;Program: ModSeven.ASM
;Update:  20 February 2003
;By:      Dr. Marcus O. Durham, PhD, PE
;         Tulsa, OK, USA
;         mod@superb.org
;         www.ThewayCorp.com
;Copyright (c)1991, 2003.  All rights reserved

;Purpose:
;  Data is stored in GapA, GapB, GapC.
;  Data is displayed on 3 7-segment displays.
;  Display-out latch shows the data.
;  The 3 displays are selected by port 3.2, 3.3, & ;  3.4
;
;#################################################
;                     ASSIGNMENTS
;#################################################
P35       EQU   0B5h  ;switch input

GapD      EQU   37H   ;general purpose variables
GapC      EQU   36H 
GapB      EQU   35H 
GapA      EQU   34H

LoopC     EQU   07H   ;loop counter 
;R0       EQU   00H   ;destination indirect addr 

;#################################################
;                     PROGRAM
;#################################################
          ORG   00h
START:    LJMP  INITIAL

          ORG   0033h
          DB    25, 1,'Marcus O. Durham, PhD, PE'

;-------------------------------------------------
          ORG   0080H         ;get past interrupt
INITIAL:
;-------------------------------------------------
          MOV   SP,#5Fh       ;start stack @ 5f+1
          SETB  P35           ;make input

          LCALL HELLO         ;start display
          LCALL SEVBCD        ;BCD to 7 segment

;-------------------------------------------------
MAIN:
;-------------------------------------------------
          LCALL SEVSEG        ;output seven segmen

MAN9:     JNB   P35,MAIN      ;Repeat

;-------------------------------------------------
HELLO:
;-------------------------------------------------
;  Preload a value into the general purpose bytes.

                              ;PRELOAD
          MOV   GapA,#0Fh     ;data byte
          MOV   GapB,#0Ah
          MOV   GapC,#0Bh
          RET

;-------------------------------------------------
SEVBCD:
;-------------------------------------------------
;  Convert the binary coded decimal to seven seg.
;  Use a table look up.

                              ;INITIAL
          MOV   DPTR,#TabSeven ;seven seg table

          MOV   R0,#GapA      ;base of digits
          MOV   LoopC,#3      ;number of digits

                              ;CONVERT
SBCD1:    MOV   A,@R0         ;get BCD
          MOVC  A,@A+DPTR     ;table offset to A
          MOV   @R0,A         ;replace w/ 7 seg

                              ;NEXT DIGIT
          INC   R0            ;next 
          DJNZ  LoopC,SBCD1   ;>=0, repeat process

          RET                 ;else, exit  

;-------------------------------------------------
SEVSEG:
;-------------------------------------------------
;  Seven Seg is a routine that operates in 4 steps
;  1. Select the digit stored in GapS
;  2. Select the digit to turn on
;  3. Send the data
;  4. Wait briefly for persistence of the led.
;  5. Repeat

                              ;INITIAL BYTE LOCATE
          MOV   R0,#GapA      ;least sig display

                              ;SELECT LINES
          CLR   92h           ;turnoff all selects
          CLR   93h
          CLR   94h

                              ;DIGIT 1
          SETB  92h           ;Bit is hi, select A
          LCALL SEVOUT        ;output & delay
          CLR   92h

                              ;DIGIT 2
                              ;DIGIT 3

                              ;TERMINATE
          RET                 ;back to Hotlanta

;#################################################
;                     TABLES
;#################################################
TabSeven:
;-------------------------------------------------
;  Seven-segment display 
          DB    3Fh   ;0
          DB    06h   ;1
          DB    5Bh   ;2
                      ;complete the table

;*************************************************
          END                     ;Program end


Alternative output can be on a port or via memory mapped io.

;-------------------------------------------------
SEVOUT:
;-------------------------------------------------
                              ;BYTE OUT
          MOV   A,@R0         ;data byte
          INC   R0            ;next info to show

                              ;PERSISTENCE WAIT
          MOV   R2,#200       ;Nested loop counter
ZDEL1:    MOV   Pio,A         ;display digit
          DJNZ  R2,ZDEL1      ;Nested loop, 256 x

          RET 


;-------------------------------------------------
SEVOUT:
;-------------------------------------------------
                              ;BYTE OUT
          MOV   A,@R0         ;data byte
          INC   R0            ;next info to show

                              ;PERSISTENCE WAIT
          MOV   DPTR,#8002h   ;addr for disp latch

          MOV   R2,#200       ;Nested loop counter
ZDEL1:    MOVX  @DPTR,A       ;display digit
          DJNZ  R2,ZDEL1      ;Nested loop, 256 x
          RET