PDM / DMIC not giving any sensible output

[timonsku]

I'm trying to capture audio data from a PDM microphone.
Using the dmic sample I output the captured buffer via serial. Though the captured PCM data is 8x the same data. So each time the dmic sample captures one "block" is receives the same data though this is not what I see in my logic analyser.
What is even more puzzling is that I keep getting successful data reads when I disconnect the microphone. That happens with the unmodified dmic sample.

The issue sounds very similar to this old ticket: RE: Trouble extracting data using DMIC 

Used hardware:
nrF5340DK
Microphone: https://www.adafruit.com/product/4346 ( https://cdn-learn.adafruit.com/assets/assets/000/049/977/original/MP34DT01-M.pdf)

Software:
NCS2.5.0 via VSCode Extension
Windows 10

The received output. 1. Zoomed in 2. Full capture

[Maria Gilje]

Hello,

I have picked this up and will get back to you soon. But let's start with some clarification questions:

What is even more puzzling is that I keep getting successful data reads when I disconnect the microphone.

How to you disconnect the microphone?

How do you define "successful data reads"?

Best regards,

Maria

[timonsku]

Hi Maria,

I remove the data and clock line from the development kit, then I press the reset button, the firmware reboots and still prints out sample data. This also happens with the unmodified dmic sample where it reports successful reads from the interface. E.g. this line is not triggered because dmic_read returned with 0 which it should not have with no physical device present to return audio data.
https://github.com/nrfconnect/sdk-zephyr/blob/main/samples/drivers/audio/dmic/src/main.c#L58C12-L58C12

It tried both nrf5340dk_nrf5340_cpuapp and nrf5340dk_nrf5340_cpuapp_ns build targets, same outcome.

[timonsku]

I also just tested with NCS 2.4.2. I can do a cold boot of the devkit and it still reports successful reads with nothing attached to the devkit.

[timonsku]

Hi Maria Gilje were you able to reproduce the problem? I saw that the Thingy:53 is also using a PDM microphone but the firmware is very complex and seems to be very specific for use with Edge Impulse.
I've seen that they do not really make use of Zephyr's DMIC subsystem but talk to the nordic pdm driver directly.
Is this expected procedure? If so do you have any simpler samples that demonstrate basic use of the pdm driver?