SAADC Multiple Channel Sampling and Reset with PPI

RE: SAADC Multiple Channel Sampling and Reset with PPI

JyriLehtinen — Mon, 03 Feb 2025 08:56:35 GMT

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]);
    }
}

RE: SAADC Multiple Channel Sampling and Reset with PPI

JyriLehtinen — Mon, 03 Feb 2025 08:49:16 GMT

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

RE: SAADC Multiple Channel Sampling and Reset with PPI

Jeffrey Haynes — Sat, 01 Feb 2025 18:39:19 GMT

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

RE: SAADC Multiple Channel Sampling and Reset with PPI

Jeffrey Haynes — Sat, 01 Feb 2025 15:55:20 GMT

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.

RE: SAADC Multiple Channel Sampling and Reset with PPI

JyriLehtinen — Fri, 31 Jan 2025 10:19:40 GMT

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

RE: SAADC Multiple Channel Sampling and Reset with PPI

Jeffrey Haynes — Thu, 30 Jan 2025 21:28:05 GMT

Ah, ok. Thank you for the explanation. I'll try that.

Just to clarify, I can't do oversampling in multiple channel mode, correct? That is to say, I can't oversample on multiple channels and average per channel.

And to extend that a bit, what would be the best way to average multiple samples across multiple channels?

RE: SAADC Multiple Channel Sampling and Reset with PPI

Hieu — Thu, 30 Jan 2025 21:25:28 GMT

Could you be confusing the DONE event and the END event? They are different events.

The Scan Mode documentation only says that the RESULTDONE and the DONE events have the same meaning when there is no oversampling.

You can think of the event in this way: DONE is generated when a sampling is done, and RESULTDONE is generated when all samples necessary for averaging a result is done. When you don't use oversampling, averaging is done over just one sample, so DONE and RESULTDONE are generated at the same time.

Meanwhile, END is an entirely different event and is tied to result buffer. When EasyDMA has filled the configured buffer, END is generated.

RE: SAADC Multiple Channel Sampling and Reset with PPI

Jeffrey Haynes — Thu, 30 Jan 2025 20:52:24 GMT

Sorry, I clearly don't understand the different events.

My understanding was that when multiple channels are being sampled, this forces the SAADC into scan mode. In scan mode, the documentation states that END and RESULTSDONE have the same meaning. What am I missing?

RE: SAADC Multiple Channel Sampling and Reset with PPI

Hieu — Thu, 30 Jan 2025 20:40:45 GMT

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.

RE: SAADC Multiple Channel Sampling and Reset with PPI

JyriLehtinen — Thu, 30 Jan 2025 07:58:20 GMT

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 );