Higher rate PDM not working

[Geoffr]

Hi,

We are trying to provide example support for our RAK4631 (nRF52840 based) and an ultrasonic PDM microphone (RAK18032). We have been able to get a sample frequency of 100 kHz at 16bits working, but not 200 kHz. We are trying to do this in Mono mode. At 200 kHz we don't seem to get any valid data (FFTs are completely wrong)

We are using the PDM library from Arduino which provides support for up to 200 kHz.

100 kHz PCM = 8000K PDM
200 kHz PCM = 16000K PDM

Has anyone been able to get this working? We need to get this working for customers trying to monitor wildlife (eg Bats).

If this has been confirmed to work I can post our sample code as necessary. 

Thanks!

[hmolesworth]

Perhaps use a debugger to see what the PDM library is setting the PDMCLKCTRL control register to, as 1600kHz is not listed as a recommended setting. However, it may work using this helpful information from a recent post:

nrf52-pdm-frequency-calculation posted by guilherme