Bug #1066: Add helper function to send SysEx/MTC via usb-device-midi

[mungewell]

I was using USB-device-midi to send MTC to the host, which is essentially SysEx. But was slowed by the missing helper function to send the packet of SysEx. Anyhow, I wrote one...

Full example script here:
https://github.com/orgs/micropython/discussions/18450#discussioncomment-15043069

[dpgeorge]

Thanks for the contribution.

But I'm a bit confused, I thought system exclusive messages were a completely different format, namely:

• they start with 0xF0
• they have an number of data bytes with the most-sig-bit clear
• they end with 0xF7

How does that differ to the sys ex added here?

[mungewell]

You are correct, SysEx generally starts with 0xF0 and ends with 0xF7.

The helper function takes an array of bytes (say "0xf07f7f010160000000f7") and fragments it to pass over USB-midi protocol, then the USB host reassembles it to pass to client application.

The example script (for Pico) generates both the long and short messages that MTC uses.

[dpgeorge]

There is a lot of duplicated code in this function. Do you think you can factor it to be simpler? For example:

def send_sysex(self, p):
    while p:
        w = self._tx.pend_write()
        if len(w) < 4:
           return False
        if len(p) > 3:
            w[0] = _CIN_SYSEX_START
            ...
        elif len(p) > 2:
            w[0] = _CIN_SYSEX_END_3BYTE
            ...
        elif len(p) > 1:
            ...
        else:
            w[0] = _CIN_SYSEX_END_1BYTE
        self._tx.finish_write(4)
        self._tx_xfer()

(And what's supposed to happen for when len(p) is greater than 7?)