audioio initialisation has strange DAC outputs on at least two SAMD51 boards

[kevinjwalters]

I've put CircuitPython on a PyGamer. My understanding is the audioio libraries nowadays put the output at the midpoint 32768 (3.3V/2 =1.65V) by default.

From REPL on 4.1.0-beta.1 I'm running

import board
import audioio
dacs = audioio.AudioOut(board.A0, right_channel=board.A1)

As the third command runs both outputs jump to around 1.65V as expected. They then drift slowly (perhaps 5 seconds) to A0=0.03V and A1=2.46V (multimeter and oscilloscope show same thing). That drift is almost linear but there is a slight jump towards end.

A0 and A1 in isolation behave similarly although maybe exact, final resting values differ a little:

import board, audioio
dacs = audioio.AudioOut(board.A0)  # just DAC0

import board, audioio
dacs = audioio.AudioOut(board.A1)  # just DAC1

These two independent versions have similar output

PyGamer does have some hardware attached to A0/A1. I have the speaker attached but that's other side of amplifier which I am not enabling. I have nothing in the 3.5mm socket.

Other on-going SAMD51/M4 DAC related discussions:

• analogWrite() oddness on M4 when using both DACs ArduinoCore-samd#87
• Adafruit Forums: SAMD51 M4 DAC has problems getting it up
• Adafruit Forums: PyGamer DACs limited to 2.5V - 2.6V ?
• Adafruit Forums: What's fastest practical sample_rate for M4 DACs?

There's mention of a refresh setting somewhere. It sounds like that's needed when you want the output to hold and are not outputting samples constantly at a reasonable rate. I think @dhalbert may have set this to 2 in #649 - I wonder if that's used for both analogio and audioio? The latter might be expected to write constant values but given that wavs/etc are not always played then there will be no activity phases where it would be expected to hold midpoint value (the value can be customised with quiescent_value).

To my surprise Feather M4 (mine's running 4.0.1) does same thing and that has nothing connected to A0/A1 AFAIK so rules out any significant effect from external components. I tested analogio.AnalogOut too on Feather. That drifts slowly from ~ 0V to ~ 2.2V (then jumps further to 3.3V if oscilloscope probe is briefly removed) when initialised but as soon as a value is assigned it holds that output voltage.