#define PERIOD 500    // control loop period [ms] - max 4194 ms !!!

boolean state = LOW;

void setup() {
  cli();  // disable interrupts

  pinMode(0, OUTPUT);
  digitalWrite(0, LOW);

  // Configure Timer 4 to generate interrupt each PERIOD
  // (CTC mode; 1024 prescaler; enable interrupt on compare match)
  TCCR4A = 0;
  TCCR4B = (1 << WGM42) | (1 << CS42) | (1 << CS40);
  TCNT4  = 0;
  // output compare register
  OCR4A = (16000000 / 1024) * PERIOD / 1000;
  // Output Compare A Match Interrupt Enable
  TIMSK4 = (1 << OCIE4A);

  sei();  // enable interrupts
}

void loop() {
  // nothing to do
}

// Controll loop goes here
ISR(TIMER4_COMPA_vect)
{
  state = !state;
  digitalWrite(0, state);
}