#define __AVR_ATmega2560__
#define F_CPU 16000000UL

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

int tick_timer_start =  0xffff - (F_CPU / 1024); // 1 second

void start_timer() {
    TCCR1B |= _BV(CS10) | _BV(CS12); // clk/1024

    // Set the TOIE (Timer Overflow Interrupt Enable) bit
    // on TIMSK1 (Timer 1 Interrupt Mask Register).
    TIMSK1 |= _BV(TOIE1);

    // Sets the current timer value
    TCNT1 = tick_timer_start;

    // Enable interrupts
    sei();
}

// ISR is used to handle interrupts. TIMER1_OVF_vect
// is triggered whenever timer 1 (16 bit) overflows.
ISR(TIMER1_OVF_vect) {
    TCNT1 = tick_timer_start;
    PORTB ^= 0b10000000;
}

void main() {
    DDRB = 0b10000000;

    start_timer();
    while(1);
}