void setupTimer3() {
  noInterrupts();
  // Clear registers
  TCCR3A = 0;
  TCCR3B = 0;
  TCNT3 = 0;

  // 1 Hz (16000000/(15624+1)*1024))
  OCR3A = 15624;
  // CTC
  TCCR3B |= (1 << WGM32);
  // Prescaler 1024
  TCCR3B |= (1 << CS32) | (1 << CS30);
  // Output Compare Match A Interrupt Enable
  TIMSK3 |= (1 << OCIE3A);
  interrupts();
}

void setup() {
  DDRC = 0xFF;
  setupTimer3();
}

void loop() {
  PORTC = (1 << PC4) | (1 << PC5) | (1 << PC6) | (1 << PC7);
  delay(2000);
  PORTC = (0 << PC4) | (0 << PC5) | (0 << PC6) | (0 << PC7);
  delay(2000);
}

ISR(TIMER3_COMPA_vect) {
  PORTC ^= (1 << PC0) | (1 << PC1) | (1 << PC2) | (1 << PC3);
}