Example
// This is a MiDI clk generator. This takes a #defined BPM and
// makes the appropriate clk rate. The queue is used to let other messages
// through, but allows a clock to go immediately to reduce clock jitter
#define QUEUE_DEPTH 128
#define BPM 121
#define MIDI_SYSRT_CLK 0xF8
// clock tracking and calculation
unsigned long lastClock;
unsigned long captClock;
unsigned long clk_period_us;
// Queue Logic for storing messages
int headQ = 0;
int tailQ = 0;
unsigned char tx_queue[QUEUE_DEPTH];
void setup() {
// Set MIDI baud rate:
Serial.begin(31250);
clk_period_us = 60000000 / (24 * BPM);
lastClock = micros();
}
// getQDepth checks for roll over. Folks have told me this
// is not required. Feel free to experiment.
int getQDepth() {
int depth = 0;
if (headQ < tailQ) {
depth = QUEUE_DEPTH - (tailQ - headQ);
} else {
depth = headQ - tailQ;
}
return depth;
}
void addQueue (unsigned char myByte) {
int depth = 0;
depth = getQDepth();
if (depth < (QUEUE_DEPTH-2)) {
tx_queue[headQ] = myByte;
headQ++;
headQ = headQ % QUEUE_DEPTH; // Always keep the headQ limited between 0 and 127
}
}
unsigned char deQueue() {
unsigned char myByte;
myByte = tx_queue[tailQ];
tailQ++;
tailQ = tailQ % QUEUE_DEPTH; // Keep this tailQ contained within a limit
// Now that we dequeed the byte, it must be sent.
return myByte;
}
void loop() {
captClock = micros();
if (lastClock > captClock) {
// we have a roll over condition - Again, maybe we don't need to do this.
if (clk_period_us <= (4294967295 - (lastClock - captClock))) {
// Add a the ideal clock period for this BPM to the last measurement value
lastClock = lastClock + clk_period_us;
// Send a clock, bypasing the transmit queue
Serial.write(MIDI_SYSRT_CLK);
}
} else if (clk_period_us <= captClock-lastClock) {
// Basically the same two commands above, but not within a roll over check
lastClock = lastClock + clk_period_us;
// Send a clock, bypasing the transmit queue
Serial.write(MIDI_SYSRT_CLK);
}
if (getQDepth>0) {
Serial.write(deQueue());
}
}
// The little function that gets called each time loop is called.
// This is automated somwhere in the Arduino code.
void serialEvent() {
if (Serial.available()) {
// get the new byte:
addQueue((unsigned char)Serial.read());;
}
}