STM32H7 DFSDM – Best way to read two microphones with one filter?

[MannyMarc35]

Hi everyone,

I'm working on an audio acquisition project using the STM32H743ZIT6 microcontroller. The goal is to build a microphone array with 8 digital MEMS microphones (MP34DT05-A), connected via DFSDM and streamed over USB to a PC using the USB Audio Class (UAC). At the moment, I'm starting small and testing with one or two microphones.

So far, I have successfully configured one DFSDM filter and one channel using regular conversion with DMA, and I'm receiving valid audio data.

Now I would like to add a second microphone, which is connected to a second DFSDM channel (with opposite clock edge). Both channels should ideally be handled by the same DFSDM filter.

I'm currently wondering:

• What is the best approach to sample two channels using a single DFSDM filter?

• Is it necessary or recommended to use injected conversion for the second channel in this case?

• How would I properly configure DMA and interrupts if both regular and injected conversions are used?

• Are there any known working examples for this kind of setup?

Any tips, experiences, or suggestions would be greatly appreciated!

Thanks in advance

[Saket_Om]

Hello @MannyMarc35 

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@MannyMarc35 wrote:

• What is the best approach to sample two channels using a single DFSDM filter?

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The best approach is to use the injected conversion with scan conversion enabled. 

 

[MannyMarc35]

Hi @Saket_Om ,

thanks for your reply.

I understand that both channels can be configured by using regular and injected conversion and triggered sequentially using one trigger event. However, I have a few open questions before proceeding:

 

• Which trigger is most suitable for injected conversions in this setup?
  I see options like TIM1_TRGO, but I'm unsure whether that's the best choice or if I should use software trigger (trigger from regular conversion)

• Is DMA supported for injected scan conversions?
  If yes, does it automatically copy both values (one per channel) into the buffer? Or would I need to manage the transfers manually (e.g. by reading via HAL_DFSDM_FilterGetInjectedValue() in the callback)?

• Do I need a separate DMA stream for injected conversions?
  Currently, regular conversion uses DMA1_Stream0. Can injected use the same stream, or must I set up an additional DMA handle?

• What’s the best way to synchronize the trigger and buffer handling?
  For regular conversions, the DMA transfer completes when the buffer is full. With injected conversions triggered periodically (e.g. by a timer), how should I design the timing so I don’t miss data or cause overlaps?

Of course, some kind of example would be best, but I appreciate any help. Thanks in advance.

 

[Saket_Om]

Hello @MannyMarc35 

For audio applications, it is recommended to use the regular mode, which is typically used for continuous conversions from a single channel.

 

Please refer to the document below for more details:

STM32L4_System_DFSDM.pdf

In your application, you need to configure 8 channels and 8 filters and then synchronize all the filters with filter 0. This allows all conversions to start simultaneously.

To use DMA, you also need to configure 8 DMA streams, one for each filter.

Please refer to the example project: Projects/STM32H743I-EVAL/Examples/DFSDM/DFSDM_AudioRecord,
which demonstrates the use of 2 DFSDM channels, 2 filters, and 2 DMA streams. To add an additional DFSDM channel, you need to configure it in the same way as filter 1 in the example.

  

 

[MannyMarc35]

Hello @Saket_Om ,

So, is it not possible or recommended to use a filter for two channels (regular + injected conversion)?
My problem is that the STM32H743ZIT6 only offers four filters and eight channels. Therefore, I wanted to use a filter for two channels.