RAK18000 PDM works standalone but PDM.read() returns 0 when Edge Impulse SDK is linked

WisBlock WisBlock Audio
1

Hi all,

Running into an issue with RAK18000 + Edge Impulse on RAK4631 and hoping someone has hit this before.

Setup

  • RAK4631 + RAK19007 + RAK18000 v.C (in IO slot)
  • Arduino IDE 2.3.8
  • BSP: RAKwireless nRF Boards 1.3.3
  • USB Stack: TinyUSB

Standalone PDM works

Following the RAK 18000 stereo mic example:

cpp

PDM.setPins(21, 4, -1);
PDM.setBufferSize(512);
PDM.onReceive(callback);
PDM.setGain(20);
PDM.begin(2, PCM_16000);

Result: ISR fires at 64 Hz, PDM.available() returns valid byte count, audio data captured (RMS varies with input).

Fails when Edge Impulse SDK linked

Same exact PDM init code, but with #include <my_project_inferencing.h> added:

  • ISR still fires (now at 127 Hz, double rate)
  • PDM.available() returns 0 every call
  • PDM.read() returns 0 bytes
  • Buffer never fills

What I’ve tried

  • Multiple buffer sizes (512–4096)
  • Different gain values
  • PDM.end() before re-init
  • Capture-then-classify pattern (avoiding continuous classifier)
  • Both run_classifier() and run_classifier_continuous()
  • Bypassing PDM.available() and reading fixed buffer size — still returns 0

Static buffer test confirms model works

Edge Impulse static_buffer example compiles and runs inference correctly on RAK4631 (5 ms inference, correct classification on test data). So the SDK + BSP base layer work — the issue is specifically live PDM capture when EI SDK is linked.

Diagnostic output

ISR/s:127 | Avail0:1263 Avail+:0 | LastSamples:0 | BufCount:0/16000

ISR firing, but PDM reports zero data available continuously.

Questions

  1. Has anyone successfully run RAK18000 + Edge Impulse on BSP 1.3.3?
  2. Is there a known interaction issue between BSP PDM library and Edge Impulse’s nRF52 porting layer?
  3. Is a newer BSP version available that addresses this? (Boards Manager only shows 1.3.3)
  4. Recommended path for audio classification on RAK4631 — should I be using nrfx_pdm directly, or external I2S mic?

Happy to share full diagnostic sketches if useful.
Thanks!