SAADC double buffering not working

[Andy]

Hi,

I'm setting up SAADC as explained in the examples: First start conversions with buffer 0, then with buffer 1. On the SAADC callback I queue the current buffer for conversions again.

I'm working on an NRF52832 using SDK15.2.

NRF_LOG_INFO("ADC samples[0] pointer = %p", &m_adc_samples[0]);
NRF_LOG_INFO("ADC samples[1] pointer = %p", &m_adc_samples[1]);

// Start conversions with buffer 0
err_code = nrf_drv_saadc_buffer_convert((nrf_saadc_value_t *)(&m_adc_samples[0]), get_adc_sampling_size());
APP_ERROR_CHECK(err_code);

// Queue conversions with buffer 1
// Should start immediately after buffer 0 is full
err_code = nrf_drv_saadc_buffer_convert((nrf_saadc_value_t *)(&m_adc_samples[1]), get_adc_sampling_size());
APP_ERROR_CHECK(err_code);

static void saadc_callback(nrf_drv_saadc_evt_t const * p_event) {
	if (p_event->type == NRF_DRV_SAADC_EVT_DONE) {

		static uint32_t sampling_count = 0;
		if ((sampling_count % SAADC_CALIBRATION_INTERVAL) == 0) {
			saadc_calibrate();
		}
		sampling_count++;

		NRF_LOG_INFO("Current buffer pointer = %p", p_event->data.done.p_buffer);
		
		// Queue conversions on the current buffer again
		// Conversions on the other buffer should be ongoing while we are in the callback
		ret_code_t err_code = nrf_drv_saadc_buffer_convert(p_event->data.done.p_buffer, get_adc_sampling_size());
		APP_ERROR_CHECK(err_code);

		// Do something with data here
	}
}

When I start the conversions I see the following output:

ADC samples[0] pointer = 0x20005EE4
ADC samples[1] pointer = 0x2000613C

And then on the callbacks I see this:

Current buffer pointer = 0x20005EE4
Current buffer pointer = 0x2000613C
Current buffer pointer = 0x2000613C
Current buffer pointer = 0x2000613C
.
.
.

I would expect the logs on the callbacks to look like this:

Current buffer pointer = 0x20005EE4
Current buffer pointer = 0x2000613C
Current buffer pointer = 0x20005EE4
Current buffer pointer = 0x2000613C
Current buffer pointer = 0x20005EE4
Current buffer pointer = 0x2000613C
.
.
.

What am I doing wrong? I read the documentation and a bunch of threads in the devzone but I still don't see my problem, maybe I have tunnel vision already.

Thanks for your help!

EDIT: After reading a couple more threads I realized I needed to provide more information. Apparently the issue has something to do with the way the START and SAMPLE tasks are triggered.

I'm sampling more than one channel and I'm using PPI to trigger the SAMPLE task when a COMPARE0 event happens on a timer. As far as I can see, the START task is triggered by the nrfx_saadc module automatically when the low power mode is not configured (my case). I'm having the buffering problem even on first initialization, no need to stop and start to get the problem.