Hello,
short quick question, since I am building a little bit of gear by myself.
I do connect the usually 5V midi signals over a Optocoupler 6N137 or 138 to my MSP430 processor, at leats that is the plan 
Now I ran into some troubleshooting on my own boards, which lead me to take a look at some other gear i got lying around, like the good old axoloti.
I realized form the schematics you also run the Midi signal with 5 V
1: without an pull up on the 6N137, which is a bit odd I think, cause all my other gear does have it.
2: it your STM32 processor 5V tolerant? your Optocoupler 5V output signal directly goes to an input pin right?
Thanks for reading, best reagrds.