#include "pitches.h"

const uint8_t buzzerPin = 9;
const int iterFrequency = 62500;
const float freq = 261.63;
const int iterPeriod = 128;

volatile unsigned long iter = 0;
int period;
bool buzzerState = false;

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

  period = round(1e6 / freq / iterPeriod);

  noInterrupts();
  TCCR1A = 0;
  TCCR1B = 0;
  TCNT1 = 0;
  OCR1A = 249;
  TCCR1B |= (1 << WGM12);
  TCCR1B |= (1 << CS11);
  TIMSK1 |= (1 << OCIE1A);
  interrupts();
}

void loop() {
  
}

ISR(TIMER1_COMPA_vect) {
  iter++;

  if (iter % period == 0 || iter % period == round(period / 2)) {
    buzzerState = !buzzerState;
    digitalWrite(buzzerPin, buzzerState);
  }
}