[sdiy] MIDI to CV/Gate on a microcontroller?

[Robert Lorentz]

I'm looking for compiled or source code (both free to do whatever I
want with) to put on any kind of microprocessor (smaller/simpler the
better) that can do MIDI note on/off to CV/Gate signals.  If it can do
4 note poly then that's (way) better.

Can anyone recommend a solution to me?  This doesn't need to be
nanosecond timing or have a CV up to 0.000001% precision or anything
ridiculous like that (I'd just be using CV if it mattered here), just
a good lightweight solution up to snuff with say, 1980's commercial
gear