Simple example of MIDI beat clock

examples/MIDIUSB_clock/MIDIUSB_clock.ino

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+/*
+ * MIDIUSB_clock.ino
+ * 
+ * Simple example of beat clock based on MIDI pulse messages
+ * received from software. 
+ * 
+ * Tested on Leonardo with Ableton.
+ * 
+ * In preferences go to MIDI Sync. Select device Output
+ * and toggle Sync button, change clock type to Pattern.
+ * Usually changing Sync Delay is required.
+ * 
+ * Created: 19/12/2016
+ * Author: Ernest Warzocha
+ */
+
+#include "MIDIUSB.h"
+
+//Pulse per quarter note. Each beat has 24 pulses.
+//Tempo is based on software inner BPM.
+int ppqn = 0;
+
+void noteOn(byte channel, byte pitch, byte velocity) {
+  midiEventPacket_t noteOn = {0x09, 0x90 | channel, pitch, velocity};
+  MidiUSB.sendMIDI(noteOn);
+}
+
+void noteOff(byte channel, byte pitch, byte velocity) {
+  midiEventPacket_t noteOff = {0x08, 0x80 | channel, pitch, velocity};
+  MidiUSB.sendMIDI(noteOff);
+}
+
+void setup() {
+  Serial.begin(115200);
+}
+
+void loop() {
+
+  midiEventPacket_t rx;
+
+  do {
+    rx = MidiUSB.read();
+
+    //Count pulses and send note 
+    if(rx.byte1 == 0xF8){
+       ++ppqn;
+
+       if(ppqn == 24){
+          noteOn(1,48,127);
+          MidiUSB.flush();      
+          ppqn = 0;
+       };
+    }
+    //Clock start byte
+    else if(rx.byte1 == 0xFA){
+      noteOn(1,48,127);
+      MidiUSB.flush();
+      ppqn = 0;
+    }
+    //Clock stop byte
+    else if(rx.byte1 == 0xFC){
+      noteOff(1,48,0);
+      MidiUSB.flush();
+      ppqn = 0;
+    };
+
+  } while (rx.header != 0);
+
+}