[sdiy] MIDI I/O c code..

eidorian at aladan.net eidorian at aladan.net
Fri Mar 20 01:52:03 CET 2020

Sorry, I don't have example code for you, but here is some analysis that
I hope might help point you in the right direction: 

20MHz / 31,250Hz => ~640 instructions per MIDI data bit, or 6,400
instructions per MIDI data byte.  That's quite a lot of instruction
execution time between notes, so I'd be checking the
efficiency/optimisation of the code. 

How long is it taking to update the analog outputs and do other
processing?  If it's updating all 8 CVs via a "slow" protocol like I2C
every time it loops then that might be taking a while, and maybe you
need a different approach (i.e. only update a CV output when it

How have you implemented the FIFO?  Do you have locking to ensure the
interrupt isn't writing to it while the main loop is updating it? 

Create a debug version that has checks on the FIFO size in the
interrupt, and sets an LED on if it overflows, so you can confirm for
certainly whether or not that's the problem. 

What I do (with 30+ years of programming experience) is pretend I'm the
CPU, and "execute" the code in my head (or on paper if complex) in order
to try to work out what unplanned or unexpected things might be
happening to cause the problematic behaviour. 



On 20-03-2020 11:14, Jean-Pierre Desrochers wrote:


> I'm working on a new polyphonic MIDI to CV module (8 voices). 
> So far I did my tests with an old PIC16F887 @ 20Mhz micro I had on hand. 
> I use a USART interrupt driven c function to 'catch' all the incoming MIDI bytes. 
> My code is pretty fast, but still, since I only read one MIDI channel (1-16) at the time 
> when I play very fast chords on an external keyboard 
> all the notes are read in the incoming queue but sometimes 
> I get stuck notes or unread ones.. 
> Same thing happens when playing MIDI files on Cakewalk SONAR 
> feeding my prototype with  a MIDI cable.
> The interrupt function grabs each incoming byte and put them 
> In a receive buffer of 32 bytes and the main () reads and treats them in a FIFO manner 
> Later in the main loop. 
> _-Would a 32Mhz micro do a difference in the USART interrupt reading speed ?_ 
> _-And is 32 bytes long enough for the RxBuffer to handle a 6 voices chords ?_ 
> _-And finaly I checked the web for a 'decent' C code examples_ 
> for MIDI reception (MIDI Tx is much easier to implement) with no success..
> 'Obscure' Arduino libraries all around with no explainations of its inner code. 
> Did anybody use good C code available ? 
> JP 
> Synth-diy at synth-diy.org 
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