external_midi_clock.ino

#include <SdFat.h>
#include <MD_MIDIFile.h>
#include <SoftwareSerial.h>

#define MIDI_OUT_TX_PIN 9
#define SD_SELECT_PIN 10

unsigned short play = 0;
byte midiInByte;
SdFat SD;
MD_MIDIFile midiFile;
SoftwareSerial midiOutSerial = SoftwareSerial();

void midiCallback(midi_event *midiEvent);
void midiSilence(void);

void setup() {
  // MIDI in
  Serial.begin(31250);
  while (!Serial);

  // MIDI out
  midiOutSerial.setTX(MIDI_OUT_TX_PIN);
  midiOutSerial.begin(31250);
  while (!midiOutSerial);
  
  if (!SD.begin(SD_SELECT_PIN, SPI_HALF_SPEED)) {
    while (1);
  }

  midiFile.begin(&SD);
  midiFile.setMidiHandler(midiCallback);
  midiFile.looping(true);
  midiFile.setFilename("TEST.mid");

  if (!midiFile.load()) {
    while (1);
  }
}

void loop() {
  if (Serial.available()) {
    midiInByte = Serial.read();

    if (midiInByte == 0xFA) { // Start
      midiFile.restart();
      play = 1;
    } else if (midiInByte == 0xFC) { // Stop
      midiSilence();
      play = 0;
    }

    if (play == 1) {
      if (midiInByte == 0xF8) { // clock
        // HELP :)
      }
    }
  }
}

void midiCallback(midi_event *e) {
  midiOutSerial.write(e->data[0] | e->channel);
  midiOutSerial.write(&e->data[1], e->size - 1);
}

void midiSilence(void) {
  midi_event ev;
  ev.size = 0;
  ev.data[ev.size++] = 0xb0;
  ev.data[ev.size++] = 120;
  ev.data[ev.size++] = 0;

  for (ev.channel = 0; ev.channel < 16; ev.channel++) {
    midiCallback(&ev);
  }
}