      on key press play a music file from the micro SD card from a specific folder and blink the respective key (/)
      on pressing the same key or when playback stops jump to the next file in the folder, until all music is played (/)
      read the rotary decoder to change the volume, the maximum volume can be configured (/)
+     press the rotary decoder to pause/resume (/)
      detect when a headphone is plugged in and mute the amplifier for the internal speakers (/)
      switch the music box off automatically after an idle period
 

void onKey(byte index) {
  nextTimeoutTick = 0;

  // Same key pressed again
  if (state == PLAY_SELECTED && playingAlbum == index) {
    stopPlaying();
    onTryNextTrack();


}

void onWatchdogPing() {
  blinkLED(BLUE_LED_PIN);
}

void blinkLED(int pin) {
  digitalWrite(pin, LOW); // LED on
  delay(1);
  digitalWrite(pin, HIGH); // LED off
}

void trellisIsr() {
