Display 0 to 9 on seven segment

Circuit assumption

A common-cathode seven-segment display is connected to Port 1: P1.0 = a, P1.1 = b, ... P1.6 = g (P1.7 = dp unused). The common pin goes to ground, and each segment has a 330 ohm resistor. A segment lights when its pin is 1.

  P1.0-P1.6 --[330R]--> a b c d e f g
  common cathode ------> GND

For a common-anode display use the complements (C0H, F9H, A4H, B0H, 99H, 92H, 82H, F8H, 80H, 90H).

Assembly program

        ORG 0000H
MAIN:   MOV R0,#0          ; digit counter 0..9
NEXT:   MOV A,R0
        MOV DPTR,#SEGTAB   ; table of segment codes
        MOVC A,@A+DPTR     ; A = code for digit
        MOV P1,A           ; send to 7-segment (P1.0=a ... P1.6=g)
        ACALL DELAY        ; show for about 0.5 s
        INC R0
        CJNE R0,#10,NEXT   ; until 10 digits shown
        SJMP MAIN          ; repeat 0..9

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
        END

The digit is used as an index into SEGTAB with MOVC A,@A+DPTR. It was assembled and tested in a simulator: successive iterations output P1 = 3FH, 06H, 5BH, ...

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)
{
    unsigned char n;
    while (1) {
        for (n = 0; n < 10; n++) {
            P1 = seg[n];                /* show digit n */
            delay_ms(500);
        }
    }
}