8051: Example 10

Writing two-digit number on LED display

It is time for time multiplexing! This is the simplest example which displays the number 23 on two displays in such a way that one of them displays units, while the other displays tens. The most important thing in the program is time synchronization. Otherwise, everything is very simple. Transistor T4 enables display D4 and at the same time a bit combination corresponding to the digit 3 is set on the port. After that, transistor T4 is disabled and the whole process is repeated using transistor T3 and display D3 in order to display digit 2. This procedure must be continuosly repeated in order to make impression that both displays are active at the same time.

;************************************************************************
;* PROGRAM NAME: 7Seg3.ASM
;* DESCRIPTION: Program displays number "23" on 7-segment LED display
;************************************************************************

;BASIC DIRECTIVES

$MOD53
$TITLE(7SEG3.ASM)
$PAGEWIDTH(132)
$DEBUG
$OBJECT
$NOPAGING

;STACK
         DSEG      AT      03FH
STACK_START:       DS      040H

;RESET VECTORS
         CSEG      AT      0
         JMP       XRESET                    ; Reset vector

         ORG       100H
XRESET:  MOV       SP,#STACK_START           ; Define Stack pointer

LOOP:    MOV       P1,#0                     ; Turn off all display segments
         MOV       P3,#20h                   ; Activate display D4
         MOV       A,#03                     ; Write digit 3 on display D4
         LCALL     Disp                      ; Find appropriate mask for that digit
         MOV       P1,A                      ; Put the mask on the port
         MOV       P1,#0                     ; Turn off all dislay segments
         MOV       P3,#10h                   ; Activate display D3
         MOV       A,#02                     ; Write digit 2 on display D3
         LCALL     Disp                      ; Find mask for that digit
         MOV       P1,A                      ; Put the mask on the port
         SJMP      LOOP                      ; Return to the label LOOP

Disp:                                        ; Subroutine for writing digits
         INC       A
         MOVC      A,@A+PC
         RET
         DB        3FH                       ; Digit 0 mask
         DB        06H                       ; Digit 1 mask
         DB        5BH                       ; Digit 2 mask
         DB        4FH                       ; Digit 3 mask
         DB        66H                       ; Digit 4 mask
         DB        6DH                       ; Digit 5 mask
         DB        7DH                       ; Digit 6 mask
         DB        07H                       ; Digit 7 mask
         DB        7FH                       ; Digit 8 mask
         DB        6FH                       ; Digit 9 mask

         END                                 ; End of program