SSADC swapping scanning values depending on the speed

RE: SSADC swapping scanning values depending on the speed

Vidar Berg — Tue, 03 Jan 2023 11:52:30 GMT

Happy new year!

Thanks for the update. I think I know what is happening now: the TIMER will continue to trigger the SAADC sample task even if the application has been prevented from processing the NRF_DRV_SAADC_EVT_DONE event and provided the SAADC with a new DMA sample buffer, same as the scenario Jørgen describes in this post: RE: Offset in SAADC samples with Easy DMA and BLE .

1. This seems to be a side effect of not following the task and event order outlined in the product specification. I.e. if the SAMPLE task gets triggered before the next DMA buffer has been supplied and the start task has been triggered by nrf_drv_saadc_buffer_convert().

2. It will do both. But it requires that you have run the nrf_drv_saadc_buffer_convert() first to prepare the SAADC for sampling.

3. Sequentially.

4. Yes, the callback is invoked when the sample buffer has been filled.

5. Yes, it will trigger the NRF_DRV_SAADC_EVT_DONE (assuming a single conversion will fill the sample buffer).

RE: SSADC swapping scanning values depending on the speed

Yano — Mon, 02 Jan 2023 12:49:03 GMT

Hi Vidar,

Happy new year!

No we're not using that.

However I think we've found the issue.

We setup a test array and on each saadc call back we filled the array with the adc buffer.

When analysing what we saw was that the buffer skip happened on the 6th call back.

We then looked at a trace (see below), the top line indicates timer interrupts and the next two i2c operation in the saadc callback. As you can see there was a period after the 5th where there is no call back.

It turns out this is BLE setup, as we set up the timer and ADC before BLE setup.

Moving it fixed the issue.

We still have a few questions:

1. Why would the BLE blocking shift the SAADC buffer pointer?

2. Does nrfx_saadc_sample() only start the ADC acquisition (not conversion)?

3. If there are multiple channels are they all acquired at the same time or sequentially?

4. Is the ADC callback called when the acquisition is completed?

5. When we call nrfx_saadc_sample() to convert is there a way to know when its completed?

Thanks

RE: SSADC swapping scanning values depending on the speed

Vidar Berg — Mon, 02 Jan 2023 12:01:31 GMT

Hello,

As far as I am aware, the only known issue which can lead to this behavior you describe is if you use the calibrate offset task when the SAADC is active ([252] SAADC: Unexpected behavior when TASKS_CALIBRATEOFFSET is used during sampling). Are you doing calibration anywhere in your code outside of the saadc_callback()?

Thanks,

Vidar