float c0 = 16.352;
float d0 = 18.354;
float e0 = 20.602;
float f0 = 21.827;
float g0 = 24.500;
float a0 = 27.500;
float b0 = 30.868;

float c4 = c0 * 16;
float d4 = d0 * 16;
float e4 = e0 * 16;
float f4 = f0 * 16;
float g4 = g0 * 16;
float a4 = a0 * 16;
float b4 = b0 * 16;

void setup() {
  // put your setup code here, to run once:
  pinMode(2, OUTPUT);
}

void play_tone(int port, float f, long t) {
  const long T = 1000000 / f;
  long d = T / 2;
  int count = t / T;
  for (int i = 0; i < count; i++) {
    digitalWrite(port, HIGH);
    delayMicroseconds(d);
    digitalWrite(port, LOW);
    delayMicroseconds(d);
  }
}

void loop() {
  const int BPM = 100;
  const long MINUTE = 60000000;
  const long MEASURE = (MINUTE / BPM) * 4; // microseconds

  play_tone(2, c4, MEASURE / 4);
  delay(10);
  play_tone(2, c4, MEASURE / 4);
  delay(10);
  play_tone(2, g4, MEASURE / 4);
  delay(10);
  play_tone(2, g4, MEASURE / 4);
  delay(10);

  play_tone(2, a4, MEASURE / 4);
  delay(10);
  play_tone(2, a4, MEASURE / 4);
  delay(10);
  play_tone(2, g4, MEASURE / 2);
  delay(10);
}