// Arduino uno default clock freqency 16Mhz

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

unsigned long n_overflows = 0;

// Timer 2 Overflow Interrupt Service Routine
ISR(TIMER2_OVF_vect) {
  // Task: toggle pin 12 at overflow and led (pin 13) at 1 sec.
  n_overflows++;
  PORTB ^= _BV(PORTB4);
  if (n_overflows >= 62500){
    n_overflows = 0;
    PORTB ^= _BV(PORTB5);
  }
}

int main() {
  // Set pinmode 
  DDRB= _BV(DDB4) | _BV(DDB5);

  // setting clock source
  TCCR2B = _BV(CS21); 

  // enabling timer2 overflow interrupt
  TIMSK2 = _BV(TOIE2);
  
  // enabling global interrupts
  sei();

  while(1); // empty loop
}