Programming 8251A

Programming 8251A

To implement serial communication the CPU must inform the 8251A all details such as mode, baud rate (in case of asynchronous mode), stop bits, parity etc. Therefore, prior to data transfer, a set of control words must be loaded into the mode instruction and control instruction registers of 8251A.

Example Write the sequence of instructions required to initialize 8251A at address 80H and 81H for the configuration given below

i) Character length - 6 bits

ii) Parity even

iii) Stop bit 1

iv) Baud rate 64 X

v) DTR and RTS asserted

vi) Error flag reset

vii) Transmitter enable

Solution : In the example, number of stop bits and baud rate is specified, therefore, it is necessary to initialize 8251A in the asynchronous mode.

Mode word for given specification is as follows :

Command word for given specification is as follows :

Program :

MVI A, 00H

OUT 81H ; Dummy mode word

OUT 81H

OUT 81H

MVI A, 40 H ; Reset command word

OUT 81H ; Reset 8251A

MVI A, 77H ; Mode Word initialization

OUT 81H

MVI A, 33 H ; Command word initialization

OUT 81H

Note Before initialization of the 8251A, the dummy mode word and the reset command are sent to the control register. Initially control register may have any random word; therefore, it is a good practice to reset the 8251A. However, it expects the instruction as a mode word followed by the command word. Therefore, the reset command is sent after sending three dummy mode words, which are recommended to avoid problems when it is turned on.

Lab Experiment 11.9.1 ; Transmit message using 8251.

Statement : Draw interfacing diagram and write a assembly program to transmit a message from an 8085 to a CRT terminal for the following requirements.

i) A message of 50 characters is stored as ASCII characters (without parity) in memory locations starting at 2200H.

ii) Baud rate × 16

iii) Stop bits 2

Solution : CRT terminal uses normal RS 232C standard serial communication interface. Therefore to transmit data to CRT it is necessary to have RS 232C interface at the sending end. Fig. 11.9.3 shows the interfacing of 8251 with RS 232C to 8085.

As shown in the Fig. 11.9.3 three RS-232C signals (TxD, RXD are Ground) are used for serial communication between the CRT terminal and the 8085 system. Line drivers and receivers are used to transfer logic levels from TTL logic to RS-232C logic. For RS-232C the voltage level +3 V to +15 V is defined as logic 0 and voltage level from -3 V to -15 V is defined as logic 1. The line driver, MC 1488, converts logic 1 of TTL to approximately -9 V and logic 0 of TTL to approximately +9 V. These levels at the receiving end are again converted by the line receiver, MC1489, into TTL compatible logic.

I/O Map: 

Mode word necessary for the given specification is as follows :

Command word necessary for the given specification is as follows :

To transmit characters on the TxD line it is necessary for 8085 check whether transmitter is ready or not. This can be checked by reading status word as shown in the Fig. 11.9.6.

If bit 0 of the status word is logic '1' then transmitter is ready to accept the character.

Program :

LXI H, 2200H ; Initialize memory pointer to

; point the message

MVI C, 32H ; Initialize counter to send

; 50 characters

MVI A, 00H

OUT FFH

OUT FFH ; Dummy mode word

OUT FFH

MVI A, 40H ; Reset command word

OUT FFH ; Reset 8251A

MVI A, CAH ; Mode word initialization

OUT FFH

MVI A, 11H ; Command word initialization

OUT FFH

CHECK : IN FFH

ANI 01H ; Check TxRDY

JZ CHECK ; Is TxRDY 1 ? if not, check

; again

MOV A, M ; Get the character in

; accumulator

OUT FEH ; Send character to the transmitter

INX H ; Increment memory pointer

DCR C ; Decrement counter

JNZ CHECK ; if not zero, send next character

HLT ; Stop program execution

 

Lab Experiment 11.9.2 : Receive message using 8251.

Statement : For the interfacing in lab experiment 66, write an assembly language program to receiver 25 bytes from the CRT terminal.

Solution : Mode word necessary for the given specification is as follows :

 Command word necessary for the given specification is as follows :

To receive character from RxD line it is necessary for 8085 to check whether receiver is ready to give data or not. This can be checked by reading status word as shown in the Fig. 11.9.9.

If bit 1 of the status word is logic '1' then receiver is ready to give the character.

Program :

LXI H, 2300 H ; Initialize memory pointer

MVI C, FFH ; Initialize counter to accept 25

; characters

MVI A, 00H

OUT FFH ; Dummy mode word

OUT FFH

MVI A, 40 H ; Reset command word

OUT FFH ; Reset 8251A

MVI A, CAH ; Mode word initialization

OUT FFH

MVI A, 14H ; Command word initialization

OUT FFH

CHECK : IN FFH

ANI 02H ; Check RxRDY

JZ CHECK ; Is RxRDY 1 ? If not, check

; again

IN FEH ; Get the character

MOV M, A ; save the character

INX H ; Increment memory pointer

DCR C ; Increment memory pointer

OUT FEH ; Send character to the

; transmitter

JNZ CHECK ; If not zero, accept next

; character

HLT ; Stop program execution

Review Question

1. Explain the operation and programming of 8251 USART in detail.