SAADC scan mode sample order is not always consistent

[jprofit]

I am seeing a problem using the nRF52 SAADC with two analog channels enabled in scan mode. Essentially, channel 0 is measuring an analog signal with gain ~= 15 while channel 1 is measuring the same signal with gain = 1.

The ADC is sampled in scan mode at 4kHz into a buffer that holds 2048 samples for each channel. When I receive the interleaved buffer, I usually see that the channel 0 ADC data is at address offset 0, 4, 8, etc and the channel 1 data is at address offset 2, 6, 0xa, etc.

Unfortunately, sometimes these channels are swapped. (ie. the channel 0 data is at address offset 2, 6, 0xa,... and the channel 1 data is at address offsets 0, 4, 8, ....).

Is there anything I can do to ensure that channel 0 data starts at offset 0 and channel 1 data starts at offset 2?

Here is a printf from the code that shows the peak value of each channel as well as the buffer address:

Capture 1:

base_ptr=0x20003028, peak=1363.000000

base_ptr=0x2000302a, peak=86.000000

Capture 2:

base_ptr=0x20005028, peak=1342.000000

base_ptr=0x2000502a, peak=96.000000

Capture 3:

base_ptr=0x20003028, peak=92.000000

base_ptr=0x2000302a, peak=1352.000000

As you can see, the first two captures have the higher gain signal in the 0 offset position as expected. However, the third capture has the lower gain signal in the 0 offset position.

Is there something I need to do to the EasyDMA module to reset between captures? Currently, I am just calling nrf_drv_saadc_buffer_convert() to start a new capture.

I am using SDK version 11.0.0 and S132 version 2.0.1

[Olaf]

I would like to share my findings on this issue. When using UART and SAADC at the same or different IRQ levels, I was experiencing this issue reproducible during startup. It happened always after a SAADC calibration while UART was processing a lot of data.

Changing the IRQ priority did not help. Neither for UART nor SAADC. The -DNRF52_PAN_28 did not help as well.

What solved the issue for me was using this sequence to stop the ADC before a next calibration:

void stop_adc()
{
	nrf_drv_saadc_uninit();
	nrf_drv_ppi_uninit();
	nrf_drv_timer_uninit(&m_timer);

	while (nrf_drv_saadc_is_busy())
		;

}

[jakub]

Indeed this solution solves the problem, Thank you Olaf for sharing that.

[Hemabas]

hey olaf,

can you please elaborate to where exactly this piece of code needs to be called.

thanks