A simple sketch that sends a midi clock signal

midi_clock.ino

int tempo = 50;
void setup() {
  // put your setup code here, to run once:
  Serial.begin(31250);
  Serial.write(250);
}

void loop() {
  // put your main code here, to run repeatedly:
  for (int i = 0; i < 10; i++) {
      Serial.write(248);
      delay(tempo);
  }
  if (tempo > 3) tempo--;

}