PDM microphone on nrf54l15

[Marko_P]

I am unable to read PDM microphone on my custom board or nrf54l15-dk.

My environment:

- nRF Connect SDK v3.0.2

- nrf54l15-dk

- custom board with Fanstel BM15C - nrf54l15 CPU

- T3902 PDM mic - setup as right channel.

Using the sample code I created a simple app that should read the PDM microphone using Zephyr DMIC API on my custom board. But, after a few seconds, I am only getting the same sample value all the time, -320.

- I want to use 400 kHz clock with 50x decimation -> 8000 samples per second, driver calculates correct values, also returns correct number of bytes per second (16000 bytes). I am assuming I am getting int16_t values, soon after start sample values converge to -320 and then stay -320.

- I moved the code to nrf54l15-dk. I have no microphone connected there, so I just output the clock to P1.11 and read pin P1.12 (left unconnected). A few seconds after start, I get the same value as on my custom board (that has the mic connected), -320

- On the DK I raised the P1.12 pin to match the rail voltage, then PDM returns samples +319.

pdm.zip

What am I doing wrong? I tried left and right channel, different clocks, ...

I am attaching a simple app for nrf54l15-dk. On the DK, shouldn't I get sample values close to +32767 when I force the data pin high and probably sample value close to -32768 when I leave the data pin unconnected?

[Vidar Berg]

Hello,

The reason you don't see the full scale output ( -32768 or 32768) when tying the input to gnd or vdd is likely because of the decimation filter. I got the same result here as well. However, I think it may be more relevant to look at the output from the mic to understand why the sample value converges to -320. This suggests that the output goes low and remains low. Are you able to confirm this with an oscilloscope or logic analyzer?

Best regards,

Vidar

[Marko_P]

Hi Vidar,

Thank you for your response. I already spent a lot of time on this with no result - and the API seems pretty straightforward...

So:

- It seems I do not understand then how PDM works - constant low data signal will not produce -32000 and constant high data signal will not produce +32000. Do you know of any online explanation on this subject?

- I connected logic analyzer to the DK PDM clock pin (running the previously attached sample) to confirm that it is ticking at 400 kHz. It is...

- On custom board I cannot connect to clock or data pins. But I tried 3 samples of the same board, all behave the same.

- Can you please confirm that the attached app correctly reads PDM microphone? 

So, I need to find somebody able to connect an oscilloscope to traces on the custom board. The other thing I can do is get an evaluation board for the mic and connect it to DK to confirm that my app works.  

BR,
Marko