Leonardo fade

// Fade: hardware PWM on pin 13, which on this board is timer 4.
//
// Timer 4 is the 32U4's ten-bit high-speed timer and it is unlike anything on
// an Uno: its TOP is a register (OCR4C) rather than a mode, its channels are
// A, B and D with no C, and each channel needs a PWM4x enable of its own on
// top of the COM bits. analogWrite hides all of that (the sketch is the
// ordinary one), but the waveform on pin 13 is coming out of a different kind
// of timer from the one that drives pin 9.
//
// Pin 9 gets the same ramp a quarter of a period behind, off timer 1, so the
// two PWM frequencies this board has are both on screen: 976.5625 Hz on 13 and
// 490.196 Hz on 9.

int level = 0;
int step = 5;

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  pinMode(9, OUTPUT);
}

void loop() {
  analogWrite(LED_BUILTIN, level);
  analogWrite(9, 255 - level);

  level += step;
  if (level <= 0 || level >= 255) {
    step = -step;
    level += step;
  }
  delay(12);
}