PDM microphone on nrf54l15

RE: PDM microphone on nrf54l15

Vidar Berg — Mon, 08 Sep 2025 07:06:50 GMT

Hi Marko,

I'm glad to hear you found the problem, thank you for the update. Yes, adding the nfct-pins-as-gpios; property to the uicr node is the correct approach for now even though the 54l does not have a register for this in UICR like the 52/53 series did. We are doing the same in our tests as you can see here:

The NRF_CONFIG_NFCT_PINS_AS_GPIOS flag will be set when you have the nfct-pins-as-gpios property defined in your devictree ensuring that this line is executed on every boot: https://github.com/zephyrproject-rtos/hal_nordic/blob/54f33f10a0b826174fb145f155afa61ce5a44b93/nrfx/mdk/system_nrf54l.c#L226

Please refer to the pin assigments chapter of the nRF54l15 datasheet for more details: https://docs.nordicsemi.com/bundle/ps_nrf54L15/page/chapters/pin.html

Best regards,

Vidar

RE: PDM microphone on nrf54l15

Marko_P — Fri, 05 Sep 2025 20:23:48 GMT

Hi Vidar,

1. Well, I accidentaly managed to get my mic to work. The issue was that I used P1.03 pin as clock, and on nrf54l this pin is by default assigned to NFC. So adding this to my DTS file solved the problem:

&uicr {

nfct-pins-as-gpios;

};

Please confirm that this is the correct way to do this on nrf54l - the dts yaml file talks only about nrf52 and nrf53 chips.

If so - are there any other pins that need this kind of "let me be normal gpio pin" treatment except for reset pin?

2. I found the above setting while reading about UICR.

- Is this setting persistent on nrf54l or does the appropriate startup code need to be executed every time?

- Is there any high(er) level API for using FICR and UICR or are NRF_UICR and NRF_FICR structs the way to go?

BR, Marko

RE: PDM microphone on nrf54l15

Marko_P — Thu, 04 Sep 2025 11:27:44 GMT

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

RE: PDM microphone on nrf54l15

Vidar Berg — Thu, 04 Sep 2025 09:27:42 GMT

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