[SAADC] Zephyr's SAADC Driver (DMA Design choices analysis)

[Pawel(embeddedsolutions.pl)]

Setup:
NCS v2.8.0
nRF52840

Hi,

I would like to ask about the implementation of Zephyr's SAADC driver (`adc_nrfx_saadc.c`).

In the project I'm working on, I need to sample the ADC at 16 kHz.
However, I don't need to process every individual sample in the application. Instead, I would prefer to collect around 320 samples (equivalent to 20 ms of data) and then raise an interrupt. DMA seems like a perfect fit for this use case.

Initially, I thought that by using the `struct adc_sequence_options`—specifically the `extra_samplings` field—the SAADC driver would configure the `RESULT.MAXCNT` register to match the total number of samples (including `extra_samplings`), and thus trigger an interrupt only after all samples had been collected.

However, this does not appear to be the case. For each `adc_sequence` triggered by `adc_read()` or `adc_async_read()`, the driver calls `nrf_saadc_buffer_init()` with a buffer size (`RESULT.MAXCNT`) equal only to the number of channels selected for that sequence.
In my case, with a single channel, this results in `RESULT.MAXCNT = 1`, which causes the SAADC interrupt to fire after each individual sample—even if `extra_samplings` is set to a higher value.

This means the MCU is being interrupted every 62.5 μs, which is far too frequent for my application.

**My questions are:**

0. Am I correct in my understanding above?

1. Why was this design decision made in the SAADC driver implementation? What benefit does it provide?

2. What would be the best way to extend the SAADC driver to fully support single-channel continuous conversion mode, without needing to fork the driver?

3. What happens if an ADC sequence is triggered while another is still running? Will the previous sequence be aborted?

I look forward to your response.

Best regards,
Pawel

[Pawel(embeddedsolutions.pl)]

Hi again, 

I am back here to share thoughts and decisions I finally made. 

I ended up with implementing NRFX_SAADC_PPI driver compatible with Zephyr's ADC API.
The driver utilizes the NRFX_SAADC, NRFX_TIMER and NRFX_GPPI APIs.
SAADC works in scan mode with double-buffering. All defined channels are sampled with specified frequency.
Users of the driver pull samples associated with requested channel/channels.

It is important to know which timer instances are used by other components of the system.
On my side it turned out that I was using the same timer instance as BLE controller, so I had to change it to different one.
Next challenge I encountered was validation of acquisition time, resolution, oversampling during build-time via preprocessor macros.
I failed at this point and the validation is done on runtime at initialization.

Thank you all for pointing me into right direction