Serial Communication Supported by 8085

Serial Communication Supported by 8085

The 8085 microprocessor communicates serially using two lines : SOD and SID. The Serial Output Data (SOD) line is used to send 1-bit data on the SOD Pin of 8085 and RIM instruction is used to receive 1-bit data on the SID pin of 8085.

Lab Experiment 11.1.1 : Output byte from SOD pin.

Statement : Write a program to output contents of B register LSB to MSB on the SOD pin.

Source program

MVI C, 08H ; Initialize count with 8

BACK : MOV A,B ;

RRC ; Rotate B register contents right

MOV B,A ; Save contents of register B

JNC SKIP  ; If no carry skip

MVI A, C0H

SIM ; If carry, send high on SOD

JMP NEXT

SKIP : MVI A, 40H

SIM ; If no carry, send high on SOD

NEXT : CALL DELAY ; Wait for specific time

DCR C ; Decrement count by 1

JNZ BACK ; if count = 0, if not repeat

HLT ;  Stop program execution

 

Lab Experiment 11.1.2 : Output square wave from SOD pin.

Statement : Write a program to output square wave of 1 kHz frequency on the SOD pin of 8085 for 5 seconds. Operating frequency of 8085 is 2 MHz.

LXI SP, 27FFH ; Initialize stack pointer

LXI H, 2000H ; Initialize counter with count

; 5000.

BACK : MVI A, C0H

SIM ; Send high on SOD pin

CALL DELAY ; Wait for 0.5 msec (Refer delay

; program of program 46)

MVI A, 40 H ; Send low on SOD pin

CALL DELAY ; Wait for 0.5 msec

DCX B ; Decrement count by 1

MOV A, C

ORA B ; Check if count = 0

JNZ BACK ; If not, repeat

HLT ; Stop program execution

 

Lab Experiment 11.1.3 : Receive ASCII character through SID pin.

Statement : An ASCII character is being received on SID pin of 8085. Write a program in assembly language of 8085 to assemble this character and store it in memory. Write comment for each instruction.

Source program

LXI SP, 27FFH   

LXI H, 2000H ; Memory pointer

RIM  ; Read SID

ANI 80H ; Check D₇ bit of Accumulator

CALL Delay ; 1/2 bit time delay for stop bit

MVI B, 08H ; Initialize bit counter

MVI D,00H          ; Clear data register

UP1 ALL Dealy ; 1bit time

RIM ; Read SID line

ANI 80H ; Mask bits B₆-B₀

ORA D ; OR data bit with previous

; bits

RRC

MOV D, A ; Store data bit at

; appropriate position

DCR B

JNZ UP1

RLC ; Shift left to correct result

MOV M, A ; Store result

RIM ; Read stop bit

ANI 80H

CZ error ; If not stop bit call error

HLT ; Terminate program.

From above examples it is clear that 8085 has two pins for serial communication. However, it does not support built-in serial to parallel converter and parallel-to-serial converter. To transmit byte data it is necessary to convert byte into eight serial bits. This can be done by using the parallel to serial converter. Similarly at the reception these serial bits must be converted into parallel 8 bit data. The serial to parallel converter is used to convert serial data bits into the parallel data.

In 8085, we have to achieve this with the help of programming as done in previous examples.

The Intel has designed special devices for this purpose are called Universal Asynchronous Receiver-Transmitter (UART). The devices which provide synchronous as well as asynchronous transmission and reception are called Universal Synchronous Asynchronous Receiver-Transmitter. A good example of UART is 8250 and USART is 8251. These devices are software programmable for number of data bits, parity and number of stop bits. In the next section we discuss IC 8251 (USART).

Review Questions

1. Compare parallel and serial type of data transfer.

2. Write a short note on serial communication supported by 8085.

3. Write a program to output square wave of 1 kHz frequency on the SOD pin of 8085 for 5 seconds.