void setup() {
  // set pin 9 (OC1A) as output
  pinMode(9, OUTPUT);

  // stop Timer1 while configuring
  TCCR1A = 0;
  TCCR1B = 0;

  // set CTC mode (Clear Timer on Compare Match)
  TCCR1B |= (1 << WGM12);

  // toggle OC1A on compare match
  TCCR1A |= (1 << COM1A0);

  // set prescaler to 1
  TCCR1B |= (1 << CS10);

  // set compare match value
  // frequency = 16MHz / (2 * (OCR1A + 1))
  OCR1A = 7; // ~1 MHz
}

void loop() {
}