Using PDM causes BLE connection to drop

Question

We have a custom board using a nRF52840 module and a PDM peripheral. Some of our recently produced boards are experiencing issues with dropped BLE connections when we turn on PDM. The boards continue to work otherwise and the PDM signals the boards are receiving are fine. There are no issues with BLE communication as long as PDM is not running. Our firmware is based on nRF5 SDK 15 and we are using the NRFX PDM driver. 
 On the affected boards , the behavior seems to be temperature dependent. We can communicate over BLE while PDM is running at higher temperatures of >35°C, but as we lower the temperature, we start having BLE communication issues. Once a board starts experiencing the issue, the radio stops working completely over the span of about 3°C. Different boards encounter the issue at different temperatures, and most of them are fine all the way down to 0°C which is where we stopped testing. 
 We're using pins 0.24 and 0.25 for PDM, which don't have any high frequency restrictions in the datasheet. Also, we have found that if we remove the PDM peripheral from an affected board, the BLE issue doesn't occur anymore even if we turn on PDM. 
 I tried searching the forum for similar issues, but the closest I was able to find was this one, where BLE was causing problems with the PDM signal. We have the inverse issue. https://devzone.nordicsemi.com/f/nordic-q-a/53823/pdm-conflict-with-bluetooth-transceiver

brianatcarrot · Accepted Answer

Here's an update for anybody who stumbles across this post in the future. We were unable to figure out the root cause of the problem. However, we did figure out two workarounds. 
 Workaround one is moving your PDM peripheral to a different pin seems to fix it. That requires hardware changes though, which isn't always possible. 
 Workaround two can be done entirely in firmware and takes advantage of the fact that the temperature where the BLE communication issues start is apparently dependent on the PDM clock rate. Higher clock rates generally decreased the temperature where we started seeing the issue, but it wasn't universal. That means that if your application can handle using a different PDM clock rate, you might be able to get the temperature cutoff to outside of your operating range. Downsampling or changing the PDM decimation ratio can help if you're going to a higher clock rate. The other way to take advantage of this is for screening your units in manufacturing. If you can find a PDM clock rate that's affected at higher temperatures than the rate you use in the field, you can use that for screening at room temperature and be pretty sure you caught anything that would be affected at cold temperatures.

Using PDM causes BLE connection to drop

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