main.c

#include <stdint.h>
#include <mcs51/at89x051.h>

#define OSCILLATOR_FREQ 12000000

// assuming 8-bit counter in timer
#define INTERRUPTS_PER_SECOND (OSCILLATOR_FREQ / ((1 << 8) * 12))

// display multiplexing
#define DISPLAY_FREQ 1000 * 3
#define INTERRUPTS_PER_DISPLAY (INTERRUPTS_PER_SECOND / DISPLAY_FREQ)
#define NUM_DIGITS 3
volatile uint8_t display_digits[NUM_DIGITS] = {0xf, 0xf, 0xf};
volatile uint8_t display_current = NUM_DIGITS;
volatile uint16_t display_counter = INTERRUPTS_PER_DISPLAY;

// buttons
volatile uint8_t button_last = 0;
volatile uint8_t button_pressed = 0;

volatile uint16_t num = 0;
volatile uint8_t counter = 120;
void display(uint16_t val);
void isr_int0(void) __interrupt (0) {
    counter--;
    if (counter == 0) {
        counter = 120;
        num++;
    }
}

void isr_timer0(void) __interrupt (1) {
    display_counter--;
    if (display_counter == 0) {
        display_counter = INTERRUPTS_PER_DISPLAY;
        display_current--;

        // sample and debounce buttons
        uint8_t button_now = (P1 & 0b01100000) >> 5;
        button_pressed |= button_last & ~button_now;
        button_last = button_now;

        // turn off all digits
        P3 |= 0b111000;

        // set new display state
        P1 &= 0xf0;
        P1 |= display_digits[display_current] & 0xf;

        // turn on the current digit
        P3 &= ~(1 << (3 + display_current));

        if (display_current == 0) {
            display_current = NUM_DIGITS;
        }
    }
}

void setup() {
    // disable interrupts
    EA = 0;

    // safe defaults (caps are active low)
    // P1  aUDB3210 (adc data, up/down button, buzzer, display out)
    P1 = 0b01110000;
    // P3  H_321CSa (heat, digit select (low), PSU timing, adc select, adc clk)
    P3 = 0b10111110;

    // we use timer0 to drive the display multiplexer
    // set timer 0 to 8 bit, no prescaler
    TMOD = 0x3;
    // clear timer
    TL0 = 0;
    TH0 = 0;
    // turn timer on
    TR0 = 1;
    // enable timer interrupt
    ET0 = 1;

    // turn on edge-triggered external interrupt 0
    IT0 = 1;
    EX0 = 1;

    // enable interrupts
    EA = 1;
}

void display(uint16_t val) {
    uint8_t d;
    display_digits[0] = val % 10;
    val /= 10;
    d = val % 10;
    display_digits[1] = d ? d : 0xf;
    val /= 10;
    d = val % 10;
    display_digits[2] = d ? d : 0xf;
}

int main() {
    setup();
    display(num);
    while (1) {
        if (button_pressed & 0b01) {
            // down
            button_pressed &= ~0b01;
            num--;
        }
        if (button_pressed & 0b10) {
            // up
            button_pressed &= ~0b10;
            num++;
        }
        display(num);
    }
}