I want to use pin PA2 as aditional midi out on an axoloti based footcontroller project. But I think this should be fairly compatible to ksoloti.
So I want to have 2 expression pedal jacks, and a midi jack on a little DIY breadboard adapter and a pinheader going to gpios on axoloti.
The expression pedals would go to PA0 and PA1 GND and VDDA. The midi tx from PA2 through a 220ohm resistor. Midi also needs GND and 5V through 220ohm resistor.
My question: Could you also do 3.3V Midi (with different resistor values)
If so, could you do tap the same VDDA power pins that the expression pedals use?
These are the midi specs I found:
Lower resistance is recommended when running MIDI on 3.3V. The Teensy MIDI circuits for 3.3v have always worked for me. You probably don’t need to go as low as 47 ohms, 100 ohms for both resistors will work (and happens to be possible to realize using two 220 ohms in parallel).
https://www.pjrc.com/teensy/td_libs_MIDI.html
(Diagram under “This schematic is recommended for Teensy 4.x & 3.x & LC”)
So in short, what you wrote makes sense and seems possible.
If you use VDDA to power MIDI out, I am wondering if you will get digital MIDI noise creeping into the VDDA (as this is an analog voltage meant for the ADCs) but likely nothing noticeable.
Hey thanks for the quick answer!
Yes this diagram is quite clear. I was wondering about the resistor values. I made it work yesterday with 2 25ohm resistors. It even worked with 3.3v and 220ohm resistors. I think I should have 47ohm around actually, so I’ll just stick to that.
Ok I’ll have to test this. This would be some kind of jitter on the adsc readings of the expression pedals?
I guess it could make the readings more noisy. I just saw that the Axoloti Core uses VDDA both for the ADC reference and to power the audio codec and amplifier IC. So strictly speaking there is some path where that noise might creep into the audio signals as well.
So in short, if you notice weird noises during MIDI activity, you’ll know where to look. Use VDD for MIDI out and the expression pedals then.
Ok, thanks for clarifiyng. I plan on using this board mainly as midi footcontroller, with the option of using it as a backup, if any of my main axolotis don’t work for whatever reason.
But maybe it makes more sense if I just tap the 5V line?
but even in that scenario I have experienced, noise from USB midi controllers bleeding into the audio path. The LEDs from midi controllers would add an audible drone in the audio. I was only able to solve that by running usb midi controllers through a usb isolators that is powered by another output on my isolated multi power supply.
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