AVR-Template/main.c

#include <avr/interrupt.h>
#include <avr/io.h>
#include <avr/sleep.h>
#include <avr/wdt.h>

#define LED_PIN PB5  // Define the pin connected to the LED

ISR(TIMER1_COMPA_vect) {
    // Static variable to keep track of the number of interrupts
    static uint16_t counter = 0;

    PORTB ^= _BV(LED_PIN);  // Toggle the LED

    // Modulo magic, see below for explanation
    if (++counter % 2 == 0) {
        OCR1A = 15624 / 2;
    } else {
        OCR1A = 15624 / 16;
    }

    // Clear the interrupt flag
    TIFR1 |= (1 << TOV1);
}

// We will use Timer1 for the interrupt
void configure_interrupt() {
    TCNT1 = 0;   // Initialize the counter value to 0
    TCCR1A = 0;  // Set the timer to normal mode
    TCCR1B = 0;  // Set the timer to normal mode
    OCR1A = 15624 / 16;  // 31250 for a 2s delay, 15624 for a 1s delay
    TCCR1B |= (1 << WGM12);               // Set the timer to CTC mode
    TCCR1B |= (1 << CS02) | (1 << CS00);  // Set the prescaler to 1024
    TIMSK1 |= (1 << OCIE1A);              // Enable overflow interrupt
    sei();  // Enable global interrupts (cli() to disable)
}

int main(void) {
    // Configure the LED pin as an output
    DDRB |= _BV(LED_PIN);

    configure_interrupt();

    // Set the watchdog timer to 8 seconds
    wdt_enable(WDTO_8S);

    // Set the sleep mode to idle
    set_sleep_mode(SLEEP_MODE_IDLE);

    // Pat the dog and sleep until the next interrupt
    while (1) {
        wdt_reset();
        sleep_mode();
    }

    return 0;
}