First, let me say that I accept this very well maybe a hardware issue of my own creation, but before I start tearing things apart:
I have both gpio pins (PA4 and PA5) wired to stereo 1/4" jacks. They are wired: gpio to tip, 3.3v to ring, and ground to sleeve. Power and ground are wired in series with 8 other pots I have connected. Here's the results I get:
Expression pedal into PA4: Pedal controls a disp/dial p with almost the full range from 0-64, but also afftects a dial connected to PA5
Expression pedal into PA5:Pedal controls a disp/dial p with almost the full range from 0-64, no affect on PA4
I have a pedal with an expression in, designed to take 3.3v, and wired per TRS the connections I note above.
PA5 works to send control signals to the pedal, which reacts as expected, except I have to phyically disconnect the wires from the board to PA4. If PA4 is also wired up, it kills the pedal when plugged in.
PA4 kills the pedal when plugged in, even if PA5 is unhooked.
I talked to the guy who made the pedal and from the pedal being "dead" he guesses that the expression is somehow shorting power to ground. Unplug the expression jack from Axo and it works normally again.
Like I said -- it's very possible I have a solder bridge somewhere, but when I load the gpio/analog/out object I see both PA4 and PA5 which makes me assume they share a bus internally on the board and this could be related.