USBMIDI Callback

Mbed OS Best practice
5

What error are you getting? Mutex error?

I have never used USBMIDI and cannot provide MIDI examples. But, if you haven’t tried EventQueue, you may want to look into it.

https://os.mbed.com/docs/mbed-os/v5.15/tutorials/the-eventqueue-api.html

If your error occurs because your program executes codes in an interrupt context, you may be able to solve that by deferring the execution to a different context using EventQueue.

Below is an example. This postpones an execution of myCallback function (which printf and writes to SPI port) from ISR context to a user context.

#include "mbed.h"

#define MOSI p5
#define MISO p6
#define SCLK p7
#define CS p8

EventQueue *queue = mbed_event_queue(); // event queue

RawSerial pc(USBTX, USBRX);
SPI spi(MOSI, MISO, SCLK);
DigitalOut cs(CS);

void onDataReceived();

void myCallback() {
    char c = pc.getc();
    printf("received: %02x\r\n", c);
    cs = 0;
    spi.write(c);
    cs = 1;
    pc.attach(&onDataReceived, Serial::RxIrq);  // reattach interrupt
}

void onDataReceived() {
    pc.attach(NULL, Serial::RxIrq); // detach interrupt
    queue->call(myCallback);        // process in a different context
}

int main() {
    pc.attach(&onDataReceived, Serial::RxIrq);
    queue->dispatch_forever();
}

See queue->call(myCallback) in onDataReceived. If I have just myCallback() there instead, it crashes because it is not allowed to do printf and spi write in ISR context.