SAADC Multiple Channel Sampling and Reset with PPI

[Jeffrey Haynes]

I have a situation where I need to sample from multiple channels using SAADC on a timer (similar to maximum performance example).  However, the added catch is that at the end I want to use a timer to reset an external circuit.  I have it somewhat working but I'm currently using the RESULTDONE event to trigger the reset.  I've noticed that event gets fired multiple times, once for each channel.  The documentation is slightly vague on this point but it looks like that is expected behavior.  Is there an event I can use to tell me when ALL channels have finished?

[JyriLehtinen]

Have you tried using EVENTS_END instead?

// Enable interrupt for buffer full
NRF_SAADC_NS->INTENSET = SAADC_INTENSET_END_Set << SAADC_INTENSET_END_Pos;
NRF_SAADC_NS->INTEN |= SAADC_INTEN_END_Enabled << SAADC_INTEN_END_Pos;
NRF_SAADC_NS->EVENTS_END = 0;

NVIC_EnableIRQ( SAADC_IRQn );

[Hieu]

I agree that using EVENTS_END would help you achieve the desired result here. Please just remember that EVENTS_END is generated when the buffer is full, so the RESULT.MAXCNT need to be set to the appropriate value for the END event to be generated at the timing you desire.

[JyriLehtinen]

Simple approach would be to set the buffer large enough to collect multiple samples, then calculate the average per channel after EVENTS_END.

[Jeffrey Haynes]

Is there an example of this being done?  I haven't seen an example with multiple channels and multiple samples and so far I haven't gotten it to work.

[Jeffrey Haynes]

I got it working.  I did have to "overclock" the timer driving the SAADC to get the desired behavior.  Hopefully that's right :)

[JyriLehtinen]

There's the internal ADC @16 MHz which can be used to sample at 8 kHz or higher, or you can use a PPI  task for lower or more accurate timing.

I think here's some code (I haven't tested this) that uses the internal timer:
modules/saadc/saadc.ino · 5f44ca1fab29243f3871e2a8ddfa1851a8fbc384 · pub / hello-world / nrf52 · GitLab

[JyriLehtinen]

Once the buffer is full, I'd "unzip" it into arrays per channel, then do whatever math you want, like taking the average:

// Assuming int16_t sample_buffer[3*8] is the EasyDMA buffer for SAADC,

// 3 channels with 8 samples each.
int16_t adc_ch_buffer[3][8];

// Split ADC buffer into individual channels
for (int i = 0; i < 8; i++) {
    for(int j=0; j < 3; j++) {
        adc_ch_buffer[j][i] = (*sample_buffer)[i*3+j]);
    }
}