Display 99 to 00 on seven segment

Circuit

Two common-cathode seven-segment displays: units digit on Port 1, tens digit on Port 2, each segment through a 330 ohm resistor.

 8051                    Tens display   Units display
 P2.0-P2.6 --[330R x7]--> a b c d e f g
 P1.0-P1.6 --[330R x7]------------------> a b c d e f g
                          common cathode  common cathode
                               |                |
                              GND              GND

The segment code for each digit (gfedcba, common cathode) is: 0=3F, 1=06, 2=5B, 3=4F, 4=66, 5=6D, 6=7D, 7=07, 8=7F, 9=6F. The count is kept as packed BCD in R0 and decremented using ADD A,#99H followed by DA A (adds 99H, which equals subtracting 1 in BCD).

Assembly program

        ORG 0000H
MAIN:   MOV R0,#99H          ; BCD count, start at 99
SHOW:   MOV DPTR,#SEGTAB
        MOV A,R0
        ANL A,#0FH           ; units digit
        MOVC A,@A+DPTR
        MOV P1,A             ; units display on Port 1
        MOV A,R0
        SWAP A
        ANL A,#0FH           ; tens digit
        MOVC A,@A+DPTR
        MOV P2,A             ; tens display on Port 2
        ACALL DELAY          ; about 0.5 s
        MOV A,R0
        JZ MAIN              ; after 00 start again from 99
        ADD A,#99H           ; A = A + 99H, then DA A gives BCD A - 1
        DA A
        MOV R0,A
        SJMP SHOW

DELAY:  MOV R5,#4
D1:     MOV R6,#250
D2:     MOV R7,#250
D3:     DJNZ R7,D3
        DJNZ R6,D2
        DJNZ R5,D1
        RET

SEGTAB: DB 3FH,06H,5BH,4FH,66H,6DH,7DH,07H,7FH,6FH   ; common cathode codes
        END

Checked in a simulator: the sequence 99, 98, ... 90, 89, ... 00 is displayed, then restarts at 99.

C program

#include <reg51.h>

unsigned char code seg[10] = {0x3F,0x06,0x5B,0x4F,0x66,0x6D,0x7D,0x07,0x7F,0x6F};

void delay_ms(unsigned int ms)
{
    unsigned int i, j;
    for (i = 0; i < ms; i++)
        for (j = 0; j < 120; j++);          /* about 1 ms at 12 MHz */
}

void main(void)
{
    signed char n;                          /* must be signed: loop ends at -1 */
    while (1) {
        for (n = 99; n >= 0; n--) {
            P2 = seg[n / 10];               /* tens digit on Port 2  */
            P1 = seg[n % 10];               /* units digit on Port 1 */
            delay_ms(500);
        }
    }
}