majenko:
Then you can just use the attiny as a MIDI instrument manually sending the note on / note off commands over serial.
I forgot to mention that I don't really need the MIDI library on the attiny2313a, all I need is a way for buttons hooked up to the attiny to call functions located on the 328P. It does not matter to me what type of message the attiny sends to the 328P, be it midi or something else. I actually would prefer a way where the attiny would send a short string according to the button pushed and the 328 would read that string and call the appropriate function from the MIDI library.
btw, is it complicated to send messages over the serial port without using the Serial library?
I reckon that dedicated serial ShiftIn chips like the CD4021BE have a simplified mini version of the serial library?