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What to do with unused pins in audio application?
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What to do with unused pins in audio application?

by OutstandingBill on Sat Jul 17, 2021 9:21 am

I'm trying to design a guitar effects pedal with an Itsy M4 at its heart. The Itsy uses its A0 to drive an analog audio component. I've got it working nicely, apart from some intermittent noise. The effect can be noise-free for minutes at a time, then super noisy (Shot noise) for minutes at a time. I'm trying to figure out where the noise could be coming from, and one thought is that it might be from unused pins picking up environmental noise and passing it through to A0.

So my question is: what should I do with unused pins? I'd like to know how I should configure them both in CircuitPython and physically.

Many thanks

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Re: What to do with unused pins in audio application?

by OutstandingBill on Sat Jul 17, 2021 7:52 pm

I should point out that my A0 pin is providing an LFO control voltage, not the audio.

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Re: What to do with unused pins in audio application?

by OutstandingBill on Mon Jul 19, 2021 9:41 pm

Any advice would be warmly welcomed : )

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Re: What to do with unused pins in audio application?

by adafruit_support_mike on Wed Jul 21, 2021 11:34 pm

The usual solution is to tie any unused pins to GND with 10k resistors. That's enough to hold them at a fixed voltage, but won't damage anything if one of the pins wants to go high for some reason.

Shot noise is usually a semiconductor barrier issue though.. electrons pass through any barrier at random times, and older transistors had a tendency to trap charge at imperfections in the crystal lattice. The defects would accumulate energy until the level got high enough to break free, then the whole bundle of energy would go in a burst.

Modern processes have mostly eliminated those defects, so my guess would be a connection somewhere in the wiring. Those can trap charger and generate noise too. Connectors are always suspect.. a layer of oxide can cause all sorts of trouble, and microscopic make-and-break connections can flex under self-heating under moderate current.

Solder joints are also worth checking.. cold joints, fractures, and buried oxide are all playgrounds of unwanted physics.

An oscilloscope would be handy for tracking the source of the problem, but those still cost enough to be uncommon. Try swapping parts of the signal path to see if the problem follows any specific device.

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