Seven Segment Display Interface (Eight Digits

Statement: Interface an 8-digit 7 segment LED display using 8255 to the 8085 microprocessor system and write an 8085 assembly language routine to display message on the display.

HARDWARE FOR EIGHT DIGIT SEVEN SEGMENT DISPLAY INTERFACE

Fig. shows the multiplexed eight 7-segment display connected in the 8085 system using 8255. In this circuit port A and port B are used as simple latched output ports. Port A provides the segment data inputs to the display and port B provides a means of selecting a display position at a time for multiplexing the displays. A0-A7 lines are used to decode the addresses for 8255. For this circuit different addresses are:
PA = 00H PB = 01H
PC = 02H CR = 03H.
The register values are chosen in Fig. such that the segment current is 80 mA. This current is required to produce an average of 10 mA per segment as the displays are multiplexed. In this type of display system, only one of the eight display position is 'ON' at any given instant. Only one digit is selected at a time by giving low signal on the corresponding control line. Maximum anode current is 560 mA (7-segments x 80 mA = 560 mA), but the average anode current is 70 mA.

Source program:
SOFTWARE TO INITIALIZE 8255:
MVI A, 80H : Load control word in AL
OUT CR : Load control word in CR
SUBROUTINE TO DISPLAY MESSAGE ON MULTIPLEXED LED DISPLAY:
SET UP REGISTERS FOR DISPLAY:
MVI B, 08H : load count
MVI C, 7FH : load select pattern
LXI H, 6000B : starting address of message
DISPLAY MESSAGE:
DISP 1: MOV A, C : select digit
OUT PB
MOV A, M : get data
OUT PA : display data
CALL DELAY : wait for some time
DISP 1: MOV A, C
RRC
MOV C, A : adjust selection pattern
INX H
DCR B : Decrement count
JNZ DISP 1 : repeat 8 times
RET
Note: This "display message subroutine" must be called continuously to display the 7-segment coded message stored in the memory from address 6000H.
INTERFACING SCHEME


Delay subroutine:
Delay: LXI D, Count
Back: DCX D
MOV A, D
ORA E
JNZ Back
RET