[sdiy] MIDI I/O c code..

Jean-Pierre Desrochers jpdesroc at oricom.ca
Fri Mar 20 15:15:23 CET 2020

Looking at my code I remembered this note I put in 

To be sure I won’t  forget it:



Voice Message           Status Byte      Data Byte1          Data Byte2

-------------           -----------   -----------------   -----------------

Note off                      8x      Key number          Note Off velocity

Note on                       9x      Key number          Note on velocity

Polyphonic Key Pressure       Ax      Key number          Amount of pressure

Control Change                Bx      Controller number   Controller value

Program Change                Cx      Program number      None

Channel Pressure              Dx      Pressure value      None            

Pitch Bend                    Ex      MSB                 LSB


Channel mode messages are a special case of the Control Change message ( 0xBx or 1011nnnn). 

The difference between a Control message and a Channel Mode message, 

which share the same status byte value, is in the first data byte. 

Data byte values 121 through 127 have been reserved in the 

Control Change message for the channel mode messages. 


Channel mode messages determine how an instrument will process MIDI voice messages. 


1st Data Byte      Description                Meaning of 2nd Data Byte

-------------   ----------------------        ------------------------

     79        Reset all  controllers            None; set to 0

     7A        Local control                     0 = off; 127  = on

---->7B        All notes off   <------           None; set to 0

     7C        Omni mode off                     None; set to 0

     7D        Omni mode on                      None; set to 0

     7E        Mono mode on (Poly mode off)      **

     7F        Poly mode on (Mono mode off)      None; set to 0



De : Synth-diy [mailto:synth-diy-bounces at synth-diy.org] De la part de Mikko Helin
Envoyé : 20 mars 2020 01:39
À : synth-diy at synth-diy.org List
Objet : Re: [sdiy] MIDI I/O c code..


>Running status are ignored here

So you are ignoring the data bytes that belong to next note on which is using running status, meaning the sender does not send the same status byte again but just skips it and just sends the key and velocity bytes?



pe 20. maalisk. 2020 klo 3.29 Jean-Pierre Desrochers <jpdesroc at oricom.ca> kirjoitti:

> Are you handling running status and note off sent as zero velocity note on?

Yes I handle both.

Running status are ignored here,

And I consider both NOTE OFF and NOTE ON with zero velocity

As the end of a note.



De : dougall [mailto:dougalli at gmail.com] 
Envoyé : 19 mars 2020 21:06
À : eidorian at aladan.net
Cc : Jean-Pierre Desrochers; Synth-diy at synth-diy.org
Objet : Re: [sdiy] MIDI I/O c code..


Are you handling running status and note off sent as zero velocity note on?




On Fri, 20 Mar 2020 at 11:55, <eidorian at aladan.net> wrote:

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 changes).

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 ?



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