topic Re: RT595 FreeRTOS DMIC recording: FIFO/DMA buffer only first sample non-zero (bare-metal works fine in i.MX RT Crossover MCUs

RT595 FreeRTOS DMIC recording: FIFO/DMA buffer only first sample non-zero (bare-metal works fine)

Weitin — Thu, 25 Sep 2025 04:04:35 GMT

Hi community,

I am working on audio recording with DMIC + DMA on RT595 under FreeRTOS, but I ran into an issue:

When I read from FIFO_DATA, only the very first value may be non-zero. All following values are always 0.
Similarly, in the DMA buffer, only the first sample sometimes has a non-zero value, but all other samples are always 0.

My setup:

Environment: RT595 EVK, FreeRTOS
Interrupt priority follows configLIBRARY_MAX_SYSCALL_INTERRUPT_PRIORITY
DMIC clock and PDM_DATA signals are toggling, so input seems valid

Important note:
With the same configuration and same function call sequence, the DMIC + DMA works correctly on bare-metal (no RTOS). In that case, both FIFO_DATA and DMA buffer contain valid non-zero audio samples. The issue only happens under FreeRTOS.

Question:

Has anyone faced a similar issue with DMIC recording under FreeRTOS on RT595?
Could this be related to DMA configuration, cache/memory section, or DMIC FIFO handling in an RTOS context?

Any suggestions, hints, or working references would be greatly appreciated.

Thanks!

Re: RT595 FreeRTOS DMIC recording: FIFO/DMA buffer only first sample non-zero (bare-metal works fine

Sam_Gao — Thu, 25 Sep 2025 07:42:40 GMT

Hi,

I don't think this issue is from FreeRTOS, it seems like a sw bug from my side.

Would you please double check the functions running on both bare-metal or RTOS are 100% same?

Re: RT595 FreeRTOS DMIC recording: FIFO/DMA buffer only first sample non-zero (bare-metal works fine

Weitin — Fri, 26 Sep 2025 09:12:34 GMT

Hi Sam,

Thanks for your feedback.
I have already double-checked, and I can confirm that the functions running on both bare-metal and RTOS are 100% the same.
The same call flow works correctly in the bare-metal project, but in the FreeRTOS project the buffer data is always 0x00000000.

Do you think there could be any other project settings (for example in MCUXpresso IDE, such as linker script, memory regions, or compiler options) that might cause this kind of difference?

Thanks again for your suggestions.

Weitin

Re: RT595 FreeRTOS DMIC recording: FIFO/DMA buffer only first sample non-zero (bare-metal works fine

Sam_Gao — Tue, 30 Sep 2025 03:54:19 GMT

Hi,

If the logic code is the same, it seems only involves the RTOS scheduling configuration, you can try to modify the priority of RTOS.