EC7451: MICROPROCESSOR AND MICROCONTROLLER IV- Microcontroller_Final_4March2020.pdf8051...

EC7451: MICROPROCESSOR AND MICROCONTROLLER

Dr. V. SATHIESH KUMAR Department of Electronics Engg., MIT1

Presentation Slides:

www.sathieshkumar.com/tutorials

Unit IV : MICROCONTROLLER1. 8051 – Architecture and pindetails2. Special Function Registers (SFRs)3. I/O Ports, Timers / counters4. Interrupts and serial communication5. Instruction set 6. Addressing modes7. Assembly language programming TEXT BOOKMuhammad Ali Mazidi, Janice Gillispie Mazidi and Rolin D. McKinley, "The 8051Microcontroller and Embedded Systems", Second Edition, Pearson Education 2008. Fifthimpression 2011

Presented By,

Dr. V. Sathiesh Kumar Department of Electronics Engg.,

MIT-Anna University

EC7451: MICROPROCESSOR AND MICROCONTROLLER UNIT IV – Microcontroller


8051 Microcontroller:

•In 1981, Intel introduced an 8-bit microcontroller called the 8051.

•It was referred as system on a chip because it had 128 bytes of RAM, 4K byte of on-

chip ROM, two timers, one serial port, and 4 ports (8-bit wide), all on a single chip.

Feature 8051 8052 8031

ROM(bytes) 4K 8K 0K

RAM(bytes) 128 256 128

Timers 2 3 2

I/O pins 32 32 32

Serial port 1 1 1

Interrupt sources 6 8 6



8051 Microcontroller: Features

•An 8051 microcontroller comes with the following features −

1. 4KB bytes on-chip program memory (ROM)

2. 128 bytes on-chip data memory (RAM)

3. Four register banks

4. 8-bit bidirectional data bus

5. 16-bit unidirectional address bus

6. 32 general purpose registers, each of 8-bit

7. 16 bit Timers (usually 2, but may have more or less)

8. Three internal and two external Interrupts

9. Four 8-bit ports

10.16-bit program counter and data pointer



8051 Microcontroller: Block Diagram



8051 Microcontroller: Architecture



8051 Microcontroller: Pin Details

• In 8051, I/O operations are done using four ports and 40 pins.

• I/O operation port reserves 32 pins where each port has 8 pins.

•The other 8 pins are designated as Vcc, GND, XTAL1, XTAL2, RST, EA*, ALE/PROG*,

and PSEN*.

8051



8051 Microcontroller: Pin Details - I/O Port and their functions

• The four ports P0, P1, P2, and P3, each use 8 pins, making them 8-bit ports.

•Upon RESET, all the ports are configured as inputs, ready to be used as input ports.

• When the first 0 is written to a port, it becomes an output.

•To reconfigure it as an input, a 1 must be sent to a port.

Port 0 (Pin No 32 – Pin No 39):

•It has 8 pins (32 to 39).

•It can be used for input or output.

•Unlike P1, P2, and P3 ports, we normally connect P0 to 10K-ohm pull-up resistors

to use it as an input or output port being an open drain.

•It is also designated as AD0-AD7, allowing it to be used as both address and data.

•ALE (Pin no 31) indicates if P0 has address or data.

•When ALE = 0, it provides data D0-D7, but when ALE = 1, it has address A0-A7.




Port 1 (Pin No 1 – Pin No 8):

•It is an 8-bit port (pin 1 through 8) and can be used either as input or output.

•It doesn't require pull-up resistors because they are already connected internally.

•Upon RESET, Port 1 is configured as an input port.

•If Port 1 is configured to be used as an output port, then to use it as an input port again,

program it by writing 1 to all of its bits.

Port 2 (Pins 21 through 28):

•Port 2 occupies a total of 8 pins (pins 21 through 28) and can be used for both input

and output operations.

• P2 also doesn't require external Pull-up resistors because they are already

connected internally.

•It must be used along with P0 to provide the 16-bit address for the external

memory.




•When the 8051 is connected to an external memory, it provides path for upper 8-

bits of 16-bits address, and it cannot be used as I/O.

•Upon RESET, Port 2 is configured as an input port.

•If Port 2 is configured to be used as an output port, then to use it as an input port again,

program it by writing 1 to all of its bits.

Port 3 (Pins 10 through 17):

• It is also of 8 bits and can be used as Input/Output.

•P3.0 and P3.1 are RxD (Receiver) and TxD (Transmitter), respectively and are

collectively used for Serial Communication.

•P3.2 and P3.3 pins are used for external interrupts.

•P3.4 and P3.5 are used for timers T0 and T1, respectively.

•P3.6 and P3.7 are Write (WR*) and Read (RD*) pins.




•Vcc − Pin 40 provides supply to the Chip and it is +5 V.

•Gnd − Pin 20 provides ground for the Reference.

XTAL1, XTAL2 (Pin no 18 & Pin no 19): 8051 has on-chip oscillator but requires

external clock to run it.

• A quartz crystal is connected between the XTAL1 and XTAL2 pin of the chip.

•This crystal also needs two capacitors of 30pF for generating a signal of desired

frequency.

•One side of each capacitor is connected to ground.

•8051 IC is available in various speeds and it all depends on this Quartz crystal.

•For example, a 20 MHz microcontroller requires a crystal with a frequency no more

than 20 MHz.




RST (Pin No. 9) : It is an Input pin and active High pin.

•Upon applying a high pulse on this pin, that is 1, the microcontroller will reset and

terminate all activities. This process is known as Power-On Reset.

•Activating a power-on reset will cause all values in the register to be lost. It will set a

program counter to all 0's.

•To ensure a valid input of Reset, the high pulse must be high for a minimum of two

machine cycles before it is allowed to go low.




EA or External Access (Pin No. 31) : It is an input pin.

•This pin is an active low pin; upon applying a low pulse, it gets activated.

•In case of microcontroller (8051/52) having on-chip ROM, the EA* pin is connected

to Vcc.

•But in an 8031 microcontroller which does not have an on-chip ROM, the code is

stored in an external ROM and then fetched by the microcontroller. In this case, we must

connect the (pin no 31) EA* to GND to indicate that the program code is stored

externally.




PSEN* or Program store Enable (Pin No 29) : This is also an active low pin, i.e., it

gets activated after applying a low pulse.

•It is an output pin and used along with the EA* pin in 8031 based (i.e. ROMLESS)

Systems to allow storage of program code in external ROM.

ALE or (Address Latch Enable) : This is an Output Pin and is active high.

•It is especially used for 8031 IC to connect it to the external memory.

•It can be used while deciding whether P0 pins will be used as Address bus or Data

bus.

•When ALE = 1, then the P0 pins work as Address bus and when ALE = 0, then the

P0 pins act as Data bus.



8051 Microcontroller: Little Endian vs Big Endian

•Although numbers are always displayed in the same way, they are not stored in

the same way in memory.

•Big-Endian machines store the most significant byte of data in the lowest memory

address.

•A Big-Endian machine stores 0x12345678 as −

ADD+0: 0x12

ADD+1: 0x34

ADD+2: 0x56

ADD+3: 0x78



8051 Microcontroller: Little Endian vs Big Endian

•Little-Endian machines, on the other hand, store the least significant byte of data in

the lowest memory address.

•A Little-Endian machine stores 0x12345678 as,

ADD+0: 0x78

ADD+1: 0x56

ADD+2: 0x34

ADD+3: 0x12



8051 Microcontroller: Registers

•Registers are used in the CPU to store information on temporarily basis which

could be a data to be processed, or an address pointing to the data.

•In 8051, there is one data type is of 8-bits, from the MSB (most significant bit) D7 to

the LSB (least significant bit) D0.

•With 8-bit data type, any data type larger than 8-bits must be broken into 8-bit

chunks before it is processed.

•The most widely used registers of the 8051 are,

1. Accumulator (A) – 8 bits

2. R register (R0 to R7) - 8 bits

3. B register - 8 bits

4. Data Pointer (DPTR) - 16 bits

5. Program Counter (PC) - 16 bits

6. Stack Pointer (SP) - 8 bits

•All these registers are of 8-bits, except DPTR and PC are 16-bit registers.




Accumulator (A):

•The accumulator, register A, is used for all arithmetic and logic operations.

•If the accumulator is not present, then every result of each calculation (addition,

multiplication, shift, etc.) is to be stored into the main memory.

•Access to main memory is slower than access to a register like the accumulator

because the technology used for the large main memory is slower (but cheaper)

than that used for a register.

The ‘R’ Registers:

•The „R‟ registers are a set of eight registers, namely, R0, R1 to R7.

•These registers function as auxiliary or temporary storage registers in many

operations.




The ‘B’ Register:

•The „B‟ register is very similar to the Accumulator in the sense that it may hold an 8-

bit (1-byte) value.

•The "B" register is used only by two 8051 instructions: MUL AB and DIV AB.

•To quickly and easily multiply or divide A by another number, you may store the other

number in „B‟ and make use of these two instructions.

•Apart from using MUL and DIV instructions, the "B" register is often used as yet

another temporary storage register, much like a ninth R register.

The Data Pointer:

•The Data Pointer (DPTR) is the 8051‟s only user-accessible 16-bit (2-byte) register.

•DPTR is meant for pointing to data in external memory.




The Program Counter:

•The Program Counter (PC) is a 2-byte address (16-bit register) which tells the 8051

where the next instruction to execute can be found in the memory.

•PC starts at 0000H when the 8051 initializes and is incremented every time after an

instruction is executed.

•PC is not always incremented by 1.

•Some instructions may require 2 or 3 bytes; in such cases, the PC will be

incremented by 2 or 3.

•Branch, jump, and interrupt operations load the Program Counter with an

address other than the next sequential location.

•Activating a power-on reset will cause all values in the register to be lost. It means

the value of the PC is 0 upon reset, forcing the CPU to fetch the first opcode from the

ROM location 0000H.




The Stack Pointer (SP):

•Stack is implemented in RAM and a CPU register is used to access it called SP

(Stack Pointer) register.

•Stack is a upward growing stack (Memory address increase with every insertion

of data into stack).

•SP register is an 8-bit register and can address memory addresses of range 00H to

FFH.

•The Stack Pointer tells the location from where the next value is to be removed

from the stack.

•When the content of a CPU register is stored in a stack, it is called a PUSH

operation.

•When a value is pushed onto the stack, the value of SP is incremented and then

the value is stored at the resulting memory location.




•When the content of a stack is stored in a CPU register, it is called a POP operation.

•When a value is popped off the stack, the value is returned from the memory

location indicated by SP, and then the value of SP is decremented.

•SP will be initialized to 07H when the 8051 is initialized.

•If a value is pushed onto the stack at the same time, the value will be stored in the

internal RAM address 08H.

•SP is modified directly by the 8051 by six instructions: PUSH, POP, ACALL, LCALL,

RET, and RETI.



8051 Microcontroller: ROM Space

•Some family members of 8051 have only 4K bytes of on-chip ROM (e.g. 8751,

AT89C51); some have 8K ROM like AT89C52, and there are some family members with

32K bytes and 64K bytes of on-chip ROM such as Dallas Semiconductor.

•The point to remember is that no member of the 8051 family can access more than

64K bytes of opcode since the program counter in 8051 is a 16-bit register (0000

to FFFF address).



8051 Microcontroller: Reset Vector

•The significance of the reset vector (0000H in ROM) is that it points the processor to

the memory address which contains the firmware's first instruction.

•Without the Reset Vector, the processor would not know where to begin execution.

•Upon RESET, the processor loads the Program Counter (PC) with the reset vector

value from a predefined memory location.



8051 Microcontroller: Flag Bits and PSW

•The program status word (PSW) register is an 8-bit register, also known as flag

register.

•It is of 8-bit wide but only 6-bit of it is used.

•The two unused bits are user-defined flags.

•Four of the flags are called conditional flags, which means that they indicate a

condition which results after an instruction is executed.

•These four are CY (Carry), AC (auxiliary carry), P (parity), and OV (overflow).

•The bits RS0 and RS1 are used to change the bank registers.

•The PSW Register contains that status bits that reflect the current status of the

CPU.

CY AC F0 RS1 RS0 OV - P




CY PSW.7 Carry Flag

AC PSW.6 Auxiliary Carry Flag

F0 PSW.5 Flag 0 available to user for general purpose.

RS1 PSW.4 Register Bank selector bit 1

RS0 PSW.3 Register Bank selector bit 0

OV PSW.2 Overflow Flag

- PSW.1 User definable FLAG

P PSW.0 Parity FLAG. Set/ cleared by hardware during instruction cycle to indicate even/odd number of 1 bit in accumulator.




•We can select the corresponding Register Bank select bit using RS0 and RS1 bits.

CY, the carry flag : This carry flag is set (1) whenever there is a carry out from the

D7 bit.

•It is affected after an 8-bit addition or subtraction operation.

•It can also be reset to 1 or 0 directly by an instruction such as "SETB C" and "CLR C"

where "SETB" stands for set bit carry and "CLR" stands for clear carry.

RS1 RS0 Register Bank Address

0 0 0 00H-07H

0 1 1 08H-0FH

1 0 2 10H-17H

1 1 3 18H-1FH




AC, auxiliary carry flag : If there is a carry from D3 and D4 during an ADD or SUB

operation, the AC bit is set; otherwise, it is cleared.

•It is used for the instruction to perform binary coded decimal arithmetic.

P, the parity flag : The parity flag represents the number of 1's in the accumulator

register only.

•If the A register contains odd number of 1's, then P = 1; and for even number of 1's,

P = 0.

OV, the overflow flag : This flag is set whenever the result of a signed number

operation is too large causing the high-order bit to overflow into the sign bit.

•It is used only to detect errors in signed arithmetic operations.



8051 Microcontroller: RAM memory allocation

•The 8051 microcontroller has a total of 128 bytes of RAM.

•The 128 bytes of RAM inside the 8051 are assigned the address 00 to 7FH.

•They can be accessed directly as memory locations and are divided into three

different groups as follows,

1. 32 bytes from 00H to 1FH locations are set aside for register banks and the

stack.

2. 16 bytes from 20H to 2FH locations are set aside for bit-addressable read/write

memory.

3. 80 bytes from 30H to 7FH locations are used for read and write storage; it is

called as scratch pad. These 80 locations RAM are widely used for the purpose

of storing data and parameters by 8051 programmers.




7FH




Register Banks in 8051:

•A total of 32 bytes of RAM are set aside for the register banks and the stack.

•These 32 bytes are divided into four register banks in which each bank has 8

registers, R0–R7.

•RAM locations from 00H to 07H are set aside for bank 0 of R0–R7 where R0 is RAM

location 00, R1 is RAM location 01, R2 is location 02, and so on, until the memory

location 07, which belongs to R7 of bank 0.

•The second bank of registers R0–R7 starts at RAM location 08H and goes to

locations 0FH.

•The third bank of R0–R7 starts at memory location 10H and goes to location to

17H.

• Finally, RAM locations 18H to 1FH are set aside for the fourth bank of R0–R7.




Default Register Bank:

•If RAM locations 00–1F are set aside for the four registers banks, which register bank

of R0–R7 do we have access to when the 8051 is powered up?

•The answer is register bank 0; that is, RAM locations from 0 to 7 are accessed with

the names R0 to R7 when programming the 8051.

•Because it is much easier to refer these RAM locations by names such as R0 to

R7, rather than by their memory locations.




How to Switch Register Banks?

•Register bank 0 is the default when the 8051 is powered up.

•We can switch to the other banks using PSW register.

•D4 and D3 bits of the PSW are used to select the desired register bank, since they

can be accessed by the bit addressable instructions SETB and CLR.

•For example, "SETB PSW.3" will set PSW.3 = 1 and select the bank register 1.

RS1 RS0 Bank Selected

0 0 Bank0

0 1 Bank1

1 0 Bank2

1 1 Bank3




Stack and its Operations:

•The stack is a section of a RAM used by the CPU to store information such as data

or memory address on temporary basis.

•The CPU needs this storage area considering limited number of registers.

How Stacks are Accessed?

•As the stack is a section of a RAM, there are registers inside the CPU to point to it.

•The register used to access the stack is known as the stack pointer register.

•The stack pointer in the 8051 is 8-bits wide, and it can take a value of 00H to FFH.

•When the 8051 is initialized, the SP register contains the value 07H. This means

that the RAM location 08 is the first location used for the stack.

•The storing operation of a CPU register in the stack is known as a PUSH, and

getting the contents from the stack back into a CPU register is called a POP.




Pushing into the Stack:

•In the 8051, the stack pointer (SP) points to the last used location of the stack.

•When data is pushed onto the stack, the stack pointer (SP) is incremented by 1.

•When PUSH is executed, the contents of the register are saved on the stack and

SP is incremented by 1.

•To push the registers onto the stack, we must use their RAM addresses.

•For example, the instruction "PUSH 1" pushes register R1 onto the stack.

Popping from the Stack:

•Popping the contents of the stack back into a given register is the opposite to the

process of pushing.

•With every pop operation, the top byte of the stack is copied to the register

specified by the instruction and the stack pointer is decremented once.


External Memory Interface with 8051:

The 8051 microcontroller has the internal program memory.

In this chip, there is EA pin. It indicates the External Access.

So by using this pin, we can check whether the internal program memory is used

or not.

So when there is a low signal in this pin, the internal 4K bytes of program memory

is not used, but in this situation, it can access only the ROM.

When we are trying to access the external data memory, then the read RD or

write WR will be the output from 8051.

So for reading the data from external sources, there PSEN (Program Strobe

Enable) pin. This is an active low output pin.

When this pin is active low, it indicates that the microcontroller reads from the

external ROM.

The 16-bit ROM address will be floated on Port 2 (MS Byte) and Port 0 (LS Byte).

The program code is received on Port 0 pins.




So there is two case of accessing the program memory.

The first case is when EA is low, and the second case is when EA is high.

For the low state of EA, it can access the entire external memory from location

0000H to FFFFH.

And in the second case (EA = 1). In this case, the internal memory is accessed by

the address 0000H to 0FFFH. And also the external memory is accessed from

location 1000H to FFFFH.




The external program memory address is always 16-bit address.

If the memory is very small compared to 64 KB memory, then also the address

will be 16-bit.

So when we use the external memory, we cannot use the Port0 and Port 2 as I/O

ports.

The instruction execution times are not depending on the on-chip ROM or

external EPROMs.




8051 Microcontroller: Timers/Counters

•In Intel 8051, there are two 16-bit timer registers. These registers are known as

Timer0 and Timer1.

•The timer registers can be used in two modes,

1. Timer mode

2. Counter mode

•The only difference between these two modes is the source for incrementing the

timer registers.

Timer Mode:

•In the timer mode, the internal machine cycles are counted. So this register is

incremented in each machine cycle.

•So when the clock frequency is 12 MHz, then the timer register is incremented in

each millisecond.

•In this mode it ignores the external timer input pin.




Counter Mode:

•In the counter mode, the external events are counted.

•In this mode, the timer register is incremented for each 1 to 0 transition of the

external input pin. This type of transitions is treated as events.

•The external input pins are sampled once in each machine cycle, and to

determine the 1 to 0 transitions, another machine cycle will be needed to increment

the register.

•When the frequency is 12 MHz, then the maximum count frequency will be 12

MHz/12= 1 MHz. So for event counting the time duration is 1 µs..




•There are four different modes of the Timer or Counter.

•The Mode 0 to Mode 2 are for both of the Timer/Counter.

•Mode 3 has a different meaning for each timer register.

•There is a register called TMOD. This register can be programmed to configure these

timers or counters.

•The Serial port is used for serial communication in mode 1 and 3.

•Timer1 is used for generating the baud rate.

•So only Timer0 is available for timer or counter operations.




TMOD Register:

•TMOD(Timer Mode) is an Special Function Register (SFR).

•The address of this register is 89H.

•This is not bit-addressable.

Timer Timer1 Mode Timer0 Mode

Bit Details Gate (G) C/T* M1 M0 Gate (G) C/T* M1 M0

Bit Details High Value(1) Low Value(0)

C/T* Configure for the Counter operations Configure for the Timer operations

Gate (G) Timer0 or Timer1 will be in Run Mode when TRX bit of TCON register is high and INT0 or INT1 is high.

Timer0 or Timer1 will be in Run Mode when TRX bit of TCON register is high.




M1 M0 Mode Operation

0 0 0 (13-bit timer mode) 13-bit timer/counter, 8-bit of THx & 5-bit of TLx

0 1 1 (16-bit timer mode) 16-bit timer/counter, THx cascaded with TLx

1 0 2 (8-bit auto reload mode)

8-bit timer/counter (auto reload mode), TLx

reload with value held by THx each time TLx

overflow

1 1 3 (split timer mode)

Timer 0: Split 16-bit timer X two 8-bit timer i.e.

THx and TLx like two 8-bit timer

Timer 1: used for Baud Rate generation in Serial

Communication




TCON Register: It is a 8-bit control register and contains timer and interrupt flags.

Bit 7, 5 - TF1/TF0: Timer1/Timer0 Overflow Flag – It is cleared through software.

1 = Timer1/Timer0 overflow (i.e. Register goes to its max and rolls back to zero).

0 = Timer1 overflow not occurred.

Bit 6, 4 - TR1/TR0: Timer1/Timer0 Run Control Bit – Set and Cleared by Software.

1 = Timer1/Timer0 start.

0 = Timer1/Timer0 stop.

Bit 3, 1 - IE1/IE0: External Interrupt1/External Interrupt0 Edge Flag

1 = External interrupt1/External interrupt0 occurred.

0 = External interrupt1/External interrupt0 Processed.

•It is set and cleared by hardware.




TCON Register: It is a 8-bit control register and contains timer and interrupt flags.

Bit 2, 0 - IT1/IT0: External Interrupt1/External Interrupt0 Trigger Type Select Bit

1 = Interrupt occur on falling edge at INT1/INT0 pin.

0 = Interrupt occur on low level at INT1/INT0 pin.



8051 Microcontroller: Timers/Counter Modes

Mode 0 (13-bit timer mode):

•Mode 0 is 13-bit timer mode for which 8-bit of THx and 5-bit of TLx (as Prescaler)

are used.

•As shown in above figure, 8-bit of THx and lower 5-bit of TLx used to form a total 13-bit

timer.

•Higher 3-bits of TLx should be written as zero while using timer mode0, or it will

affect the result.




Mode 1 (16-bit timer mode):

•Mode 1 is 16-bit timer mode used to generate a delay, it uses 8-bit of THx and 8-bit

of TLx to form a total 16-bit register.




Mode 2 (8-bit auto-reload timer mode):

•Mode 2 is 8-bit auto-reload timer mode.

•In this mode we have to load the THx-8 bit value only.

•When the Timer gets started, THx value gets automatically loaded into the TLx

and TLx starts counting from that value.

•After the value of TLx overflows from the 0xFF to 0x00, the TFx flag gets set and

again value from the THx gets automatically loaded into the TLx register.




Mode 3 (Split timer mode):

•Mode 3 is different for Timer0 and Timer1.

•When the Timer0 is working in mode 3, the TL0 will be used as an 8-bit

timer/counter.

•It will be controlled by the standard Timer0 control bits.

•The TH0 is used as an 8-bit timer but not the counter. This is controlled by Timer1

Control bit TR1.

•When the TH0 overflows from FFH to 00H, then TF1 is set to 1.

•When the Timer1 is working in Mode 3, it simply holds the count but does not run.

•When Timer0 is in mode 3, the Timer1 is configured in one of the mode 0, 1 and 2.

•In this case, the Timer1 cannot interrupt the microcontroller.

•When the TF1 is used by TH0 timer, the Timer1 is used as Baud Rate Generator.



8051 Microcontroller: Interrupts

• Interrupts are basically 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 five interrupts. These interrupts are INT0*, INT1*,T0 ,T1 and TI/RI.

•All of the interrupts can be enabled or disabled by using the IE (interrupt enable)

register.

•The interrupt addresses of these interrupts are like below,

Interrupt Address

INT0* 0003H

INT1* 000BH

T0 0013H

T1 001BH

TI/RI 0023H




Interrupt Enable (IE) Register:

•This register can be used to enable or disable interrupts programmatically.

•This register is an SFR.

•The address is A8H.

•This byte is bit addressable.

•So it can be programmed by the user.

Bit Address AF AE AD AC AB AA A9 A8

Bit Details EA X X ES ET1 EX1 ET0 EX0


EA Least significant 5 bits can decide enable or disable of these five interrupts.

Disable all five interrupts. It just ignores the rest five bits.

ES Enable Serial Port Interrupt Disable Serial Port Interrupt

ET1 Enable Timer1 interrupt Disable Timer1 interrupt

EX1 Enable external interrupt 1 (INT1*) Disable external interrupt 1 (INT1*)

ET0 Enable Timer0 interrupt Disable Timer0 interrupt

EX0 Enable external interrupt 0 (INT0*) Disable external interrupt 0 (INT0*)




Interrupt Priority (IP) Register:

•The five interrupts can be in one or two interrupt level.

•The priority levels are level 1 and level 0.

•Priority level 1 indicates the higher priority, and level 0 indicates lower priority.

•This IP register can be used to store the priority levels for each interrupt.

•This is also a bit addressable SFR. Its address is B8H.

Bit Address BF BE BD BC BB BA B9 B8

Bit Details X X X PS PT1 PX1 PT0 PX0


PS Set level 1 priority of Serial port interrupt

Set level 0 priority of Serial port interrupt

PT1 Set level 1 priority of Timer1 interrupt Set level 0 priority of Timer1 interrupt

PX1 Set level 1 priority of external interrupt 1 (INT1*)

Set level 0 priority of external interrupt 1 (INT1*)

PT0 Set level 1 priority of Timer0 interrupt Set level 0 priority of Timer0 interrupt

PX0 Set level 1priority of external interrupt 0 (INT0*)

Set level 0 priority of external interrupt 0 (INT0*)




•When all of the five interrupts are in same priority level, and if all of the interrupts

are enabled, then the sequence of interrupts will be INT0*, T0, INT1*, T1, TI/R I.

•Some specific priority register value can be used to maintain the priorities of the

interrupts.

•Let the value of Priority register is xxx00101 indicates the sequence INT0*, INT1*,

TI/RI, T1, T0.

External Interrupts:

•The external interrupts of 8051 are INT0* and INT1*.

•These interrupts can be programmed to either edge-triggered or level triggered.

•The TCON register can be used to program external interrupts to edge or level

triggered.

•The TCON is Timer Control.

•TCON is another bit addressable SFR. Here the address is 88H.




Bit Address 8F 8E 8D 8C 8B 8A 89 88

Bit Details TF1 TR1 TF0 TR0 IE1 IT1 IE0 IT0


IT0 Set ( INT0*) as negative edge triggered input.

Set ( INT0*) as active low level triggered input.

IT1 Set ( INT1*) as negative edge triggered input.

Set ( INT1*) as active low level triggered input.

IE0 This will be 1, when INT0* is activated as level triggered.

This will be 0, when INT0* is activated as edge triggered.

IE1 This will be 1, when INT1* is activated as level triggered.

This will be 0, when INT1* is activated as edge triggered.

TR0 Set Timer0 as run mode Set Timer0 as stop mode.

TR1 Set Timer1 as run mode Set Timer1 as stop mode.

TF0 High when Timer T0 overflow occurs. After resetting the timer T0 this will also be changed to 0 state

TF1 High when Timer T1 overflow occurs. After resetting the timer T1 this will also be changed to 0 state.

TCON




•The IT0 and IT1 are stands for Interrupt Type. These bits are used to decide whether

the INT0* and INT1* will be level trigged or edge triggered.

•IE0 and IE1 bits are used to indicate the status of external interrupts. These bit can

be set or reset by the microcontroller itself.

•The first four bits are the status information about timers.

•When TR0 and TR1 are 1, it indicates the running mode of the timers. These bits

provide software control over the running of timers. Timers can also be controlled by the

hardware. The priority of hardware mode is higher than the software mode.

•The TF0 and TF1 are used to indicate the overflow of timer T0 and T1, respectively.

•When overflow occurs these flags are set to 1.

•When the interrupt is handled by some interrupt service subroutine (ISS), these will be

0.




Serial Port Interrupt:

•The serial ports can be used either Transmitting mode or reception mode.

•The interrupt status for the Transmission is provided by TI, and status for

Reception is provided by RI.

•These are two bits of SCON(Serial Control).

•This is also a bit addressable SFR. The address is 98H.


Bit Details SM0 SM1 SM2 REN TB8 RB8 TI RI

Bit Details Description

SM0 This is Serial Port Mode 0 shift register

SM1 This is Serial Port Mode 1 (8-bit UAR + variable)

SM2 Enable multiprocessor communication in the mode 2 or 3

REN Set or reset by the software to enable or disable the Reception

TB8 Indicates the 9th

bit (transmitted in mode 2 or 3). Set or reset by the software

RB8 In mode 2 or 3, the 9th

bit was received in mode 1.

TI The transmission interrupt flag. It can be set by hardware.

RI The receiver interrupt flag. Set by hardware but must be reset by software.

SCON


8051 Microcontroller: Serial Communication

•One of the 8051s many powerful features is its integrated UART, otherwise known as

a serial port.

•We need to configure the serial ports operation mode and baud rate.

•Once configured, all we have to do is write to an SFR to write a value to the serial

port or read the same SFR to read a value from the serial port.

•The 8051 will automatically let us know when it has finished sending the character

we wrote and will also let us know whenever it has received a byte so that we can

process it.



8051 Microcontroller: Serial Communication – Configuration

•This lets us tell the 8051 how many data bits we want, the baud rate we will be using,

and how the baud rate will be determined.

•"Serial Control" (SCON) SFR represents,

MICROPROCESSOR AND MICROCONTROLLER UNIT IV & V – Microcontroller & it Applications

Bit Name Bit Address Explanation of Function

7 SM0 9FH Serial port mode bit 0

6 SM1 9EH Serial port mode bit 1.

5 SM2 9DH Multiprocessor Communications Enable

4 REN 9CHReceiver Enable. This bit must be set in order to receive

characters.

3 TB8 9BH Transmit bit 8. The 9th bit to transmit in mode 2 and 3.

2 RB8 9AH Receive bit 8. The 9th bit received in mode 2 and 3.

1 TI 99HTransmit Flag. Set when a byte has been completely

transmitted.

0 RI 98H Receive Flag. Set when a byte has been completely received.


Bit Details SM0 SM1 SM2 REN TB8 RB8 TI RI



•Additionally, it is necessary to define the function of SM0 and SM1 by,

(*) Note: The baud rate indicated in this table is doubled if PCON.7 (SMOD) is set.

SM0 SM1 Serial Mode Explanation Baud Rate

0 0 0 8-bit Shift Register Oscillator / 12

0 1 1 8-bit UART Set by Timer 1 (*)

1 0 2 9-bit UART Oscillator / 64 (*)

1 1 3 9-bit UART Set by Timer 1 (*)




•The first four bits (bits 4 through 7) are configuration bits.

•Bits SM0 and SM1 let us set the serial mode to a value between 0 and 3, inclusive.

Selecting the Serial Mode selects the mode of operation (8-bit/9-bit, UART or Shift

Register) and also determines how the baud rate will be calculated.

In modes 0 and 2 the baud rate is fixed based on the oscillators frequency.

In modes 1 and 3 the baud rate is variable based on how often Timer 1 overflows.

• The next bit, SM2, is a flag for "Multiprocessor communication."

Generally, whenever a byte has been received the 8051 will set the "RI" (Receive

Interrupt) flag. This lets the program know that a byte has been received.

However, when SM2 is set, the "RI" flag will only be triggered if the 9th bit

received was a "1". That is to say, if SM2 is set and a byte is received whose 9th bit is

clear, the RI flag will never be set. This can be useful in certain advanced serial

applications.




•The next bit, REN, is "Receiver Enable.“

If you want to receive data via the serial port, set this bit.

You will almost always want to set this bit.

•The last four bits (bits 0 through 3) are operational bits. They are used when actually

sending and receiving data--they are not used to configure the serial port.

•The TB8 bit is used in modes 2 and 3.

In modes 2 and 3, a total of nine data bits are transmitted.

The first 8 data bits are the 8 bits of the main value, and the ninth bit is taken from

TB8.

If TB8 is set and a value is written to the serial port, the data bits will be written to

the serial line followed by a "set" ninth bit. If TB8 is clear the ninth bit will be "clear.”




•The RB8 also operates in modes 2 and 3 and functions essentially the same way as

TB8, but on the reception side.

When a byte is received in modes 2 or 3, a total of nine bits are received.

In this case, the first eight bits received are the data of the serial byte received

and the value of the ninth bit received will be placed in RB8.

•TI means "Transmit Interrupt."

When a program writes a value to the serial port, a certain amount of time will

pass before the individual bits of the byte are "clocked out" the serial port.

The 8051 lets the program know that it has "clocked out" the last byte by setting

the TI bit.

When the TI bit is set, the program may assume that the serial port is "free" and ready

to send the next byte.




•The RI bit means "Receive Interrupt."

It functions similarly to the "TI" bit, but it indicates that a byte has been received.

That is to say, whenever the 8051 has received a complete byte it will trigger the RI

bit to let the program know that it needs to read the value quickly, before another

byte is read.



8051 Microcontroller: Serial Communication – Baud Rate Setting

•In mode 0, the baud rate is always the oscillator frequency divided by 12. This

means if the crystal frequency is 11.059 MHz, mode 0 baud rate will always be

921,583 baud.

•In mode 2, the baud rate is always the oscillator frequency divided by 64, so a

11.059 MHz crystal speed will yield a baud rate of 172,797.

•In modes 1 and 3, the baud rate is determined by how frequently timer 1

overflows.

•The more frequently timer 1 overflows, the higher the baud rate.

•There are many ways one can cause timer 1 to overflow at a rate that determines a

baud rate, but the most common method is to put timer 1 in 8-bit auto-reload mode

(timer mode 2) and set a reload value (TH1) that causes Timer 1 to overflow at a

frequency appropriate to generate a baud rate.




•To determine the value that must be placed in TH1 to generate a given baud rate,

we may use the following equation (assuming PCON.7 is clear).

TH1 = 256 - ((Crystal / 384) / Baud)

•If PCON.7 is set then the baud rate is effectively doubled, thus the equation

becomes,

TH1 = 256 - ((Crystal / 192) / Baud)

•Example:Assume11.059 MHz crystal and to configure the serial port to 19,200 baud,

TH1 = 256 - ((Crystal / 384) / Baud)

TH1 = 256 - ((11059000 / 384) / 19200 )

TH1 = 256 - ((28,799) / 19200)

TH1 = 256 - 1.5 = 254.5

•To obtain 19,200 baud on a 11.059 MHz crystal we have to set TH1 to 254.5. If we set

it to 254 we will have achieved 14,400 baud and if we set it to 255 we will have

achieved 28,800 baud.




•To achieve 19,200 baud we simply need to set PCON.7 (SMOD).

TH1 = 256 - ((Crystal / 192) / Baud)

TH1 = 256 - ((11059000 / 192) / 19200)

TH1 = 256 - ((57699) / 19200)

TH1 = 256 - 3 = 253

•Therefore, to obtain 19,200 baud with an 11.059 MHz crystal we must,

1. Configure Serial Port mode 1 or 3.

2. Configure Timer 1 to timer mode 2 (8-bit auto-reload).

3. Set TH1 to 253 to reflect the correct frequency for 19,200 baud.

4. Set PCON.7 (SMOD) to double the baud rate.



8051 Microcontroller: Serial Communication – Writing to Serial Port

•To write a byte to the serial port one must simply write the value to the SBUF (99H)

SFR.

•For example, if you wanted to send the letter "A" to the serial port, it could be

accomplished as easily as MOV SBUF, #A

•Obviously transmission is not instantaneous, it takes a measureable amount of time

to transmit. And since the 8051 does not have a serial output buffer we need to be

sure that a character is completely transmitted before we try to transmit the next

character.

•The 8051 lets us know when it is done transmitting a character by setting the TI bit

in SCON.

•Consider the following code segment,

CLR TI ; Be sure the bit is initially clear

MOV SBUF, #A ; Send the letter A to the serial port

JNB TI,$ ; Pause until the TI bit is set.



8051 Microcontroller: Serial Communication – Reading from Serial Port

•To read a byte from the serial port one just needs to read the value stored in

the SBUF (99H) SFR after the 8051 has automatically set the RI flag in SCON.

•For example, if your program wants to wait for a character to be received and

subsequently read it into the Accumulator, the following code segment may be used,

JNB RI,$ ; Wait for the 8051 to set the RI flag

MOV A,SBUF ; Read the character from the serial port

•So as long as the bit is not set the program repeats the "JNB" instruction continuously.

•Once the RI bit is set upon character reception the above condition automatically fails

and program flow falls through to the "MOV" instruction which reads the value.



8051 Microcontroller: Addressing Modes

•In 8051 there are 1-byte, 2-byte instructions and very few 3-byte instructions.

•The opcodes are 8-bit long.

•As the opcodes are 8-bit data, there are 256 possibilities.

•Among 256, 255 opcodes are implemented.

•If the clock frequency is 12 MHz, so 64 instruction types are executed in just 1 µs,

and rest are in 2 µs.

•The Multiplication and Division operations take 4 µs to execute.

•There are six types of addressing modes:

1. Immediate Addressing Mode

2. Register Addressing Mode

3. Direct Addressing Mode

4. Register Indirect Addressing Mode

5. Indexed Addressing Mode

6. Implied Addressing Mode



8051 Microcontroller: Addressing Modes - Immediate

•In Immediate Addressing Mode, the data is provided in the instruction itself.

•The data is provided immediately after the opcode.

•These are some examples of Immediate Addressing Mode:

MOV A, #0AFH;

MOV R3, #45H;

MOV DPTR, #FE00H;

•In these instructions, the # symbol is used for immediate data.

•In the first instruction, the immediate data is AFH, but one 0 is added at the beginning.

•So when the data is starting with A to F, the data should be preceded by 0.



8051 Microcontroller: Addressing Modes - Register

•In register addressing mode, the source or destination data should be present in a

register (R0 to R7).

•These are some examples of Register Addressing Mode:

MOV A, R5;

MOV R0, A;

•n 8051, there is no instruction like MOVR5, R7.

•But we can get the same result by using this instruction MOV R5, 07H, or by

using MOV 05H, R7.

•But this two instruction will work when the selected register bank is RB0.



8051 Microcontroller: Addressing Modes - Direct

•In Direct Addressing Mode, the source or destination address is specified by using

8-bit address in the instruction.

•Only the internal data memory can be used in this mode.

•Here some of the examples of direct Addressing Mode:

MOV 80H, R6;

MOV R2, 45H;

MOV R0, 05H;

• The first instruction will send the content of register R6 to port P0 (Address of Port 0

is 80H).

•The second one is for getting content from 45H to R2.

•The third one is used to get data from Register R5 (When register bank RB0 is

selected) to register R0.



8051 Microcontroller: Addressing Modes – Register Indirect

•In this mode, the source or destination address is given in the register.

•By using register indirect addressing mode, the internal or external addresses can be

accessed.

•The R0 and R1 are used for 8-bit addresses, and DPTR is used for 16-bit

addresses, no other registers can be used for addressing purposes.

•Let us see some examples of this mode:

MOV 0E5H, @R0;

MOV @R1, 80H

• In the instructions, the @ symbol is used for register indirect addressing.

•In the first instruction, if the content of R0 is 40H, then the instruction will take the

data from location 40H of the internal RAM.

•In the second one, if the content of R1 is 30H, then it indicates that the content of

port P0 will be stored at location 30H in the internal RAM.



8051 Microcontroller: Addressing Modes – Register Indirect

MOVX A, @R1;

MOVX @DPTR, A;

•In these two instructions, the X in MOVX indicates the external data memory.

•The external data memory can only be accessed in register indirect mode.

•In the first instruction, if the R1 is holding 40H, then A will get the content of external

RAM location 40H.

•And in the second one, the content of A is overwritten in the location pointed by

DPTR.



8051 Microcontroller: Addressing Modes – Indexed

•In the indexed addressing mode, the source memory can only be accessed from

program memory.

•The destination operand is always the register A.

•These are some examples of Indexed addressing mode:

MOVC A, @A+PC;

MOVC A, @A+DPTR;

•The C in MOVC instruction refers to code byte.

•For the first instruction, let us consider A holds 30H. The PC value is 1125H.

•The contents of program memory location 1155H (30H + 1125H) are moved to

register A.



8051 Microcontroller: Addressing Modes – Implied

•In the implied addressing mode, there will be a single operand.

•These types of instruction can work on specific registers only.

•These types of instructions are also known as register specific instruction.

•Here are some examples of Implied Addressing Mode:

RLA;

SWAP A;

•These are 1- byte instruction.

•The first one is used to rotate the A register content to the Left.

•The second one is used to swap the nibbles in A.



8051 Microcontroller: Instruction Set

•Based on the operation they perform, all the instructions in the 8051 Microcontroller

Instruction Set are divided into five groups. They are:

1. Data Transfer Instructions

2. Arithmetic Instructions

3. Logical Instructions

4. Boolean or Bit Manipulation Instructions

5. Program Branching Instructions



8051 Microcontroller: Instruction Set


DATA

TRANSFERARITHMETIC LOGICAL BOOLEAN

PROGRAM

BRANCHING

MOV ADD ANL CLR LJMP

MOVC ADDC ORL SETB AJMP

MOVX SUBB XRL MOV SJMP

PUSH INC CLR JC JZ

POP DEC CPL JNC JNZ

XCH MUL RL JB CJNE

XCHD DIV RLC JNB DJNZ

DA A RR JBC NOP

RRC ANL LCALL

SWAP ORL ACALL

CPL RET

RETI

JMP


8051 Microcontroller: Instruction Set – Data Transfer Instructions

•The Data Transfer Instructions are associated with transfer of data between registers

or external program memory or external data memory.

•The mnemonics associated with Data Transfer are given below,

MOV

MOVC

MOVX

PUSH

POP

XCH

XCHD





Ri – R0 or R1

Rn – R0 to R7


8051 Microcontroller: Instruction Set – Arithmetic Instructions

•Using Arithmetic Instructions, perform addition, subtraction, multiplication and

division. Also include increment by one, decrement by one and a special instruction

called Decimal Adjust Accumulator.

•The mnemonics associated with the Arithmetic Instructions of the 8051 are,

ADD

ADDC

SUBB

INC

DEC

MUL

DIV

DA A

•The operations performed by the arithmetic instructions affect flags like carry,

overflow, zero, etc. in the PSW Register.



8051 Microcontroller: Instruction Set – Logical Instructions

•The Logical Instructions, perform logical operations like AND, OR, XOR, NOT, Rotate,

Clear and Swap.

•Logical operations are performed on Bytes of data on a bit-by-bit basis.

•Mnemonics associated with Logical Instructions are as follows,

ANL

ORL

XRL

CLR

CPL

RL

RLC

RR

RRC

SWAP



8051 Microcontroller: Instruction Set – Boolean or Bit manipulation Instructions

•Boolean or Bit Manipulation Instructions deal with bit variables.

•We know that, there is a special bit-addressable area in the RAM and some of the

Special Function Registers (SFRs) are also bit addressable.

•The Mnemonics corresponding to the Boolean or Bit Manipulation instructions are,

CLR SETB

MOV JC

JNC JB

JNB JBC

ANL ORL

CPL



8051 Microcontroller: Instruction Set – Program Branching Instructions

•These instructions control the flow of program logic.

•The mnemonics of the Program Branching Instructions are as follows,

LJMP AJMP

SJMP JZ

JNZ CJNE

DJNZ NOP

LCALL ACALL

RET RETI

JMP

•All these instructions, except the NOP (No Operation) affect the Program Counter

(PC) in one way or other.

•Some of these instructions has decision making capability before transferring

control to other part of the program.



Summary

1. 8051 – Architecture

2. Pin details

3. Special Function Registers (SFRs)

4. I/O Ports,

5. Timers / counters

6. Interrupts and serial communication

7. Instruction set

8. Addressing modes


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