volatile bool ledState =0 false;

ISR(TIMER1_COMPA_vect)
{
  ledState = !ledState;
  digitalWrite(13, ledState);
}

void setup()
{
  pinMode(13, OUTPUT);

  noInterrupts();

  TCCR1A = 0;
  TCCR1B = 0;
  TCNT1  = 0;

  OCR1A = 15624;

  TCCR1B |= (1 << WGM12);

  TCCR1B |= (1 << CS12) | (1 << CS10);

  TIMSK1 |= (1 << OCIE1A);

  interrupts();
}

void loop()
{
}