topic rt685 HiFi4, erratic DSP code bahaviour when loaded from MCU in i.MX RT Crossover MCUs

rt685 HiFi4, erratic DSP code bahaviour when loaded from MCU

witali_z — Mon, 30 Jun 2025 13:08:08 GMT

Hi,
I am working on a audio processing demo that runs on a R685-AUD-EVK. I have followed the getting started resources, the MCU and DSP FW is mostly based on "audio_demo_bm" from the SDK.

MCU FW only does basic initialization of GPIOS for a simple user interface.
DSP FW intializes the I2S and DMA for audio input/output, data is buffered using the "triple buffer" approach as suggested in one of the NXP application notes.
The actual audio processing function is called from the I2S/DMA receive ISR once 1ms of audio data (i.e. 48 left and 48 right samples) has been received.
Processing of audio data uses the natureDSP library functions.

So far everything runs as expected when MCU FW and DSP FW are built and run in Debug mode, but I am observing issues when the DSP FW is loaded from the MCU (Release build, DSP_IMAGE_COPY_TO_RAM = 1).

When the audio processing function (called from the ISR) is placed in ".iram0.text" (instruction TCM memory) the DSP FW runs into an exception.
xt-ocd prints:

Error: Exception details: exccause:0x0, excvaddr:0xff0042, epc:0x240205bc, epcdbg:20e022

This does not happen if the function is placed in non-TCM memory, or if the DSP FW is loaded in debug mode.

Is there anything to look for that may explain the observed issue?
Could this be related to alignment in memory?

Re: rt685 HiFi4, erratic DSP code bahaviour when loaded from MCU

Omar_Anguiano — Fri, 04 Jul 2025 20:51:47 GMT

Alignment issue could be a potential issue, I see that this does not happen if it is built for debug.
When building for release, the compiler may add some optimization levels, which can affect how the example works.
If the MCU doesn't properly copy .iram0.text or align it, the DSP may fetch invalid instructions—leading to the exccause: 0x0 (illegal instruction).

Best regards,
Omar

Re: rt685 HiFi4, erratic DSP code bahaviour when loaded from MCU

witali_z — Mon, 21 Jul 2025 08:23:09 GMT

@Omar_Anguiano thank you for your reply.

I found out, that the post_build script does not copy the ".iram0.text" section into the binary file which is included into the MCU code, so the actual function code was not present in the memory.
I have not looked further into alignment issue, as I can currently continue to work with "debug" mode.

BR Witali