How to Deal with MIDI Clock Signals in Arduino

It is often desirable to synchronise something like a homemade sequencing circuit or a Commodore 64 to MIDI clock signals.

I thought I might share some generic Arduino skeleton code that could be used to synchronise
many different types of things to MIDI clock (and therefore ProTools, Ableton Live etc -- any type of host sequencer).

So, what exactly is MIDI clock? Well, it is a set of specific bytes that form part of the MIDI protocol. The great thing is that MIDI clock messages are only one byte long (instead of two or three bytes like other MIDI messages), so the code is very simple.

Here are the main, useful MIDI clock bytes:

start = 0xfa
stop = 0xfc
clock tick = 0xf8
continue = 0xfb

Twenty-four clock tick bytes are sent per quarter note when the host sequencer is running. Every time the sequencer is stopped, the stop byte is sent. Every time the sequencer is started after being stopped, the start byte is sent. Every time the sequencer is started after being paused, the continue byte is sent.

Using this information, it is possible to write a simple Arduino sketch to handle MIDI clock bytes and "do something" for every clock tick. The trick is to work out what you want to do with every clock tick, and then simply adjust the code below to add this desired functionality.

byte midi_start = 0xfa;
byte midi_stop = 0xfc;
byte midi_clock = 0xf8;
byte midi_continue = 0xfb;
int play_flag = 0;
byte data;

void setup() {
Serial.begin(31250);
}

void loop() {
if(Serial.available() > 0) {
data = Serial.read();
if(data == midi_start) {
play_flag = 1;
}
else if(data == midi_continue) {
play_flag = 1;
}
else if(data == midi_stop) {
play_flag = 0;
}
else if((data == midi_clock) && (play_flag == 1)) {
Sync();
}
}
}

void Sync() {
// do something for every MIDI Clock pulse when the sequencer is running
}