button_toggle.c

#include <avr/io.h>

#define led_pin (1 << PB2)
#define button_pin (1 << PA7)

int main(void) {
    // Set the clock prescaler to 1.
    CLKPR = (1 << CLKPCE);
    CLKPR = (0 << CLKPS3) | (0 << CLKPS2) | (0 << CLKPS1) | (0 << CLKPS0);

    // Set the 8 bit timer's prescaler to 1/1024.
    TCCR0B |= 0b00000101;

    // Configure led_pin as an output.
    DDRB |= led_pin;

    // Configure button_pin as an input.
    DDRA &= ~button_pin;

    // Activate button_pin's pullup resistor.
    PORTA |= button_pin;

    int bouncy_switch_state = 0;
    int debounced_switch_state = 0;

    while (1) {
        // Use the timer to count how long it's been since button_pin changed state.
        if ((PINA & button_pin) != bouncy_switch_state) {
            bouncy_switch_state = PINA & button_pin;
            TCNT0 = 0;
        }

        // If it's been 10ms or more since PA7 changed state, it's not bouncing.
        if (TCNT0 >= 195) {
            // It's been 10ms since the switch changed.
            if (bouncy_switch_state != debounced_switch_state) {
                debounced_switch_state = bouncy_switch_state;
                // If the button is newly pressed, toggle the LED.
                if (debounced_switch_state == 0) {
                    PORTB ^= led_pin;
                }
            }
        }
    }

    return 0;
}