simple debounce routine

gistfile1.h

#include <avr/io.h>
#include <util/delay.h>

/* call every 10 ms, for buttons at pins PC0-PC3 */
uint8_t button_sample(void) {
    static uint8_t state = 0b1111;       /* initialize state, buttons are active low! */
    static uint8_t last_sample = 0b1111; /* initialize old sample */
    uint8_t new_sample = PINC & 0b1111;  /* read inputs */

    /* mark bits which are sampled with the same value */
    uint8_t same_sample = (last_sample ^ ~new_sample);
    /* all bits set in same_sample now have been sampled with the same value at
     * least two times, which means the button has settled */

    /* compare the current button state with the most recent sampled value,
     * but only for those bits which have stayed the same */
    uint8_t state_different = state ^ (new_sample & same_sample);
    /* all bits set in state_different have been sampled at least two times
     * with the same value, and this value is different from the current button state */

    /* if a bit is set in state (means: button is not pressed) AND bit is set
     * in state_different (means: input has settled and value is different from state)
     * together means: button has been pressed recently */
    uint8_t button_press = state & state_different;

    /* toggle all bits for inputs which switched state */
    state ^= state_different;

    /* store current sample for next time */
    last_sample = new_sample;

    /* if bit is set in button_press, a button has been pressed (not released yet) */
    return button_press;
}

/**
 * light leds according to parameter led_config
 * (bit0-bit4 for led1-led4) and wait 200ms
 */
static void display(uint8_t led_config) {
    /* set outputs according to led_config */
    if (led_config & 0b1)
        PORTC |= _BV(PC4);
    else
        PORTC &= ~_BV(PC4);

    if (led_config & 0b10)
        PORTD |= _BV(PD3);
    else
        PORTD &= ~_BV(PD3);

    if (led_config & 0b100)
        PORTD |= _BV(PD6);
    else
        PORTD &= ~_BV(PD6);

    if (led_config & 0b1000)
        PORTD |= _BV(PD7);
    else
        PORTD &= ~_BV(PD7);

    /* wait 10ms */
    _delay_loop_2(F_CPU/4/100);
}


int main(void) {
    /* initialisierung */

    /* led1 */
    DDRC = _BV(PC4);
    /* led2-led4 */
    DDRD = _BV(PD3) | _BV(PD6) | _BV(PD7);

    /* button pullups */
    PORTC = _BV(PC0) | _BV(PC1) | _BV(PC2) | _BV(PC3);

    uint8_t state = 0;

    while(1) {
        state ^= button_sample();
        display(state);
    }
}