POGRAMMABLE LOGIC CONTROLLERS AND MICROCONTROLLERS

LIST OF CONTENTS

Unit1 Introduction to PLC, Concept of PLC - Building blocks of PLC - Functions of various blocks,

- limitations of relays - Advantages of PLCs over electromagnetic relays - Different

programming languages - PLC manufacturer etc.

Unit 2 Working of PLC, Basic operation and principles of PLC, Architectural details

processor Memory structures - I/O structure Programming terminal - Power supply

Unit3 Instruction Set - Basic instructions bit.-Timer instruction like retentive timers, resetting of timers -

Counter instructions like up counter, down counter, resetting of counters.- Arithmetic Instructions

(ADD,SUB,DIV,MUL etc.)- MOV instruction - Comparison instructions like equal, not equal, greater,

greater than equal, less than, less than equal 4. Ladder Diagram Programming 5 Applications of PLCs 6 Micro Controller Series(MCS)-51Over View 7 Instruction Set Addressing Modes 8 Assembly language programming 9 Design and Interface

UNIT I

1.1 Introduction to PLC

The hydramatic Division of the General Motors Corporation specified the design criteria for the first

programmable controller come in 1968. Programmable logic controllers, also called programmable

controllers or PLCs, are solid-state members of the computer family, using integrated circuits instead of

electromechanical devices to implement control functions. They are capable of storing instructions, such

as sequencing, timing, counting, arithmetic, data manipulation, and communication, to control industrial

machines and processes

Purposes - Initially designed to replace relay logic boards, Sequence device actuation, Coordinate

activities accepts input from a series of switches, Sends output to devices or relays

What is PLC?

Programmable Logic Controller (PLC) is a digital computer used for the automation of various

electro-mechanical processes in industries. These controllers are specially designed to survive in harsh

situations and shielded from heat, cold, dust, and moisture etc. PLC consists of a microprocessor which

is programmed using the computer language. The program is written on a computer and is downloaded to the PLC via cable. These loaded programs

are stored in non – volatile memory of the PLC. During the transition of relay control panels to PLC, the

hard wired relay logic was exchanged for the program fed by the user. A visual programming language

known as the Ladder Logic was created to program the PLC.

1.2 Advantages of PLCs over electromagnetic relays

• Ability to interface / communicate with computers but not with reley • Simple programming is possible using ladder logic but not with relay

• Plc have High reliability • High controllability due to the use of timers and counters. • Easy maintenance

• Rugged construction - can operate in extremely harsh conditions

• Small size • Easy expandability • Economical in long term

1.3 Limitation of Relay-

• It suffer from contact wear and reliability problems • Unsuitable for designing complex control circuit • Relay occupies large space • Relay takes large time to actuate

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1.4 Building blocks of PLC

1.5 Functions of various blocks

PLCs have input/output devices like switches, push buttons, limit switches,sensors,lamp indicators,relays solenoids motors etc. block diagram of a PLC shown in above Fig. It has three

major units/sections.

• I/O (Input/Output) Modules. • CPU (Central Processing Units).

• Programmer/Monitor.

The input section converts the field signals supplied by input devices/sensors to logic-level signals that the PLC's CPU can read. The Processor Section reads these inputs, Processes the signal, and prepares the output signals. The output section converts the logic level output signals coming from processor section to high level signals and used to actuate various output field devices. The programmer/monitor is used to enter the user's program into memory and to monitor the execution of the program.

I/O Section:-

The I/O section establishes the interfacing between physical devices in the real world outside the PLC and the digital arena inside the PLC.

The input module has bank of terminals for physically connecting input devices, like push buttons,

limit switches etc. to a PLC. The role of an input module is to translate signals from input devices into a form that the PLC's CPU can understand.

The Output module also has bank of terminals that physically connect output devices like solenoids,

motor starters, indicating lamps etc. to a PLC. The role of an output module is to translate signals

from the PLC's CPU into a form that the output device can use. The tasks of the I/O section can be

classified as:

• Conditioning

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• Isolation • Termination • Indication

An electronic system for connecting I/O modules to remotely located I/O devices can be added if

needed. The actual operating process under PLC Control can be thousands of feet from the CPU and its I/O modules. CPU Section:-

The Central Processing Unit, the brain of the system is the control portion of the PLC. It has

three Subparts.

• Memory System • Processor • Power Supply

Memory System:- The memory is the area of the CPU in which data and information is stored and retrieved. The total

memory area can be subdivided into the following four Sections.

I/O Image Memory

The input image memory consists of memory locations used to hold the ON or OFF states of each input

field devices, in the input status file. The output status file consists of memory locations that stores the ON or OFF states of hardware output

devices in the field. Data is stored in the output status file as a result of solving user program and is

waiting to be transferred to the output module's switching device.

Data Memory - It is used to store numerical data required in math calculation, bar code data etc.

User Memory - It contains user's application program.

Executive Memory - It is used to store an executive program or system software. An operating system of

the PLC is a special program that controls the action of CPU and consequently the execution of the

user's program. A PLC operating system s designed to scan image memory, interprets the instruction of

user's program stored in main memory, and executes the user's application program the operating

system is supplied by the PLC manufacturer and is permanently held in memory. Processor:-The processor, the heart of CPU is the computerized part of the CPU in the form of

Microprocessor / Micro controller chip. It supervises all operation in the system and performs all

tasks necessary to fulfill the PLC function.

• It reads the information i.e. status of externally connected input devices with input module.

It stores this information in memory for later use.

It carries out mathematical and logic operations as specified in application program.

After solving the user's program, it writes the result values in the memory.

It sends data out to external devices like output module, so as to actuate field hardware.

It performs peripheral and external device communication.

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Power Supply -

The power supply provides power to memory system, processor and I/O Modules.

It converts the higher level AC line Voltage to various operational DC values.

For electronic circuitry. It filters and regulates the DC voltages to ensure proper computer operations.

Programmer/Monitor -

The Programmer/Monitor (PM) is a device used to communicate with the circuits of

the PLC. The programming unit allows the engineer/technicians to enter the edit the program to be

executed. In its simplest form it can be hand-held device with membrane keypad for program entry and a

display device (LED or LCD) for viewing program steps of functions. More advanced systems employ a

separate industrial terminal or personal computers with type-writer type keyboard and CRT monitors.

With the help of proprietary software, it allows programmer to write, view and edit the program and

download it into the PLC. It also allows user to monitor the PLC as it is running the program. With this

monitoring systems, such things as internal coils, registers, timers and other items not visible externally

can be monitored to determine proper operation. Also, internal register data can be altered, if required. to

fine tune program operation while debugging. Communication between PM and PLC is done via a cable

connected to a special programming port on PLC. Connection to the personal computer can be through a

serial port or from a dedicated card installed in the computer.

1.6 PLC Programming language - Using flowcharts, Using ladder logic and Using statement logics or

mnemonics 1.7 PLC manufacturer - Allen Bradley, Rockwell Automation, Mitsubishi, Omron, Siemens etc.

Programmable Logic Controllers Multiple Choice Questions

Q1. The acronym PLC stands for: (A) Pressure Load Control

(B) Programmable Logic Controller (C) Pneumatic Logic Capstan (D) PID Loop Controller (E) Pressure Loss Chamber

Q2. The PLC was invented by

A) Bill Gates B) Dick Morley C) Bill Landis D) Tod Cunningham

Q3. Ladder logic programming consists primarily of: (A) Virtual relay contacts and coils

(B) Logic gate symbols with connecting lines (C) Function blocks with connecting lines

(D) Text-based code (E) Hieroglyphics

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Q4. In a PLC, the scan time refers to the amount of time in which …..

(A) the technician enters the program (B) timers and counters are indexed by (C) one “rung” of ladder logic takes to complete (D) the entire program takes to execute

(E) transmitted data communications must finish

Q5. Which one of the following is not a PLC manufacturer

a. Siemens b. Mitsubishi c. Microsoft d. ABB

Short/long answer types questions

Q1 Define PLC Q2 What are the limitations of relay over PLC

Q3 Explain building blocks of PLC in detail Q4 List the names of PLC manufacturer.

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UNIT 2

2. Working of PLC (Programmable Logic Controller) The Programmable logic controller functions in four steps.

Input scan: The state of the input is scanned which is connected externally. The inputs include

switches, pushbuttons, and proximity sensors, limit switches, pressure switches. Ideally, they are

transformers and not relays. Program scan: The loaded program is executed to carry out the function appropriately.

Output scan: The input sources have a control over the output ports to energize or de-energize

them. The outputs include solenoids, valves, motors, actuator, and pumps. Depending on the

model of PLC, these relays can be transistors.

2.1 PLC ARCHITECTURE • The input sources convert the real time analog electric signals to suitable digital electric

signals and these signals are applied to the PLC through the connector rails. • These input signals are stored in the PLC external image memory in locations known as

bits. This is done by the CPU • The control logic or the program instructions are written onto the programming device

through symbols or through mnemonics and stored in the user memory. • The CPU fetches these instructions from the user memory and executes the input signals by

manipulating, computing, processing them to control the output devices. • The execution results are then stored in the external image memory which controls the

output drives. • The CPU also keeps a check on the output signals and keeps updating the contents of the input

image memory according to the changes in the output memory. • The CPU also performs internal programming functioning like setting and resetting of the

timer, checking the user memory.

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PC

Program

Loader

Switches

Printer

Processor I/O

Modules Machines

Cassette Loader

Power

EPROM Memory

Supply

Loader

Peripherals External Devices

2.2 PLC Hardware

• The hardware components of a PLC system are CPU, Memory, Input/Output, Power supply unit,

and programming device. Below is a diagram of the system overview of PLC.

•

CPU – Keeps checking the PLC controller to avoid errors. They perform functions including

logic operations, arithmetic operations, computer interface and many more.

• Memory – Fixed data is used by the CPU. System (ROM) stores the data permanently for the

operating system. RAM stores the information of the status of input and output devices, and the

values of timers, counters and other internal devices.

• I/O section – Input keeps a track on field devices which includes sensors, switches.

• O/P Section - Output has a control over the other devices which includes motors, pumps, lights

and solenoids. The I/O ports are based on Reduced Instruction Set Computer (RISC).

• Power supply – Certain PLCs have an isolated power supply. But, most of the PLCs work at

220VAC or 24VDC.

• Programming device – This device is used to feed the program into the memory of the

processor. The program is first fed to the programming device and later it is transmitted to the

PLC’s memory.

System Buses – Buses are the paths through which the digital signal flows internally of the PLC. The four system buses are:

• Data bus is used by the CPU to transfer data among different elements.

• Control bus transfers signals related to the action that are controlled internally.

• Address bus sends the location’s addresses to access the data.

• System bus helps the I/O port and I/O unit to communicate with each other

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2.3 The memory space can be divided into two broad categories:

Program and Data Memory:

Advanced ladder logic functions allow controllers to perform calculations, make decisions and do other

complex tasks. Timers and counters are examples of ladder logic functions. They are more complex than

basic inputs contacts and output coils and relay heavily upon data stored in the memory of the PLC.

The user program will account for most of the memory of a PLC system. Program files contain the logic controlling machine operation. This logic consists of instructions that are programmed in a ladder logic format.

The data file portion of memory stores input and output status, processor status, the status of various bits and numerical data.

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Multiple Choice Questions

Q.1 one of the following is an input device

a. Motor b. Light

c. Valve d. Sensor

Q.2 Solenoids, lamps, motors are connected to:

a. Analog output b. Digital output c. Analog input d. Digital input

Q.3 The type of memory which is fast and temporarily stores the data which are

immediately required for use is called as______.

a. HDD b. ROM c. RAM d. SSD

Q.4 how is the speed of operation of conventional relay system as compared to digital controllers?

a. very slow b. very fast c. same d. almost similar

Q.5 The PLC is used in _______.

a. machine tools b. automated assembly equipment c. moulding and extrusion machines

d. all of the above

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UNIT 3 INSTRUCTION SETS

3. PLC Instructions

3.1 Bit instructions

3.2 Timer and Counter instructions

3.3 Program Control Instructions

3.4 Compare Instructions

3.5 Math Instructions

3.1 Bit Instructions

Mnemonic Name Symbol Description

XIC Examine If Examines a bit for an On (set, high)

Closed condition.

XIO Examine If Examines a bit for an Off (cleared, low)

Open condition.

OTE Output When rung conditions are true, the OTE

Energize will either set or clear the data bit.

OTL Output Latch When enabled, the instruction signals to

the controller to turn on the addressed

bit. The bit remains on, regardless of the

rung condition.

OTU Output When enabled, it clears (unlatches) the

Unlatch data bit. The bit remains Off, regardless

of rung condition.

3.2 Timer and Counter Instructions

Mnemonic Name Symbol Description

TON Timer On A non-retentive timer that accumulates

Delay time when the instruction is enabled. The accumulated value is reset when

rung conditions go false.

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TOF Timer Off A non-retentive timer that accumulates

Delay time when the rung makes a true-to-

false transition.

RTO Retentive A retentive timer that accumulates time

Timer On when the instruction is enabled. Retains

its accumulated value when rung

conditions become false.

TON Count Up An instruction that counts false-to-true

rung transitions. It counts upward and

the accumulated value is incremented

by one count on each of these

transitions.

TOF Count Down This instruction counts downward on

each false-to-true rung transition. The

accumulated value is decremented by

one count on each of these transitions.

RTO Retentive A retentive timer that accumulates time

Timer On when the instruction is enabled. Retains

its accumulated value when rung

conditions become false.

CTU Count Up An instruction that counts false-to-true

rung transitions. It counts upward and

the accumulated value is incremented

by one count on each of these

transitions.

CTD Count Down This instruction counts downward on

each false-to-true rung transition. The

accumulated value is decremented by

one count on each of these transitions.

RES Reset This instruction is used to reset a timer,

counter or control structure. The

accumulated value of these instructions

are cleared when the RES instruction is

enabled.

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3.3 Move/Logical Instructions

Mnemonic

Name

Symbol

Description

MOV Move Copies the Source (which remains

unchanged) to the Destination.

MVM Masked Move Copies the Source to a Destination and

allows segments of the data to be masked.

CLR Clear Clears all the bits of the Destination.

DTR Data Passes the Source value through a Mask

Transitional and compares the result with the Reference

value.

SWPB Swap Byte Rearranges the bytes of a tag and stores the

Instruction bytes in the new order.

SWP Swap Byte Swaps the lower and higher bytes of a

Instruction specified number of words.

BTD Bit Field Copies the specified bits from the Source,

Distribute shifts the bits to the appropriate position,

and writes the bits into the Destination.

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AND Bitwise AND Performs a logical AND operation by using

the bits in Source A and Source B and

placing the result in the Destination.

OR Bitwise OR Performs a logical AND operation by using

the bits in Source A and Source B and

placing the result in the Destination.

XOR Bitwise Performs a bitwise XOR operation using

Exclusive OR the bits in Source A and Source B and

stores the result in the Destination.

NOT Bitwise NOT Executes a bitwise NOT operation by using

the bits in the Source and placing the result

in the Destination

3.4 Program Control Instructions

Mnemonic

Name

Symbol

Description

JSR Jump to This instruction jumps execution to a

Subroutine specific routine and initiates the execution

of this routine, called a subroutine.

SBR Subroutine Stores recurring sections of program logic.

RET Return Used to return to the instruction following

the a JSR operation.

JMP Jump to Label Skips sections of ladder logic.

LBL Label Target of the JMP instruction with the same

label name.

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MCR Master Cont. Used in pairs to create a program zone that

Res. can disable all rungs between the MCR

instructions.

AFI Always False Disables all instructions on a rung.

NOP No Operation This instruction functions as a placeholder.

TND Temporary End When enabled, it lets the controller execute

logic only up to this instruction.

IOT Immediate This instruction immediately updates the

Output specified output data.

3.5 Compare Instructions

Mnemonic Name Symbol Description

EQU Equal This instruction is used to test whether two

values are equal. If Source A is equal to

Source B, the instruction is logically true.

GEQ Greater Than Determines whether Source A is greater than

or Equal To or equal to Source B. If the value at Source A

is greater than or equal to the value at Source

B, then the instruction is true.

GRT Greater Than This instruction is used to test whether one

value (Source A) is greater than another value

(Source B).

LEQ Less Than or Determines whether one value (Source A) is

Equal To less than or equal to another (Source B).

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LES Less Than This instruction determines whether Source A

is less than Source B.

LIM Limit This instruction is used to test for values

within the range of the Low Limit to the High

Limit.

MEQ Mask Equal Passes the Source and Compare values

To through a Mask and compares the results.

NEQ Not Equal This instruction tests whether Source A is not

To equal to Source B.

3.6 Math Instructions

Mnemonic

Name

Symbol

Description

ADD Add Adds Source A to Source B and stores the

result in the Destination.

SUB Subtract Subtracts Source B from Source A and

places the result in the Destination.

MUL Multiply Multiplies Source A by Source B and stores

the result in the destination.

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DIV Divide Divides Source A by Source B and places

the result in the Destination.

MOD Modulo Divides Source A by Source B and stores

the remainder in the Destination.

SQR Square Root Calculates the square root of the source and

places the integer result in the Destination.

NEG Negate Changes the sign (+, -) of the Source and

stores the result in the Destination.

ABS Absolute Takes the absolute value of the Source and

places the result in the Destination.

Multiple Choice Questions

Question 1: Examine-on instruction in PLC language is symbolically represented by what shape? A. -] [- B. -( )- C. -]/[-

D. None of these Question 2: The symbol -(L)- represent what instruction in the PLC

language?

A. OUT output unlatch instruction.

B. OTL output latch instruction.

C. Examine off instruction. D. Output energizes instruction. Question 3: The address T4:6.ACC is used to address what?

A. Accumulator for timer 4 in file 6.

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B. Preset value of timer 6 in file 4. C. Accumulator for timer 6 in file 4.

D. Accumulator for counter 6 in file 4.

Q4: To reset the time for a PLC what condition must be true?

A. Reset rung of TON must be true. B. Reset rung of TON must be false.

C. RST instruction with timer address must be true.

D. RST instruction with associated timer address must be false. Question 5: To increase the number of inputs and outputs of the PLC, one can use expansion modules. A. True. B. False. C. None of the above.

Question 6: Solenoids, lamps, motors are connected to: A. Analog output.

B. Digital output.

C. Analog input.

D. Digital input Question 7: In a counter instruction if the accumulator value becomes greater than or equal to the preset value then which of the following is true?

A. CU bit goes on.

B. DN bit goes on.

C. Both CU and DN bits go on.

D. None of the above.

Short / long Answer type Questions Q1 Explain time instructions in detail Q2 Explain Counter instructions in detail

4 Ladder Logic

Let’s take a look at an example of ladder logic programming:

Figure 1. A simple ladder logic program

This ladder logic program is three rungs long. The program is “scanned” or run by the CPU from left

to right and top to bottom. The symbols placed throughout the rungs are actually graphical

instructions. The names for these instructions are:

XIC (Examine If Closed)

XIO (Examine If Open)

OTE (Output Energize).

First Rung

Looking at the first rung, notice the first two inputs I:1/1 and I:1/2. The symbol is an XIC, and the I

denotes that this is an input. This instruction represents a physical input found on one of the discrete

input cards.

Figure 2. The first rung represents a physical input found on one of the discrete input cards.

I:1 means that this input card has been placed in slot 1, directly adjacent to the processor. The /1

indicates the bit of interest. Input cards have more than one channel, and if the instruction specifies

/1, the instruction accesses channel 1.

The second input represents channel 2 on the same card. An XIC instruction really means true if

closed. That is, this instruction will be true if the input device it represents is closed. If an instruction

is true it is highlighted in green. The only way for an output to be energized is if a path of true

instructions can be traced from the left rail to the right rail. Therefore, the output on rung one will be

true because a path of true instructions, I:1/1 and I:1/2, exists. This is effectively an AND operation.

The output in this case, B: 0/1, is actually an internal bit stored in the PLC's memory. That’s why it’s

labeled B instead of O for “output.” These internal bits work great when a certain state or set of

inputs needs to be recorded without actually turning on a physical output.

Unit 5

Automation of product packaging

Product packaging is one of the most frequent cases for automation in industry. It can be encountered

with small machines (ex. packaging grain like food products) and large systems such as machines for

packaging medications. Example we are showing here solves the classic packaging problem with few

elements of automation. Small number of needed inputs and outputs provides for the use of CPM1A

PLC controller which represents simple and economical solution.

By

pushing START key you activate Flag1 which represents an assisting flag (Segment 1) that comes up

as a condition in further program (resetting depends only on a STOP key). When started, motor of an

conveyor for boxes is activated. The conveyor takes a box up to the limit switch, and a motor stops

then (Segment 4). Condition for starting a conveyor with apples is actually a limit switch for a box.

When a box is detected, a conveyor with apples starts moving (Segment 2). Presence of the box

allows counter to count 10 apples through a sensor used for apples and to generate counter CNT010

flag which is a condition for new activation of a conveyor with boxes (Segment 3). When the

conveyor with boxes has been activated, limit switch resets counter which is again ready to count 10

apples. Operations repeat until STOP key is pressed when condition for setting Flag1 is lost. Picture

below gives a time diagram for a packaging line signal.

Ladder diagram:

Traffic light control using PLC

Pump control

6. 8051 Microcontroller

A microcontroller is a small and low-cost microcomputer, which is designed to perform the specific

tasks of embedded systems like displaying microwave’s information, receiving remote signals, etc.

The general microcontroller consists of the processor, the memory (RAM, ROM, EPROM), Serial

ports, peripherals (timers, counters), etc.

Difference between Microprocessor and Microcontroller

The following table highlights the differences between a microprocessor and a microcontroller −

Microcontroller Microprocessor

Microcontrollers are used to execute a single

task within an application.

Microprocessors are used for big

applications.

Its designing and hardware cost is low. Its designing and hardware cost is high.

Easy to replace. Not so easy to replace.

It is built with CMOS technology, which

requires less power to operate.

Its power consumption is high because it

has to control the entire system.

It consists of CPU, RAM, ROM, I/O ports. It doesn’t consist of RAM, ROM, I/O ports.

It uses its pins to interface to peripheral

devices.

Types of Microcontrollers

Microcontrollers are divided into various categories based on memory, architecture, bits and

instruction sets. Following is the list of their types −

Bit

Based on bit configuration, the microcontroller is further divided into three categories.

8-bit microcontroller − This type of microcontroller is used to execute arithmetic and logical

operations like addition, subtraction, multiplication division, etc. For example, Intel 8031 and

8051 are 8 bits microcontroller.

16-bit microcontroller − This type of microcontroller is used to perform arithmetic and

logical operations where higher accuracy and performance is required. For example, Intel

8096 is a 16-bit microcontroller.

32-bit microcontroller − This type of microcontroller is generally used in automatically

controlled appliances like automatic operational machines, medical appliances, etc.

Memory

Based on the memory configuration, the microcontroller is further divided into two categories.

External memory microcontroller − This type of microcontroller is designed in such a way

that they do not have a program memory on the chip. Hence, it is named as external memory

microcontroller. For example: Intel 8031 microcontroller.

Embedded memory microcontroller − This type of microcontroller is designed in such a way

that the microcontroller has all programs and data memory, counters and timers, interrupts, I/O

ports are embedded on the chip. For example: Intel 8051 microcontroller.

Instruction Set

Based on the instruction set configuration, the microcontroller is further divided into two categories.

CISC − CISC stands for complex instruction set computer. It allows the user to insert a single

instruction as an alternative to many simple instructions.

RISC − RISC stands for Reduced Instruction Set Computers. It reduces the operational time

by shortening the clock cycle per instruction.

Applications of Microcontrollers

Microcontrollers are widely used in various different devices such as −

Light sensing and controlling devices like LED.

Temperature sensing and controlling devices like microwave oven, chimneys.

Fire detection and safety devices like Fire alarm.

Measuring devices like Volt Meter.

8051 microcontroller is designed by Intel in 1981. It is an 8-bit microcontroller. It is built with 40 pins

DIP (dual inline package), 4kb of ROM storage and 128 bytes of RAM storage, 2 16-bit timers. It

consists of are four parallel 8-bit ports, which are programmable as well as addressable as per the

requirement. An on-chip crystal oscillator is integrated in the microcontroller having crystal frequency

of 12 MHz.

In the following diagram, the system bus connects all the support devices to the CPU. The system bus

consists of an 8-bit data bus, a 16-bit address bus and bus control signals. All other devices like

program memory, ports, data memory, serial interface, interrupt control, timers, and the CPU are all

interfaced together through the system bus.

The pin diagram of 8051 microcontroller looks as follows −

Pins 1 to 8 − These pins are known as Port 1. This port doesn’t serve any other functions. It is

internally pulled up, bi-directional I/O port.

Pin 9 − It is a RESET pin, which is used to reset the microcontroller to its initial values.

Pins 10 to 17 − These pins are known as Port 3. This port serves some functions like

interrupts, timer input, control signals, serial communication signals RxD and TxD, etc.

Pins 18 & 19 − These pins are used for interfacing an external crystal to get the system clock.

Pin 20 − This pin provides the power supply to the circuit.

Pins 21 to 28 − These pins are known as Port 2. It serves as I/O port. Higher order address bus

signals are also multiplexed using this port.

Pin 29 − This is PSEN pin which stands for Program Store Enable. It is used to read a signal

from the external program memory.

Pin 30 − This is EA pin which stands for External Access input. It is used to enable/disable the

external memory interfacing.

Pin 31 − This is ALE pin which stands for Address Latch Enable. It is used to demultiplex the

address-data signal of port.

Pins 32 to 39 − These pins are known as Port 0. It serves as I/O port. Lower order address and

data bus signals are multiplexed using this port.

Pin 40 − This pin is used to provide power supply to the circuit.

8051 microcontrollers have 4 I/O ports each of 8-bit, which can be configured as input or output.

Hence, total 32 input/output pins allow the microcontroller to be connected with the peripheral

devices.

Pin configuration, i.e. the pin can be configured as 1 for input and 0 for output as per the logic

state.

o Input/output (I/O) pin − All the circuits within the microcontroller must be connected

to one of its pins except P0 port because it does not have pull-up resistors built-in.

o Input pin − Logic 1 is applied to a bit of the P register. The output FE transistor is

turned off and the other pin remains connected to the power supply voltage over a pull-

up resistor of high resistance.

Port 0 − The P0 (zero) port is characterized by two functions −

o When the external memory is used then the lower address byte (addresses A0A7) is

applied on it, else all bits of this port are configured as input/output.

o When P0 port is configured as an output then other ports consisting of pins with built-in

pull-up resistor connected by its end to 5V power supply, the pins of this port have this

resistor left out.

Input Configuration

If any pin of this port is configured as an input, then it acts as if it “floats”, i.e. the input has unlimited

input resistance and in-determined potential.

Output Configuration

When the pin is configured as an output, then it acts as an “open drain”. By applying logic 0 to a port

bit, the appropriate pin will be connected to ground (0V), and applying logic 1, the external output

will keep on “floating”.

In order to apply logic 1 (5V) on this output pin, it is necessary to build an external pullup resistor.

Port 1

P1 is a true I/O port as it doesn’t have any alternative functions as in P0, but this port can be

configured as general I/O only. It has a built-in pull-up resistor and is completely compatible with

TTL circuits.

Port 2

P2 is similar to P0 when the external memory is used. Pins of this port occupy addresses intended for

the external memory chip. This port can be used for higher address byte with addresses A8-A15.

When no memory is added then this port can be used as a general input/output port similar to Port 1.

Port 3

In this port, functions are similar to other ports except that the logic 1 must be applied to appropriate

bit of the P3 register.

Pins Current Limitations

When pins are configured as an output (i.e. logic 0), then the single port pins can receive a

current of 10mA.

When these pins are configured as inputs (i.e. logic 1), then built-in pull-up resistors provide

very weak current, but can activate up to 4 TTL inputs of LS series.

If all 8 bits of a port are active, then the total current must be limited to 15mA (port P0:

26mA).

If all ports (32 bits) are active

7 Timers / Counters

8051 has two 16-bit programmable UP timers/counters. They can be configured to operate either as timers or

as event counters. The names of the two counters are T0 and T1 respectively. The timer content is available

in four 8-bit special function registers, viz, TL0,TH0,TL1 and TH1 respectively.

In the "timer" function mode, the counter is incremented in every machine cycle. Thus, one can think of it as

counting machine cycles. Hence the clock rate is 1/12 th

of the oscillator frequency.

In the "counter" function mode, the register is incremented in response to a 1 to 0 transition at its

corresponding external input pin (T0 or T1). It requires 2 machine cycles to detect a high to low transition.

Hence maximum count rate is 1/24 th

of oscillator frequency.

The operation of the timers/counters is controlled by two special function registers, TMOD and TCON

respectively.

Timer Mode control (TMOD) Special Function Register:

Various bits of TMOD are described as follows -

Gate: This is an OR Gate enabled bit which controls the effect of on START/STOP of Timer. It is set

to one ('1') by the program to enable the interrupt to start/stop the timer. If TR1/0 in TCON is set and signal

on pin is high then the timer starts counting using either internal clock (timer mode) or external pulses

(counter mode).

It is used for the selection of Counter/Timer mode.

Mode Select Bits:

M1 and M0 are mode select bits.

Timer/ Counter control logic:

Fig .Timer/Counter Control Logic

Timer control (TCON) Special function register:

TCON is bit addressable. The address of TCON is 88H. It is partly related to Timer and partly to interrupt.

Fig TCON Register

Timer Mode-0:

In this mode, the timer is used as a 13-bit UP counter as follows.

Fig. Operation of Timer on Mode-0

The lower 5 bits of TLX and 8 bits of THX are used for the 13 bit count.Upper 3 bits of TLX are ignored.

When the counter rolls over from all 0's to all 1's, TFX flag is set and an interrupt is generated.

The input pulse is obtained from the previous stage. If TR1/0 bit is 1 and Gate bit is 0, the counter continues

counting up. If TR1/0 bit is 1 and Gate bit is 1, then the operation of the counter is controlled by input.

This mode is useful to measure the width of a given pulse fed to input.

Timer Mode-1:

This mode is similar to mode-0 except for the fact that the Timer operates in 16-bit mode.

Fig. Operation of Timer in Mode 1

Timer Mode-2: (Auto-Reload Mode)

This is a 8 bit counter/timer operation. Counting is performed in TLX while THX stores a constant value. In

this mode when the timer overflows i.e. TLX becomes FFH, it is fed with the value stored in THX. For

example if we load THX with 50H then the timer in mode 2 will count from 50H to FFH. After that 50H is

again reloaded. This mode is useful in applications like fixed time sampling.

Fig. Operation of Timer in Mode 2

Timer Mode-3:

Timer 1 in mode-3 simply holds its count. The effect is same as setting TR1=0. Timer0 in mode-3 establishes

TL0 and TH0 as two separate counters.

Fig. Operation of Timer in Mode 3

Control bits TR1 and TF1 are used by Timer-0 (higher 8 bits) (TH0) in Mode-3 while TR0 and TF0 are

available to Timer-0 lower 8 bits(TL0).

Interrupts are the events that temporarily suspend the main program, pass the control to the external

sources and execute their task. It then passes the control to the main program where it had left off.

8051 has 5 interrupt signals, i.e. INT0, TFO, INT1, TF1, RI/TI. Each interrupt can be enabled or

disabled by setting bits of the IE register and the whole interrupt system can be disabled by clearing

the EA bit of the same register.

IE (Interrupt Enable) Register

This register is responsible for enabling and disabling the interrupt. EA register is set to one for

enabling interrupts and set to 0 for disabling the interrupts. Its bit sequence and their meanings are

shown in the following figure.

EA IE.7 It disables all interrupts. When EA = 0 no interrupt will be

acknowledged and EA = 1 enables the interrupt individually.

- IE.6 Reserved for future use.

- IE.5 Reserved for future use.

ES IE.4 Enables/disables serial port interrupt.

ET1 IE.3 Enables/disables timer1 overflow interrupt.

EX1 IE.2 Enables/disables external interrupt1.

ET0 IE.1 Enables/disables timer0 overflow interrupt.

EX0 IE.0 Enables/disables external interrupt0.

IP (Interrupt Priority) Register

We can change the priority levels of the interrupts by changing the corresponding bit in the Interrupt

Priority (IP) register as shown in the following figure.

A low priority interrupt can only be interrupted by the high priority interrupt, but not

interrupted by another low priority interrupt.

If two interrupts of different priority levels are received simultaneously, the request of higher

priority level is served.

If the requests of the same priority levels are received simultaneously, then the internal polling

sequence determines which request is to be serviced.

- IP.6 Reserved for future use.

- IP.5 Reserved for future use.

PS IP.4 It defines the serial port interrupt priority level.

PT1 IP.3 It defines the timer interrupt of 1 priority.

PX1 IP.2 It defines the external interrupt priority level.

PT0 IP.1 It defines the timer0 interrupt priority level.

PX0 IP.0 It defines the external interrupt of 0 priority level.

SERIAL DATA COMMUNICATION IN 8051 MICROCONTROLLER

The fastest way of transmitting data, within a microcomputer is parallel data transfer.

• For transferring data over long distances, however, parallel data transmission requires too many

wires.

• For long distance transmission, data is usually converted from parallel form to serial form so that it

can be sent on a single wire or pair of wires.

• Serial data received from a distant source is converted to parallel form and it can be easily

transferred on the microcomputer buses.

• The types of communication systems are, 1. Simplex 2. Half-duplex 3. Full-duplex In simplex

communication, data can be transmitted only in one direction, ie. data from sensors to processor.

Eg : commercial radio stations. In half-duplex transmission, data can be transmitted in either direction

between two systems, but can occur only in one direction at a time.

Eg : two-way radio system, where one user always listens while the other talks because the receiver

circuitry is turned off during transmit. In full duplex, the data can be send and received at the same

time

. Eg : A normal phone conversation. Serial data can be sent by two ways.

They are, 1. Synchronous communication. 2. Asynchronous communication In synchronous

transmission, data are transmitted in block at a constant rate

SCON (serial control) register The SCON register is an 8-bit register used to program the start bit, stop bit, and data bits of data

framing, among other things.

The following describes various bits of the SCON register.

8 Unit

Assembler –

The Assembler is used to translate the program written in Assembly language into machine code. The

source program is a input of assembler that contains assembly language instructions. The output

generated by assembler is the object code or machine code understandable by the computer.

Compiler –

The language processor that reads the complete source program written in high level language as a whole in one

go and translates it into an equivalent program in machine language is called as a Compiler.

Example: C, C++, C#, Java

ASSEMBLER DIRECTIVES To assemble a program automatically the assembler needs information

in the form of assembler directives that controls the assembly. For example, the assembler must be told

at what address to start assembling the program. These assembler directives are command placed in the

program by the designer that provides information to the assembler. They do not become part of the

final program as they are not the part of the instruction set of the microprocessor nor did they translate

into executable code. Therefore, they are also known as pseudo-instruction on false instructions.

ORG: The origin (ORG) instruction tells the assembler the address of the memory location for the next

instruction or data byte should be assembled. ORG is entered at the beginning of a program. When

different parts of a programme (e.g. subroutines) are to be placed in different areas of memory, an

ORG pseudo instruction is used before each part of the program to specify the starting location for

assembly of that part of the program. The origin instruction has the following form

END: When an assembler scans the program to be assembled it must know, where the program ends. It

cannot depend on a HLT instruction for this because some programmes don’t contain a halt instruction

as the last instruction and other don’t contain a halt at all. An application program used, e.g., in process

monitoring on control might run continuously and, therefore, not contain a halt instruction. Thus, an

end assembly, END directive must be the last instruction. The directive has a form. END

The END statement explicitly indicates the end of the program to the assembler. If no END statement

is given, then the assembler just keeps on running through all the memory.

The ORG and END assembler directives, in effect, frame the program to be assembled. ORG 0000H

[Assembly language instructions] END

EQU: Symbolic names, which appear in assembly language programs as labels, instructions

mnemonics and operands are translated to binary values by the assembler. As discussed in

handassembly the labels are assigned the current value of the assembler’s location counter when

encountered in the first pass of the assembly. Instruction mnemonics have predefined values that the

assembler obtains from a table that is part of the assembler.

DB: When a table of fixed data values is required, memory must also be allocated. However, unlike

the DS, each memory locations must have a defined value that is assembled into it.

DW: Define word DW instruction is similar to define byte pseudo instruction. opt. name: DW list The

only difference between the DB & DW is that expression in this define word list is evaluated to 16-bit

quantity and stored as 2-bytes. It is stored with the lower order byte in the lower of the two memory

locations and the higher order byte in the next higher location

Light Emitting Diodes or LEDs are the mostly commonly used components in many applications.

They are made of semiconducting material. In this project, I will describe about basics of Interfacing

LED with 8051 Microcontroller.

Principle behind Interfacing LED with 8051

The main principle of this circuit is to interface LEDs to the 8051 family micro controller. Commonly,

used LEDs will have voltage drop of 1.7v and current of 10mA to glow at full intensity. This is applied

through the output pin of the micro controller.

Circuit Diagram

NOTE: I suggest you to connect 1KΩ Pull-up resistors to all the pins of PORT0 of 8051. I haven’t

shown that connection in this circuit diagram

Components Required

AT89C51 (8051 Microcontroller)

8 LEDs

8 Resistors – 1KΩ

Crystal oscillator – 11.0592MHz

2 Capacitors – 33pF

2 Resistors – 10KΩ

1 Capacitor – 10μF

1 Push Button

8051 Programmer

5V Power Supply

Circuit Design

The circuit mainly consists of AT89C51 microcontroller. AT89C51 belongs to the family of 8051

microcontroller. It is an 8-bit microcontroller. This microcontroller has 4KB of Flash Programmable

and Erasable Read Only Memory and 128 bytes of RAM. This can be programmed and erased a

maximum of 1000 times.

It has two 16 bit timers/counters. It supports USART communication protocol. It has 40 pins. There are

four ports are designated as P0, P1, P2, and P3. Port P0 will not have internal pull- ups, while the other

ports have internal pull-ups.

Microcontroller Questions

Question1. Introduction of 8051 Microcontroller Architecture? Answer : In 1981, Intel Corporation introduced an 8-bit microcontroller called the 8051. The 8051 became

widely popular after Intel allowed other manufacturers to make and market any flavors of the 8051.

They please with the condition that they remain code-compatible with the 8051. This has led to many,

versions of the 8051 with different speeds and amounts of on-chip ROM marketed by more than half a

dozen manufacturers. It is important to note that although there are different flavors of the 8051 in

terms of speed and amount of on-chip ROM, they are all compatible with the original 8051 as far as

the instructions are concerned. This means that if you write your program for one, it will run on any of

them regardless of the manufacturer.

Question 2. Intel 8051 Follows Which Architecture?

Answer :

Intel 8051 is Harvard Architecture.

Question 3. What Is The Difference Between Harvard Architecture And Von Neumann

Architecture?

Answer : The name Harvard Architecture comes from the Harvard Mark. The most obvious characteristic of

the Harvard Architecture is that it has physically separate signals and storage for code and data

memory. It is possible to access program memory and data memory simultaneously. Typically, code

(or program) memory is read-only and data memory is read-write. Therefore, it is impossible for

program contents to be modified by the program itself.

The von Neumann Architecture is named after the mathematician and early computer scientist John

von Neumann. Von Neumann machines have shared signals and memory for code and data. Thus,

the program can be easily modified by itself since it is stored in read-write memory.

Question 4. 8051 Was Developed Using Which Technology?

Answer : Intel’s original MCS-51 family was developed using NMOS technology, but later versions,

identified by a letter C in their name (e.g., 80C51) used CMOS technology and consume less power

than their NMOS predecessors. This made them more suitable for battery-powered devices.

Question 5. Why 8051 Is Called 8 Bit Microcontroller?

Answer : The Intel 8051 is an 8-bit microcontroller which means that most available operations are limited to

8 bits.

Question 6. What Is The Width Of Data Bus?

Answer :

8-bit data bus

Question 7. What Is The Width Of Address Bus?

Answer :

16-bit address bus

Question 8. List Out The Features Of 8051 Microcontroller?

Answer : o 40 Pin IC.

o 128 bytes of RAM.

o 4K ROM.

o 2 Timers (Timer 0 and Timer 1).

o 32 Input/ Output pins.

o 1 serial port.

o 6 Interrupts (Including Reset).

Question 9. What Location Code Memory Space And Data Memory Space Begins?

Answer :

At location 0x00 for internal or external memory

Question 10. How Much On Chip Ram Is Available?

Answer :

128 bytes of RAM (from 0x00 to 0x7F) and can be used to store data.

Question 11. List out Addressing Modes In Mcs-51?

Answer : o Direct Addressing

o Register Addressing

o Register Indirect Addressing

o Implicit Addressing

o Immediate Addressing

o Index Addressing