NRF Connect SDK DMIC Sample

[granfaloon]

I am trying to interface a PDM microphone to a NRF52840 using NRF Connect SDK v2.9.0. Does anyone have a good idea of how to get the DMIC sample to work on the NRF52840? The sample seems to run without error regardless if the pdm mic is connected or not to the 52840. I am trying to understand the behavior. Unfortunately, the documentation for this sample is lacking in my opinion. Any help would be appreciated, thanks!

[Priyanka]

Hi,

I am not sure I understand the issue well. Could you be more specific?

Here is the hardware connections for a PDM mic:

https://docs.nordicsemi.com/bundle/ps_nrf52840/page/pdm.html#ariaid-title6 

Regards,

Priyanka

[granfaloon]

Hi, thanks for the response.

I have a PDM mic connected to a NRF52840DK. I set the pins in the overlay as such:

PDM CLK - P1.06

PDM DIN - P1.07

I am running the dmic sample application at zephyr/samples/drivers/audio/dmic with SDK v2.9.0.

Line 129 of the sample calls the function do_pdm_transfer()

The function do_pdm_transfer() does 8 block reads per channel on Line 56 where dmic_read() is called.

This function call is always successful, regardless if I connect the mic or disconnect it. This is why I am confused. There is a return condition for this function, yet it never generates an error even if the mic is disconnected. Any idea why? I would think it would return a value less than 0 for an error condition if the mic is not present. It makes it difficult to gauge whether or not I am reading the mic correctly.

Thanks. 

[Priyanka]

Hi,

This has been noticed previously as well. The thing is, you always see it as successful irrespective of a mic, is because the PDM interface is still functioning correctly from a hardware perspective - it's generating the clock and sampling the input pin, regardless of whether a valid microphone is connected. The PDM module in the nRF52840 is designed to continuously sample the input pin and convert the PDM stream into PCM samples using its digital filters.

When no microphone is connected, it's still sampling whatever electrical state is present on the input pin (which could be noise, floating state, or pulled to a specific level by internal resistors), i.e. The dmic_read() function primarily checks if data is available in the buffer, but it might not verify if that data is meaningful or just noise.
To verify if your microphone is working correctly, you should examine the actual data content rather than relying on error codes. Say, you could analyse your buffer for any meaningful data, or you could try visualizing the data ( Audacity) to see if there's a pattern.

Regards,

Priyanka