nRF5340 PDM frequency range supported?

[responsible_duck]

Hi,

We are having trouble configuring our PDM peripheral to operate at 48 kHz. THe nRF5340-specific documentation on the PDM module has equations suggesting that 48 kHZ is possible with a decimation ratio of 64 at 3.072 MHz. However, there is no supported range explicitly specified. We are seeing limitations in the system where we can only record up to 24 kHz with a stereo PDM configuration. Our tests seem to point to a system limitation but we cannot find anything in the code. 

We found this post from more than 8 years ago where a Nordic engineer mentioned this for the nRF52832:

You are correct, the PDM peripheral is designed to work with higher frequencies. The reason this is not stated in the datasheet is because it has not been throughly tested on our side. For this reason we cannot guarantee that it will work with higher frequencies, especially across varying temperatures and voltages.

In another post related to the nRF52840 Nordic also mentioned the limit being 16 kHz.

Finally in a post related to nRF5340 the response from Nordic is ambiguous as to whether the system supports the desired frequency.  

If this is a driver limitation that is universally applied across Nordic systems but can be overridden somehow for the nRF5340, it would be essential to our use case. Of course we need to confirm that the nRF5340 hardware can indeed reliably support up to 48 kHz. 

Note: We are using nRF toolchain 2.5.1

Any help is appreciated thanks!

[Priyanka]

Hi Ever,
I will check internally regarding this and get back to you. 

Please expect a slight delay as we have the Easter holidays here in Norway and most of the staff are on vacation.

Regards,

Priyanka

[Priyanka]

Hi,

It looks like the required PDM clock frequency of 3.072MHz would be too fast for the "frequency doubler" to function when using APLL source. By setting FDBYPASS=1, it should be doable on the digital side, but don't think we can guarantee anything across varying temperatures and voltages.

-Priyanka

[Priyanka]

Hi,

I have inquired internally and will get back to you as soon as I hear from them.

-Priyanka

[Priyanka]

Hi,

Sorry for the delay caused. I have raised the issue once again with our internal team. I guess they missed the ticket somehow over large influx of cases during the holidays and vacation that were there. Once again, apologies for the inconvenience and I will keep you updated.

-Priyanka

[Priyanka]

Hi,

According to the discussion with the experts, it looks like we cannot reliably support the 48kHz.

-Priyanka

[responsible_duck]

What about 24 kHz? Seems doable?

[Priyanka]

Hi Ever,

The confusion has arisen from the product specification. So basically, from the documentation, it is mentioned that one of the main feature of PDM is 16kHz output sample rate:

I guess the equations confused you, or gave the idea that the legacy module is flexible for various frequencies. But it was actually designed for the following two combinations which always gives out 16kHz PCM:

So yes it might work for different combinations, but 16kHz id the one that we have designed, tested, guaranteed and qualified.

Hope this clears things.

Regards,

Priyanka