside elements necessary for operating (oscillator with condensators and the simplest

reset circuit), there are also several LEDs and one pushbutton which actually do not have any practical

application and are used only to indicate program operating.

All LEDs are polarized so that they are activated by logic zero (0) on the microcontroller’s pin.

LED Blinking

This program does not demonstrate LEDs’ operating but the speed of operation of the microcontroller!

Simply, in order to enable LED blinking be visible, sufficient amount of time must pass between on/off

states. In this example time delay is solved using a subroutine called Delay. It is a triple loop where the

program remains for approximately 0.5 seconds and decrements values in registers R0, R1 or R2. Upon

return from subroutine, the state on the pin is inverted and procedure is repeated...

;************************************************************************;* PROGRAM NAME : Delay.ASM;* DESCRIPTION: Program turns on/off LED on the pin P1.0;* Software delay is used (Delay).;************************************************************************;BASIC DIRECTIVES


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$MOD53$TITLE(DELAY.ASM)$PAGEWIDTH(132)$DEBUG$OBJECT$NOPAGING

;STACK DSEG AT 03FHSTACK_START: DS 040H

;RESET VECTORS CSEG AT 0 JMP XRESET ;Reset vector ORG 100H XRESET: MOV SP,#STACK_START ;Defining of Stack pointer MOV P1,#0FFh ;All pins are configured as inputs

LOOP: CPL P1.0 ; State on the pin P1.0 is inverted LCALL Delay ; Time delay SJMP LOOP

Delay: MOV R2,#20 ;500 ms time delayF02: MOV R1,#50 ;25 msF01: MOV R0,#230 DJNZ R0,$ DJNZ R1,F01 DJNZ R2,F02

END ;End of program

Using Watch-dog Timer

This program describes how the watch-dog timer should not operate! As a matter of fact watch-dog timer

is properly adjusted (nominal time for counting is 1024mS), but instruction for its reset is intentionally left

out so that this timer always wins the “battle for time”. As a result, the microcontroller is reset (state in

registers remains unchanged), program starts execution from the beginning, number in register R3 is

incremented by 1 and copied to port P1 afterwards.

LEDs display this number in binary format...

;************************************************************************;* PROGRAM NAME : WatchDog.ASM;* DESCRIPTION : After watch-dog reset, program increments number in;* register R3 and shows it on port P1 in binary format.;************************************************************************

;BASIC DIRECTIVES$MOD53$TITLE(WATCHDOG.ASM)$PAGEWIDTH(132)$DEBUG$OBJECT$NOPAGING

WMCON DATA 96HWDTEN EQU 00000001B ; Watch-dog timer is enabledPERIOD EQU 11000000B ; Nominal Watch-dog period in duration of 1024ms ; is defined

;RESET VECTOR CSEG AT 0 JMP XRESET ; Reset vector

CSEG ORG 100H


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XRESET: ORL WMCON,#PERIOD ; Defining of Watch-dog period ORL WMCON,#WDTEN ; Watch-dog timer is enabled

MOV A,R3 ; R3 is moved to port 1 MOV P1,A INC R3 ; Register R3 is incremented by 1

LAB: SJMP LAB ; Wait for watch-dog reset

END ; End of program

Timer T0 in mode 1

This program spends the most of its time in endless loop waiting for timer T0 to count up a full cycle.

Once it happens, interrupt is generated, the routine TIM0_ISR is executed and logic zero (0) on port P1 is

shifted right by one place. This is another way to demonstrate the speed of operation of the

microcontroller since each shift means that counter T0 has counted off 216 pulses!

;************************************************************************;* PROGRAM NAME : Tim0Mod1.ASM;* DESCRIPTION: Program rotates "0" on port 1. Timer T0 in mode 1 is;* used;************************************************************************

;BASIC DIRECTIVES

$MOD53$TITLE(TIM0MOD1.ASM)$PAGEWIDTH(132)$DEBUG$OBJECT$NOPAGING

;DEFINING OF VARIABLES

;STACK

DSEG AT 03FHSTACK_START: DS 040H

;RESET VECTORS CSEG AT 0 JMP XRESET ; Reset vector

ORG 00BH JMP TIM0_ISR ; Timer T0 reset vector

ORG 100H

XRESET: MOV SP,#STACK_START ; Defining of Stack pointer MOV TMOD,#01H ; MOD1 is selected MOV A,#0FFH MOV P1,#0FFH SETB TR0 ; Timer T0 start MOV IE,#082H ; Interrupt enabled CLR C

LOOP1: SJMP LOOP1 ; Remain here

TIM0_ISR: RRC A ; Rotate accumulator A through Carry bit MOV P1,A ; Contents of accumulator A is moved to PORT1 RETI ; Return from interrupt


Timer T0 in Split mode

Similar to the previous example, the program spends the most of its time in a loop called LOOP1. Since

16-bit Timer T0 is split into two 8-bit timers, there are also two interrupt sources, therefore.


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First interrupt is generated after timer T0 reset. It executes the routine TIM0_ISR in which logic zero (0)

bit on port P1 is rotated. Looking from outside, it seems that LED’s light shifts.

Another interrupt is generated upon Timer T1 reset. It executes the routine TIM1_ISR in which the bit

state DIRECTION is inverted. Since this bit determines direction of bit rotation then the direction of LED

shifting is also changed.

If at any moment a pushbutton T1 is pressed, logic zero (0) on output P3.2 will stop the Timer T1.

;************************************************************************;* PROGRAM NAME : Split.ASM;* DESCRIPTION: Timer TL0 rotates bit on port P1, while TL1 determines;* the direction of rotation. Both timers operate in mode;* 3. Logic 0 on output P3.2 stops rotation on port P1.;************************************************************************

;BASIC DIRECTIVES

$MOD53$TITLE(SPLIT.ASM)$PAGEWIDTH(132)$DEBUG$OBJECT$NOPAGING


BSEG AT 0

;DEFINING OF BIT-VARIABLES

SEMAPHORE: DBIT 8DIRECTION BIT SEMAPHORE


;RESET VECTORS

CSEG AT 0 JMP XRESET ; Reset vector ORG 00BH

JMP TIM0_ISR ; Timer T0 reset vector

ORG 01BH JMP TIM1_ISR ; Timer T1 reset vector

ORG 100HXRESET: MOV SP,#STACK_START ; Defining of Stack pointer MOV TMOD,#00001011B ; Defining of MOD3 MOV A,#0FFH MOV P1,#0FFH MOV R0,#30D SETB TR0 ; TL0 is turned on SETB TR1 ; TL1 is turned on MOV IE,#08AH ; Interrupt enabled CLR C CLR DIRECTION ; First rotation is to right


TIM0_ISR: DJNZ R0,LAB3 ; Slow down rotation by 256 times JB DIRECTION,LAB1 RRC A ; Rotate contents of Accumulator to the right through ; Carry bit

SJMP LAB2LAB1: RLC A ; Rotate contents of Accumulator to the left through ; Carry bit


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LAB2: MOV P1,A ; Contents of Accumulator is moved to port P1LAB3: RETI ; Return from interrupt

TIM1_ISR: DJNZ R1,LAB4 ; Slow down direction of rotation by 256 times DJNZ R2,LAB4 ; If time is ran out, change direction of ; rotation CPL SMER MOV R2,#30DLAB4: RETI


Simultaneous use of timers T0 and T1

One can take this program as extension of the previous one. The idea is the same but in this case true

timers T0 and T1 are used. In order to demonstrate operation of both timers simultaneously, the Timer T0

reset is used to shift logic zero (0) on port while Timer1 reset is used to change direction of rotation. This

program spends the most of its time in the loop LOOP1 waiting for interrupt caused by reset. By checking

the bit DIRECTION, an information on direction of rotation of both bits in Accumulator and shifting LED

on port is obtained.

;************************************************************************;* PROGRAM NAME : Tim0Tim1.ASM;* DESCRIPTION: Timer TO rotates bit on port P1 while Timer1;* changes direction of rotation. Both timers oper;* ates in mode 1.;************************************************************************

;BASIC DIRECTIVES

$MOD53$TITLE(TIM0TIM1.ASM)$PAGEWIDTH(132)$DEBUG$OBJECT$NOPAGING


BSEG AT 0

;DEFINING OF BIT-VARIABLES

SEMAPHORE: DBIT 8DIRECTION BIT SEMAPHORE



ORG 00BH ; Timer 0 Reset vector JMP TIM0_ISR

ORG 01BH ; Timer 1 Reset vector JMP TIM1_ISR

ORG 100H

XRESET: MOV SP,#STACK_START ; Defining of Stack pointer MOV TMOD,#11H ; Selecting MOD1 for both timers MOV A,#0FFH MOV P1,#0FFH MOV R0,#30D ; R0 is initialized SETB TR0 ; TIMER0 is turned on SETB TR1 ; TIMER1 is turned on


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MOV IE,#08AH ; Timer0 and Timer1 Interrupt enabled CLR C CLR DIRECTION ; First rotation is to right


TIM0_ISR: JB DIRECTION,LAB1 RRC A ; Rotate contents of Accumulator to the right through ; Carry bit SJMP LAB2LAB1: RLC A ; Rotate contents of Accumulator to the left through ; Carry bitLAB2: MOV P1,A ; Contents of Accumulator is moved to port P1 RETI ; Return from interrupt

TIM1_ISR: DJNZ R0,LAB3 ; If time is ran out, change direction of rotation CPL DIRECTION MOV R0,#30D ; Initialize R0LAB3: RETI END ; End of program

Using Timer T2

This example describes the use of Timer T2 configured to operate in Auto-Reload mode. In this very case,

LEDs are connected to port P3 while the pushbutton used for forced timer reset (T2EX) is connected to

pin P1.1.

Program execution is similar to the previous examples. When timer ends counting, interrupt is enabled

and subroutine TIM2_ISR is executed. Within it, logic zero (0) in accumulator is rotated and afterwards

content of accumulator is moved to pin P3. At the end, flags which caused interrupt are erased and

program returns to the loop LOOP1 where it remains until a new interrupt request is encountered...

If pushbutton T2EX is pressed, timer is temporarily reset. Hence, this pushbutton resets timer while

pushbutton RESET resets microcontroller.


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;************************************************************************;* PROGRAM NAME : Timer2.ASM;* DESCRIPTION: Program rotates log. "0" on port P3. Timer2 determines;* the speed of rotation and operates in auto-reload mode;************************************************************************

;BASIC DIRECTIVES

$MOD53$TITLE(TIMER2.ASM)$PAGEWIDTH(132)$DEBUG$OBJECT$NOPAGING

;DEFINITION OF VARIABLES

T2MOD DATA 0C9H;STACK DSEG AT 03FHSTACK_START: DS 040H


ORG 02BH ; Timer T2 Reset vector JMP TIM2_ISR

ORG 100H

XRESET: MOV SP,#STACK_START ; Defining of Stack pointer MOV A,#0FFH MOV P3,#0FFH MOV RCAP2L,#0FH ; 16-bit auto-reload mod is prepared MOV RCAP2L,#01H CLR CAP2 ; 16-bit auto-reload mod is turned on SETB EXEN2 ; reset through pin P1.1 is enabled SETB TR2 ; Timer2 is turned on MOV IE,#0A0H ; Interrupt is enabled CLR C


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TIM2_ISR: RRC A ; Rotate contents of Accumulator to the right through ; Carry bit MOV P3,A ; Move the content of Accumulator A to PORT3 CLR TF2 ; Erase flag TF2 of timer T2 CLR EXF2 ; Erase flag EXF2 of timer T2 RETI ; Return from interrupt


Using External Interrupt

Here is another example of interrupt execution. This time, it is about external iterrupts generated when

low logic level is present on pin P3.2 or P3.3. Depending on which input is active, one of two routines

will be executed:

Logic zero (0) on pin P3.2 starts interrupt routine Isr_Int0. The routine increments number in register R0

and copies it to port P0. Low level on pin P3.3 starts subroutine Isr_Int1which increments number in

register R1 by 1 and copies it to port P1 afterwards.

In short, each press on pushbuttons INT0 and INT1 will be counted and immediately shown in binary

format on the appropriate port (LED which emitts light represents logic zero (0)).

;************************************************************************;* PROGRAM NAME : Int.ASM;* DESCRIPTION : Program counts interrupts INT0 which are generated by;* appearance of high-to-low transition signal on pin;* P3.2 Result appears on port P0. Interrupts INT1 are;* counted off at the same time. They are generated by;* appearing high-to-low transition signal on pin P3.;* This result appears on port P1.


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;************************************************************************

;BASIC DIRECTIVES

$MOD53$TITLE(INT.ASM)$PAGEWIDTH(132)$DEBUG$OBJECT$NOPAGING

;RESET VECTORS

CSEG AT 0 JMP XRESET ; Reset vector

ORG 003H ; Interrupt routine address for INT0 JMP Isr_Int0 ORG 013H ; Interrupt routine address for INT1 JMP Isr_Int1

ORG 100HXRESET: MOV TCON,#00000101B ; Interrupt INT0 is generated by appearing ; high-to-low transition signal on pin P3.2 ; Interrupt INT0 is generated by appearing ; high-to-low transition signal on pin P3.3 MOV IE,#10000101B ; Interrupt enabled MOV R0,#00H ; Counter starting value MOV R1,#00H MOV P0,#00H ; Reset port P0 MOV P1,#00H ; Reset port P1

LOOP: SJMP LOOP ; Remain here

Isr_Int0: INC R0 ; Increment value of interrupt INT0 counter MOV P0,R0 RETI

Isr_Int1: INC R1 ; Increment value of interrupt INT1 counter MOV P1,R1 RETI END ; End of program

Using LED display

Following examples describe the use of LED display. Common chatode displays are used here, which

means that all built in LEDs are polarized so that their anodes are connected to the microcontroller pins. It

is not the way it should be but common way of thinking is that logic one (1) “turns on” something while

logic zero (0) “turns off” something. That is why Low Current displays (low consumption) and their

diodes (segments) are connected in series to resistors of relatively high resistance.

In order to save I/O pins, four LED displays are connected to operate in multiplex mode. That means that

all segments having the same name are connected to one output port each and that there is always one

display active.

By quick and synchronized activation of tranzistors and segmenats on displays, one gets impression that

all digits emit lights simultaneously.


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Write digits on LED display

This program is designed as “warming up” before real work starts. The single aim is to display something

on any of displays. This time it is not multiplex mode, instead, digit 3 is displayed on only one of them

(first one on the right).

Since the microcontroller “does not know” how man writes number 3, a small subroutine called Disp is

used (microcontroller writes it as 0000 0011). This subroutine performs as a mask for all digits in decade

system (0-9). The principle of the operation is simple. A number that should be displayed is added to the

current address and program jump is executed. Different numbers match different jump length. Precisely

determined combination of zeroes and units appears on each of these new locations (digit 1 mask, digit 2

mask...digit 9 mask). When this combination is transferred to the port, display diodes are activated as to

show desired digit.

;************************************************************************;* PROGRAM NAME : 7Seg1.ASM;* DESCRIPTION: Program shows number "3" on 7-segment LED display;************************************************************************

;BASIC DIRECTIVES

$MOD53$TITLE(7SEG1.ASM)$PAGEWIDTH(132)


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$DEBUG$OBJECT$NOPAGING



ORG 100H

XRESET: MOV SP,#STACK_START ; Defining of Stack pointer MOV P1,#0 ; Turn off all segments on displays MOV P3,#20h ; Activate display D4

LOOP: MOV A,#03 ; Send number “3” on display LCALL Disp ; Find appropriate mask for that number MOV P1,A SJMP LOOP

Disp: ; Subroutine for writing digits INC A MOVC A,@A+PC RET DB 3FH ; Digit 0 mask DB 06H ; Digit 1 mask DB 5BH ; Digit 2 mask DB 4FH ; Digit 3 mask DB 66H ; Digit 4 mask DB 6DH ; Digit 5 mask DB 7DH ; Digit 6 mask DB 07H ; Digit 7 mask DB 7FH ; Digit 8 mask DB 6FH ; Digit 9 mask END ; End of program

Write and change digits on LED display

Program in this example is only an extended verson of the previous one. There is only one digit active- the

first one on the right side, and there is no use of multiplexing. Unlike the previous case, all decade digits

are displayed (0-9). In order to enable digits to shift at rational rate, a soubroutine L2 which causes a small

time delay is executed before each shift. Basically, the whole process is very simple and takes place in the

main loop LOOP as follows:

R3 is copied to Accumulator and subroutine for masking digits Disp is executed.1.

Accumulator is copied to the port and displayed.2.

The contents of the R3 register is incremented.3.

It is checked whether 10 cycles are counted or not.

If it is counted, register R3 is reset in order to enable counting to start from 0.

4.

Instruction labeled as L2 within subroutine is executed.5.

;************************************************************************;* PROGRAM NAME: 7Seg2.ASM;* DESCRIPTION: Program writes numbers 0-9 on 7-segment LED display;************************************************************************

;BASIC DIRECTIVES

$MOD53$TITLE(7SEG2.ASM)$PAGEWIDTH(132)$DEBUG$OBJECT$NOPAGING


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ORG 100H

XRESET: MOV SP,#STACK_START ; Defining of Stack pointer MOV R3,#0 ; Counter starting value MOV P1,#0 ; Turn off all segments on display MOV P3,#20h ; Activate display D4

LOOP: MOV A,R3 LCALL Disp ; Find appropriate mask for number in ; Accumulator MOV P1,A INC R3 ; Increment number in register by 1 CJNE R3,#10,L2 ; Check whether the number 10 is in R3 MOV R3,#0 ; If it is, reset counterL2: MOV R2,#20 ; 500 mS wait timeF02: MOV R1,#50 ; 25 mSF01: MOV R0,#230 DJNZ R0,$ DJNZ R1,F01 DJNZ R2,F02 SJMP LOOP

Disp: ; Subroutine for writing digits INC A MOVC A,@A+PC RET DB 3FH ; Digit 0 mask DB 06H ; Digit 1 mask DB 5BH ; Digit 2 mask DB 4FH ; Digit 3 mask DB 66H ; Digit 4 mask DB 6DH ; Digit 5 mask DB 7DH ; Digit 6 mask DB 07H ; Digit 7 mask DB 7FH ; Digit 8 mask DB 6FH ; Digit 9 mask


Write two-digit number on LED display

It is time for time multiplex! This is the simplest example where the number 23 is displayed on two

displays which represent units and tens,. It means that digit 3 should be dispalyed on the far right display

and digit 2 on the display beside. The most important thing in the program is regular time synchronization.

Since this is the simplest case where only two digits are used and since the microcontroller does nothing

else but diaplays a number everything is very simple. Transistor T4 “turns on” display D4 and at the same

time a bits’ combination corresponding to the digit 3 is set on the port. After that, transistor T4 is “turned

off” and the whole process is repeated using transistor 3 and display 3 in order to display digit 2. This

procedure must be continuosly repeated in order to make impression that both displays are activ at the

same time.

;************************************************************************;* PROGRAM NAME: 7Seg3.ASM;* DESCRIPTION: Program displays number "23" on 7-segment LED display;************************************************************************

;BASIC DIRECTIVES

$MOD53$TITLE(7SEG3.ASM)


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$PAGEWIDTH(132)$DEBUG$OBJECT$NOPAGING



ORG 100HXRESET: MOV SP,#STACK_START ; Defining of Stack pointer

LOOP: MOV P1,#0 ; Turn off all segments on display MOV P3,#20h ; Activate display D4 MOV A,#03 ; Write digit 3 on display D4 LCALL Disp ; Find mask for that digit MOV P1,A ; Put the mask on the port MOV P1,#0 ; Turn off all segments on displays MOV P3,#10h ; Activate display D3 MOV A,#02 ; Write digit 2 on display D3 LCALL Disp ; Find mask for that digit MOV P1,A ; Put the mask on the port SJMP LOOP ; Get back to the label LOOP



Using 4-digit LED display

In this example all four displays, instead of two, are active so it is possible to write numbers 0 - 9999. In

this very case, the number 1 234 is displayed. After introductory initialization, program remains in the

loop LOOP where digital multiplexing is performed.The subroutine Disp has the purpose to convert

binary numbers into corresponding bit combinations for lighting segments activation on display.

;************************************************************************;* PROGRAM NAME : 7Seg5.ASM;* DESCRIPTION : Program displays number"1234" on 7-segment LED display;************************************************************************

;BASIC DIRECTIVES



;RESET VECTORS CSEG AT 0


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JMP XRESET ; Reset vector

ORG 100H

XRESET: MOV SP,#STACK_START ; Defining of Stack pointer

LOOP: MOV P1,#0 ; Turn off all segments on display MOV P3,#20h ; Activate display D4 MOV A,#04 ; Write digit 4 on display D4 LCALL Disp ; Find mask for that digit MOV P1,A ; Put the mask on the port MOV P1,#0 ; Turn off all segments on displays MOV P3,#10h ; Activate display D3 MOV A,#03 ; Write digit 3 on display D3 LCALL Disp ; Find mask for that digit MOV P1,A ; Put the mask on the port MOV P1,#0 ; Turn off all segments on displays MOV P3,#08h ; Activate display D2 MOV A,#02 ; Write digit 2 on display D2 LCALL Disp ; Find mask for that digit MOV P1,A ; Put the mask on the port MOV P1,#0 ; Turn off all segments on displays MOV P3,#04h ; Activate display D1 MOV A,#01 ; Write digit 1 on display D1 LCALL Disp ; Find mask for that digit MOV P1,A ; Put the mask on the port SJMP LOOP ; Return to the lable LOOP



LED display as two-digit counter

Things are getting complicated... Beside two digit multiplexing, the microcontroller performs other

operations “in the background” too. In this case, contents of registers R2 and R3 are incremented in order

to make counting 97, 98, 99, 00, 01, 02... visible on display.

This time, transistors which activate displays remains on for 25mS. The soubroutine Delay is in charge for

that. Even though digits are shifted much slower it is still not slow enough to make impression of

simultaneous operating. After 20 alternate turning on and off both digits, number on displays is

incremented by 1 and the whole procedure is repeated.

;************************************************************************;* PROGRAM NAME : 7Seg4.ASM;* DESCRIPTION: Program displays numbers 0-99 on 7-segment LED displays;************************************************************************

;BASIC DIRECTIVES


;STACK


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DSEG AT 03FHSTACK_START: DS 040H


ORG 100H

XRESET: MOV SP,#STACK_START ; Defining of Stack pointer MOV R2,#0 ; Counter starting value MOV R3,#0 MOV R4,#0

LOOP: INC R4 ;Hold before to increment the content of CJNE R4,#20d,LAB1 ;counter until display is 100 times refreshed MOV R4,#0 MOV P1,#0 ; Turn off all segments on displays INC R2 ; Increment Register with units by 1 CJNE R2,#10d,LAB1 MOV R2,#0 ; Reset units INC R3 ; Increment Register with tens by 1 CJNE R3,#10d,LAB1 ; MOV R3,#0 ; Reset tens

LAB1: MOV P3,#20h ; Activate display D4 MOV A,R2 ; Copy Register with units to A LCALL Disp ; Find mask for that digit MOV P1,A ; Write units on display D4 LCALL Delay ; 25ms wait time MOV P1,#0 ; Turn off all segments on displays MOV P3,#10h ; Activate display D3 MOV A,R3 ; Copy Register with tens to A LCALL Disp ; Find mask for that digit MOV P1,A ; Write tens on display D3 LCALL Delay ; 25ms wait time SJMP LOOP

Delay: MOV R1,#50 ; 25 mSF01: MOV R0,#250 DJNZ R0,$ DJNZ R1,F01 RET



Handling EEPROM

Program writes data to on-chip EEPROM memory. In this case, data is hexadecimal number 23 which

written to location with address 00.

To ensure that number is correctly written, the same location in EEPROM is read 10mS later and

compared with original value. In case the numbers are identical, F will be displayed on LED display.

Otherwise, E will be displayed on LED display (Error).


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;************************************************************************;* PROGRAM NAME: EEProm1.ASM;* DESCRIPTION: Programming EEPROM at address 0000hex and displaying message;* on LED display.;************************************************************************

;BASIC DIRECTIVES

$MOD53$TITLE(EEPROM1.ASM)$PAGEWIDTH(132)$DEBUG$OBJECT$NOPAGING

WMCON DATA 96HEEMEN EQU 00001000B ; Access to internal EEPROM is enabledEEMWE EQU 00010000B ; Write to EEPROM is enabledTEMP DATA 030H ; Defining of Auxilary register

THE END EQU 071H ; Write "F" on displayERROR EQU 033H ; Write "E" on display



ORG 100H

XRESET: MOV IE,#00 ; All interrupts are disabled MOV SP,#STACK_START

MOV DPTR,#0000H ; Choose location address in EEPROM ORL WMCON,#EEMEN ; Access to EEPROM is enabled ORL WMCON,#EEMWE ; Write to EEPROM is enabled MOV TEMP,#23H ; Number written to EEPROM is copied to MOV A,TEMP ; register TEMP and Accumulator MOVX @DPTR,A ; Write byte to EEPROM CALL DELAY ; 10ms wait time MOVX A,@DPTR ; Read the same location and compare to TEMP, CJNE A,TEMP,ERROR ; If they are not identical,jump to label ERROR MOV A,#KRAJ ; Write letter F on display (correct) MOV P1,A XRL WMCON,#EEMWE ; Write to EEPROM is disabled XRL WMCON,#EEMEN ; Access to EEPROM is disabledLOOP1: SJMP LOOP1 ; Remain here

ERROR: MOV A,#ERROR ; Write letter E on display (error) MOV P1,ALOOP2: SJMP LOOP2

DELAY: MOV A,#0AH ; Wait time MOV R3,ALOOP3: NOPLOOP4: DJNZ B,LOOP4LOOP5: DJNZ B,LOOP5 DJNZ R3,LOOP3 RET


Receiving data via serial communication UART

In order to enable successful serial communication using UART system, beside having correctly written

program it is also necessary to meet certain rules of RS232 connection. It is about voltage levels issued by

this standard. In accordance to it logic one (1) is represented by -10V in message, while logic zero (0) is

transferred like +10V. The microcontroller converts data serial format without error but its power supply

voltage is only 5V. It is not easy to convert 0V into 10V and 5V into -10V. Because of that, this operation


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is on both transmit and receive side left over to specialized IC circuit. In this example, MAX232 circuit

manufactured by MAXIM is used because it is widespread, cheap and reliable.

This example demonstrates message receiving which is sent from PC. Timer T1 generates boud rate.

Since quartz crystal with frequency of 11.0592 MHz is in use it is not problem to obtain standard baud

rate which amout to 9600 baud. Each received data is transferred to port P1 pins.

;************************************************************************;* PROGRAM NAME : UartR.ASM;* DESCRIPTION: Each data received from PC via UART appears on the port;* P1.;*;************************************************************************

;BASIC DIRECTIVES

$MOD53$TITLE(UARTR.ASM)$PAGEWIDTH(132)$DEBUG$OBJECT$NOPAGING


;RESET VECTORS CSEG AT 0 JMP XRESET ; Reset vector ORG 023H ; Starting address for UART interrupt routine JMP IR_SER

ORG 100H


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XRESET: MOV IE,#00 ; All interrupts are disabled MOV SP,#STACK_START ; Initialization of Stack pointer MOV TMOD,#20H ; Timer1 in mode2 MOV TH1,#0FDH ; Baud rate is 9600 baud at frequency of ; 11.0592MHz MOV SCON,#50H ; Receiving enabled, 8-bit UART MOV IE,#10010000B ; UART interrupt enabled CLR TI ; Clear transmit flag CLR RI ; Clear receive flag SETB TR1 ; Start Timer1


IR_SER: JNB RI,OUT ; If any data is received, ; copy it to the port MOV A,SBUF ; P1 MOV P1,A CLR RI ; Clear receive flagOUT RETI


Data transmission via serial communication UART

Program below describes how to use UART modul for data transmission. In concrete example, a series of

numbers (0-255) are transmitted to PC at baud rate of 9600 baud. The circuit MAX 232 is used for

voltage level converting.

;************************************************************************;* PROGRAM NAME : UartS.ASM;* DESCRIPTION: Sends values 0-255 to PC.;************************************************************************

;BASIC DIRECTIVES

$MOD53$TITLE(UARTS.ASM)$PAGEWIDTH(132)$DEBUG$OBJECT$NOPAGING



ORG 100H

XRESET: MOV IE,#00 ; All interrupts are disabled MOV SP,#STACK_START ; Initialization of Stack pointer MOV TMOD,#20H ; Timer1 in mode 2 MOV TH1,#0FDH ; Baud rate is 9600 baud at frequency of ; 11.0592MHz MOV SCON,#40H ; 8-bit UART CLR TI ; Clear transmit bit CLR RI ; Clear receive flag MOV R3,#00H ; Reset caunter SETB TR1 ; Start Timer 1

START: MOV SBUF,R3 ; Move number from counter to PCLOOP1: JNB TI,LOOP1 ; Wait here until byte transmission is ; complete CLR TI ; Clear transmit bit INC R3 ; Increment value of counter by 1

CJNE R3,#00H,START ; If 255 bytes are not sent return to the ; label START


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Write message on LCD display

The most frequent LCD version which displays text in two lines with 16 characters each is used in this

example. Since I/O ports are always valuable, a method in which only 4 lines are used for communication

is applied here. In this way each byte is transmitted in two steps: first higher one, afterwards lower nible.

You will see that, LCD needs to be initialized at the beginning (to be prepared for operating). Besides,

specific parts of the program which are repeated are separated in special totalities (subroutines). All this

may seem endlessly complicated at first sight, but the whole program basically performs several simple

operations and displays ”Mikroelektronika Razvojni sistemi”.

*************************************************************************;* PROGRAM NAME : Lcd.ASM;* DESCRIPRTION : Program for testing LCD display.4-bit communication;* is used.Program does not check BUSY flag but uses pro;* gram delay between 2 commands. PORT1 is used for con;* nection to the microcontroller.;************************************************************************

;BASIC DIRECTIVES

$MOD53$TITLE(LCD.ASM)$PAGEWIDTH(132)$DEBUG$OBJECT$NOPAGING

;Stack DSEG AT 0E0hStack_Start: DS 020h

Start_address EQU 0000h

;Reset vectors CSEG AT 0


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ORG Start_address JMP Inic

ORG Start_address+100h

MOV IE,#00 ; All interrupts are disabled MOV SP,#Stack_Start

Inic: CALL LCD_inic ; Initialize LCD

;*************************************************;* MAIN PROGRAM;*************************************************

START: MOV A,#80h ; First following character will appear on first CALL LCD_status ; location in first line on LCD display. MOV A,#'M' ; Display character ‘M’. CALL LCD_putc ; Call subroutine for character transmission. MOV A,#'i' ; Display character ‘i’. CALL LCD_putc MOV A,#'k' ; Display character ‘k’. CALL LCD_putc MOV A,#'r' ; Display character ‘r’. CALL LCD_putc MOV A,#'o' ; Display character ‘o’. CALL LCD_putc MOV A,#'e' ; Display character ‘e’. CALL LCD_putc MOV A,#'l' ; Display character ‘l’. CALL LCD_putc MOV A,#'e' ; Display character ‘e’. CALL LCD_putc MOV A,#'k' ; Display character ‘k’. CALL LCD_putc MOV A,#'t' ; Display character ‘t’. CALL LCD_putc MOV A,#'r' ; Display character ‘r’. CALL LCD_putc MOV A,#'o' ; Display character ‘o’. CALL LCD_putc MOV A,#'n' ; Display character ‘n’. CALL LCD_putc MOV A,#'i' ; Display character ‘i’. CALL LCD_putc MOV A,#'k' ; Display character ‘k’. CALL LCD_putc MOV A,#'a' ; Display character ‘a’. CALL LCD_putc

MOV A,#0c0h ; First following character will appear on first CALL LCD_status ; location in second line on LCD display. MOV A,#'R' ; Display character ‘R’. CALL LCD_putc ; Call subroutine for character transmission. MOV A,#'a' ; Display character ‘a’. CALL LCD_putc MOV A,#'z' ; Display character ‘z’. CALL LCD_putc MOV A,#'v' ; Display character ‘v’. CALL LCD_putc MOV A,#'o' ; Display character ‘o’. CALL LCD_putc MOV A,#'j' ; Display character ‘j’. CALL LCD_putc MOV A,#'n' ; Display character ‘n’. CALL LCD_putc MOV A,#'i' ; Display character ‘i’. CALL LCD_putc MOV A,#' ' ; Display character ‘ ’. CALL LCD_putc MOV A,#'s' ; Display character ‘s’. CALL LCD_putc MOV A,#'i' ; Display character ‘i’. CALL LCD_putc MOV A,#'s' ; Display character ‘s’.


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CALL LCD_putc MOV A,#'t' ; Display character ‘t’. CALL LCD_putc MOV A,#'e' ; Display character ‘e’. CALL LCD_putc MOV A,#'m' ; Display character ‘m’. CALL LCD_putc MOV A,#'i' ; Display character ‘i’. CALL LCD_putc

MOV R0,#20d ; Wait time (20x10ms) CALL Delay_10ms ; MOV DPTR,#LCD_DB ; Clear display MOV A,#6d ; CALL LCD_inic_status ; MOV R0,#10d ; Wait time(10x10ms) CALL Delay_10ms JMP START

;*********************************************;* Subroutine for wait time (T= r0 x 10ms);*********************************************

Delay_10ms: MOV R5,00h ; 1+(1+(1+2*r7+2)*r6+2)*r5 approximate ; ly. MOV R6,#100d ; (if r7>10) MOV R7,#100d ; 2*r5*r6*r7 DJNZ R7,$ ; $ indicates actual instruction. DJNZ R6,$-4 DJNZ R5,$-6 RET

;**************************************************************************************;* SUBROUTINE: LCD_inic;* DESCRIPTION: Subroutine for LCD initialization.;*;* (is used with 4-bit interface, under condition that pins DB4-7 on LCD;* are connected to pins PX.4-7 on microcontroller’s ports, i.e. four higher;* bits on a port are used).;*;* NOTE: It is necessary to define port pins for controlling LCD operating:;* LCD_enable, LCD_read_write, LCD_reg_select,similar to port for connection to LCD.;* It is also necessary to define addresses for the first character in each;* line.;**************************************************************************************

LCD_enable BIT P1.3 ; Bit for activating pin E on LCD.LCD_read_write BIT P1.1 ; Bit for activating pin RW on LCD.LCD_reg_select BIT P1.2 ; Bit for activating pin RS on LCD.LCD_port SET P1 ; Port for connection to LCD.Busy BIT P1.7 ; Port pin where Busy flag appears.

LCD_Start_I_red EQU 00h ; Address of the first message charac ; ter in the first line on LCD display.LCD_Start_II_red EQU 40h ; Address of the first message charac ; ter in the second line on LCD display.

LCD_DB: DB 00111100b ; 0 -8b, 2/1 lines, 5x10/5x7 format DB 00101100b ; 1 -4b, 2/1 lines, 5x10/5x7 format DB 00011000b ; 2 -Display/cursor shift, right/left DB 00001100b ; 3 -Display ON, cursor OFF, cursor blink off DB 00000110b ; 4 -Increment mode, display shift off DB 00000010b ; 5 -Display/cursor home DB 00000001b ; 6 -Clear display DB 00001000b ; 7 -Display OFF, cursor OFF, cursor blink off

LCD_inic: ;*****************************************

MOV DPTR,#LCD_DB

MOV A,#00d ; Triple initialization in 8-bit CALL LCD_inic_status_8 ; mode is performed at the beginning MOV A,#00d ; (in case of slow increment of CALL LCD_inic_status_8 ; power supply when power on


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MOV A,#00d lcall LCD_inic_status_8

MOV A,#1d ; Change from 8-bit into CALL LCD_inic_status_8 ; 4-bit mode MOV A,#1d CALL LCD_inic_status

MOV A,#3d ; From this point program executes in ;4-bit mode CALL LCD_inic_status MOV A,#6d CALL LCD_inic_status MOV A,#4d CALL LCD_inic_status

RET

LCD_inic_status_8: ;****************************************** PUSH B

MOVC A,@A+DPTR CLR LCD_reg_select ; RS=0 - Write command CLR LCD_read_write ; R/W=0 - Write data on LCD

MOV B,LCD_port ; Lower 4 bits from LCD port are memo ; rized ORL B,#11110000b ORL A,#00001111b ANL A,B

MOV LCD_port,A ; Data is copied from A to LCD port SETB LCD_enable ; EN=1 - EN high-to-low transition sig ; nal is generated CLR LCD_enable ; EN=0 made on EN pin of LCD display

MOV B,#255d ; Time delay in case of improper reset DJNZ B,$ ; during initialization DJNZ B,$ DJNZ B,$

POP B RET

LCD_inic_status:;**************************************************************************** MOVC A,@A+DPTR CALL LCD_status RET

;****************************************************************************;* SUBROUTINE: LCD_status;* DESCRIPTION: Subroutine for defining LCD status.;****************************************************************************

LCD_status: PUSH B MOV B,#255d DJNZ B,$ DJNZ B,$ DJNZ B,$ CLR LCD_reg_select ; RS=O: Command is sent on LCD CALL LCD_port_out

SWAP A ; Nibles are swapped in accumulator

DJNZ B,$ DJNZ B,$ DJNZ B,$ CLR LCD_reg_select ; RS=0: Command is sent on LCD CALL LCD_port_out

POP B RET


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;****************************************************************************;* SUBROUTINE: LCD_putc;* DESCRIPTION: Sending character to be displayed on LCD.;****************************************************************************

LCD_putc: PUSH B MOV B,#255d DJNZ B,$ SETB LCD_reg_select ; RS=1: Character is sent on LCD CALL LCD_port_out

SWAP A ; Nibles are swapped in accumulator

DJNZ B,$ SETB LCD_reg_select ; RS=1: Character is sent on LCD

CALL LCD_port_out POP B RET

;****************************************************************************;* SUBROUTINE: LCD_port_out;* DESCRIPTION: Sending commands or characters on LCD display;****************************************************************************

LCD_port_out: PUSH ACC PUSH B MOV B,LCD_port ; Lower 4 bits of LCD port are memo ; rized ORL B,#11110000b ORL A,#00001111b ANL A,B

MOV LCD_port,A ; Data is copied from A to LCD port

SETB LCD_enable ; EN=1 - EN high-to-low transition sig ; nal is generated CLR LCD_enable ; EN=0 made on EN pin of LCD display

POP B POP ACC RET


Binary-decimal Conversion of number

While operating with LED and LCD displays, it is often needed to convert numbers from binary to

decimal numerical system. For example, if some register contains a number in binary format that should

be displayed on three digit LED display it is necessary to convert it to decimal format. Simply, it has to be

defined what should be displayed on the far right display (units), middle display (tens) and far left display

(hundreds), respectively.

Subroutine below solves this problem in case of conversion of one byte. Binary number is stored in

Accumulator while digits of that number in decimal format are stored in registers R3, R2 and accumulator

(units, tens and hundreds).

;************************************************************************;* SUBROUTINE NAME : BinDec.ASM;* DESCRIPTION : Content of accumulator is converted into three decimal;* digits;************************************************************************

BINDEC: MOV B,#10d ; Store decimal number 10 in B DIV AB ; A:B. Remainder remains in B MOV R3,B ; Copy units to register R3 MOV B,#10d ; Store decimal number 10 in B DIV AB ; A:B. Remainder remains in B MOV R2,B ; Copy tens to register R2


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MOV B,#10d ; Store decimal number 10 in B DIV AB ; A:B. Remainder remains in B MOV A,B ; Copy hundreds to accumulator RET ; Return to the main program


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Chapter 7 : Development systems

7.1 At the end - from the beginning...

7.2 Easy8051A Development system

7.1 At the end - from the beginning...

As always, beginning is the most difficult. You have bought microcontroller, you have learned everything about its systems

and registers, you have great idea how to apply all that in practice. The only thing left over to you is to start...

How to start working?

Microcontroller is a good-natured “giant from the bottle” and there is no need for extra knowledge in order to use it.

In order to create your first device under microcontroller’s control, you need: the simplest PC, program for compiling into

machine code and simple device for “transferring” that code from PC to chip itself.

The process itself is quite logical but dilemmas are anyway common, not because it is complicated but for the fact that

there are numerous variations. Let’s start...

Writing program in assembler

In order to write a program for the microcontroller, a specialized program in Windows environment may be used. It may,

but it does not have to... On using such a software, there are numerous tools which facilitate operating (first of all simulator

tool). This is a distinct advantage, but there are other ways too. Basically, text is the only important thing. Because of that

any program for text processing can be used for writing program. The point is to write all instructions in order they should

be executed by the microcontroller. The rules of assembly language are observed and instructions are written as they are

defined. Program idea is followed. That’s all!

;RESET VECTOR

CSEG AT 0

JMP XRESET ; Reset vector

CSEG

ORG 100H

XRESET: ORL WMCON,#PERIOD ; Defining of Watch-dog period

ORL WMCON,#WDTEN ; Watch-dog timer is enabled

If a document is written for being used further by programmer then it has to have an extension, .asm in its name, for

example: Program asm.

If a program is written using a specialized program (mplab), this extension will be automatically added. If any other

program for text processing (Notepad) is used then the document should be saved and additionally renamed. For example:

Program.txt -> Program.asm. This procedure is not necessary. The document may be saved in original format while Its text

may be copied to programmer for further use.

As seen, text of the program is the only thing that matters.

Compiling into machine code

Microcontroller “does not undrestand” assembly language. That is why this program should be compiled. If a specialized

program is used- nothing simpler - a machine code compiler is a part of the software! Problem is solved by a click on the

appropriate icon. The result is a new document which has extension .hex in its name. That is the same program you have

already written, but compiled into machine language which microcontroller perfectly understands. The common name of

this document is “hex code” and represents apparently meaningless series of numbers in hexadecimal numerical system.

:03000000020100FA1001000075813F

7590FFB29012010D80F97A1479D40

90110003278589EAF3698E8EB25B

A585FEA2569AD96E6D8FED9FAD

AF6DD00000001FF255AFED589EA

F3698E8EB25BA585FEA2569AD96



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DAC59700D00000278E6D8FED9FA

DAF6DD00000001FF255AFED8FED

9FADAF6DD000F7590FFB29013278

E6D8FED9FADAF6DD00000001FF2

55AFED589EAF3698E8EB25BA585

FEA2569AD96DAC59D9FADAF6D

D00000001FF255AFED8FED9FADA

F6DD000F7590FFB29013278E6D82

78E6D8FED9FA589EAF3698E8EB2

5BA585FEA2569AD96DAF6DD000

00001FF2DAF6DD00000001FF255A

ADAF6DD00000001FF255AFED8FE

D9FA

In case some other software for writing program in assembler is used, a software especially installed for compiling into

machine code must be used. This compiler is activated, document with extension .asm is open and the appropriate

command is executed. The result is the same- a new document with extension .hex. The only problem is that it is stored in

your PC.

Copy program to a microcontroller

Acable for serial communication and a special device called programmer are necessary to transfer “hex code” to the

microcontroller. There are also several options on how to do it.

A great deal of programs and electronic circuits having this purpose can be found on Internet. Do as follows: open hex code

document, adjust a few parameters and click on the icon for compiling. After a while, a series of zeros and units will be

programmed into the microcontroller through the serial connection cable. The only thing left over is to transfer

programmed chip to the final device. In case it is necessary to change something in the program, the previous procedure

may be repeated.

How it operates?

Working with program developed by Mikroelektronika will be in short described here. Everything is very simple...

Start the program Mikroelektronika Asm51 Console. A

window appears...

...Open a new document: File -> New. Introductory header

appears. Write your program or copy text...

... Save and name your document : File -> Save As...

(Document name is limited to 8 characters!)

Finally, to compile program into HEX code select: Project

-> Build or click on the icon “play”.

If everything works properly, the computer will reward

you with a minireport!


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Program is written and successfully compiled into machine

code. It is only left over to load it to the microcontroller.

For this purpose it is necessary to have programator and

software which interfere between PC and programator

hardware.

Start the program 8051 Flash_setup.exe...

Program installation

is performed as

usual - It is

necessary to select

following

commands for

several times: Next,

Accept, Next...

...and finally -

Finish!

Program has been

installed and ready

for use. Settings are

simple and there is

no need to explain

them more (type of

the microcontroller

in use, frequency

and type of clock

oscillator and similar).

Connect PC and programmer via USB cable.

Load HEX code by command: File -> Load HEX.

Click on the “Write” pushbutton and wait...

That’s all! Program is

loaded into the

microcontroler and

everything is ready for

operating. If you are not

satisfied, make some

changes in your program

and repeat the procedure.

Until when? Until you

feel satisfied...

What are the

development systems?

A device which in testing

program phase for the

microcontroller can

simulate any device is

called- development

system. Beside

programmer, power

supply unit and

microcontroller’s socket,

development system also

contains elements to

activate input pins and

monitor state on the

output pins. In the simplest version, each pin is connected to one

pushbutton and one LED. Higher quality versions have LED displays,


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LCD displays, temperature sensors and all other elements which the final device can be supplied with. These peripherals

could be connected to MCU via miniature jumpers. In that way, a program may be checked in practice during its writing

because the microcontroller cannot “know” whether its input is activated by a pushbutton or by a sensor built in a real

machine.

7.2 Easy8051A Development system

This is one of high-quality development systems used for programming 8051 compatible microcontrollers manufactured by

Atmel. Beside chip programming, this system enables direct testing all parts of a program because it contains the most

components which are normally built in the real devices.

Easy8051A Development system consists of:

Sockets for placing microcontrollers (14, 16, 20 and 40- pin packages)

Connector for external power supply (DC 12V)

USB programmator

Power Supply Selector (external or via USB cable)

Quartz Crystal Oscillator 8 Mhz

32 LEDs for output pins’ state indication

32 pushbuttons for activating input pins

Four 7-segment LED displays in multiplex mode

Graphic LCD display

Alphanumeric LCD display (4- or 8- bit mode)

Connector and driver for serial communication RS232

Digital thermometer DS1820

12- bit A/D converter (MCP3204)

12- bit D/A converter (MCP4921)

Reference voltage source 4.096V (MCP1541)

Multiple-pin connectors for direct access to I/O ports

Several following pages describe in short some circuits within this development system- it is more illustration of its

possibilities than complete instructions. Besides, familiarizing with details of this device testifies that microcontrollers and

tools for handling them are neither privilege nor secret of the few.

Sockets

All microcontrollers manufactured by Atmel appear in a few standard DIP

packages. In order to enable their programming using one device, corresponding

pins on sockets are connected in parallel (pins having the same name). In


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accordance to that, by being placed in the appropriate socket, each microcontroller

will be automatically properly connected! Figure on left shows in detail

microcontroller in 40-pin package and connection of one of its I/O pins (P1.5) as

well. As seen, the pin can be connected to some external device (connector

PORT1), LED (microswitch SW2), pushbutton or resistor through connectors. In

the last two cases, polarity of voltage is selected using on board jumpers.

Programmer

The purpose of the programmer is to transfer HEX code from PC to the

appropriate pins and provide regular voltage levels during chip programming as

well. In this case it belongs to development system and should be connected to PC

via USB cable. Once programming is completed, pins used during that proces are

automatically available for other application.


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Development system power supply

There is a connector for external power supply source (AC/DC, 8-16V) on the

development board. Besides, voltage necessary for device operating can be also

obtained from PC via USB cable. Jumper J5 is used for power supply selecting.

8MHz Oscillator

EASY8051A Development system has got built in oscillator used as a clock signal

generator. Frequency of this oscillator is stabilized by quartz crystal. Besides,

during chip programming, internal RC oscillator can be selected instead.


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LEDs for output pins’ state indication

All I/O ports pins are connected to one LE diode each, which enables visual

indication of their logic state. In case that presence of direct polarized LEDs and

serial resistors is not acceptable in very application, DIP switch SW2 enables them

to be disconnected from the port.


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Pushbuttons for activating input pins

Similar to LEDs, each I/O port pin is connected to one pushbutton on the

development board. It enables simple activation of input pins. Jumper J6 is used

for selecting voltage polarity (+ or -). Press on the appropriate pushbuttons brings

selected voltage to the pins.


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7-segment LED displays

For being often applied in industry, four high-performance LED displays set up in

multiplex mode belong to the development board. Displays’ segments are

connected to the port P0 through resistors. Transistor drivers for activation of

individual digits are connected to the first four port P1 pins. It enables testing

programs which use 7-segment displays with minimal use of I/O ports. Similar to

LEDs, DIP switch SW2 disconnects transistor drivers from microcontroller’s pins.


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LCD displays

EASY8051A Development system

enables connecting to eather graphic or

alphanumeric LCD display. Both types are

connected by simple being placed into

appropriate connector and by switching

position of the jumper J8. If displays are

not in use, all pins used for their operation

are available for other purpose. Beside

display connector, there is also

potentiometer for contrast regulation on

the board.


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Serial communication via RS232

In order to enable testing programs which use serial communication, development

system has built in standard 9-pin SUB-D connector. The circuit MAX232 is used

as a driver for voltage adjustment.

As it is case with other embedded systems, electronics which suppports serial

communication can be enabled or disabled by using jumpers J9 and J10.


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DS1820 Digital thermometer

Temperature measurement is one of the most common tasks of devices which

operate in industry. For that reason, there is a circuit DS1820 on the EASY8051A

development system board which measure temperature in the range of -55 to

+125oC with accuracy greater than 0.50. Results of measuring are transferred via

serial communication to the pins P3.3 or P2.7. Jumper J7 is used for selecting pins

for receiving data. In case that no one jumper is installed, port pins are available

for other application.


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12-bit A/D converter MCP3204

Built in 12-bit AD Converter MCP3204 has four input channels connected to

on-board connectors. Data are interchanged with the microcontroller via SPI serial

communication using pins P1.5, P1.6, P1.7 and P3.5. If A/D converter is not in

use, these pins can be used for other purpose (DIP switch SW1). In order to check

operating, there is a potentiometer on the development board used as a variable

voltage source. It can be brought to the converter’s input pins using one of four

jumpers J12. As a particular convenience, a reference voltage source MCP1541

(4,096V) is built in. Jumper J11 is used to select whether converter will use this

voltage or 5V voltage.


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12-bit D/A converter MCP4921

Digital to analog conversion (D/A) is another operation often performed by the

microcontroller in practice. For that purpose, there is a special on-board chip

which interchanges information with the microcontroller via SPI communication.

It can also generate analog voltage in 12-bit resolution on its output pin. In case it

is not in use, all microcontroller’s pins are available for other purpose using DIP

switch SW1. As it is case with A/D converter, jumper J11 is used for selecting

reference voltage.


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Connectors for direct access to I/O ports

In order to enable direct connection between microcontrollers’ports and additional

components, each of them is connected to one on-board connector. Besides, two by

two pins on connectors are connected to power supply voltage while each pin can

be connected to + or - polarity of voltage via resistors (pull up or pull down

resistors). Presence and connecting of these resistors are defined by jumpers.

Jumper J3 which controls port P3 is shown on the figure.


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Architecture and Programming of 8051 Micro Controllers

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