About SAADC, Easy-DMA and channels (nRF5_SDK)

[Jarmo]

There's an example saadc_pca10036 in the SDK. I am wondering how this piece of code works in that example:

void saadc_init(void)
{
    ret_code_t err_code;
    nrf_saadc_channel_config_t channel_config =
            NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN0);
    err_code = nrf_drv_saadc_init(NULL, saadc_callback);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_channel_init(0, &channel_config);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_buffer_convert(m_buffer_pool[0],SAMPLES_IN_BUFFER);
    APP_ERROR_CHECK(err_code);
    
    err_code = nrf_drv_saadc_buffer_convert(m_buffer_pool[1],SAMPLES_IN_BUFFER);
    APP_ERROR_CHECK(err_code);
}

Specifically, why there are two calls of same function, nrf_drv_saadc_buffer_convert()? Does it set some kind of double-buffering?

In my case, I have four ADC-inputs I want to sample 200 times per second each. How should I configure the SAADC subsystem?

EDIT: Also, how do I know which channel was triggered when I get into the callback (below)?

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

[Jarmo]

It is still unclear for me how to set-up several channels. For example, if I need two channels, the code is maybe something like this:

#define CHANNELS       2
#define BUFFERSIZE   128
int16_t Buffers[CHANNELS][2][BUFFERSIZE];

void init_adc(void)
{
    nrf_drv_saadc_init(NULL, saadc_callback);

    // set up first channel
    nrf_saadc_channel_config_t channel_config =
         NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN0);
    nrf_drv_saadc_channel_init(0, &channel_config);
    // set-up double buffering for the first channel
    nrf_drv_saadc_buffer_convert(Buffers[0][0], BUFFERSIZE);
    nrf_drv_saadc_buffer_convert(Buffers[0][1], BUFFERSIZE);

    // set up second channel
    nrf_saadc_channel_config_t channel_config =
        NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN1);
    nrf_drv_saadc_channel_init(1, &channel_config);
    // set-up double buffering for the second channel
    nrf_drv_saadc_buffer_convert(Buffers[1][0], BUFFERSIZE);
    nrf_drv_saadc_buffer_convert(Buffers[1][1], BUFFERSIZE);
}

And the event handler:

static
  void saadc_callback(nrf_drv_saadc_evt_t const * p_event)
  {
      if (p_event->type == NRF_DRV_SAADC_EVT_DONE)
      {
          nrf_drv_saadc_buffer_convert(p_event->data.done.p_buffer, BUFFERSIZE);
          // TODO: How do I know which of the two channels was triggered?
      }
  }

As said in the TODO-line, how do I know which of the two channels was triggered? Do I need to compare the pointer p_event->data.done.p_buffer against the pointers I set earlier with the call to *_buffer_convert() at my init_adc()?

[Øyvind Karlsen]

When you send the sample command you will get data for the channel you triggered in the correct buffer, if scan mode is enabled you will get data on all channels. If you want to trigger an event when the SAADC channel goes beyond or below a certain level, have a look at limit detection which is described in the documentation for the driver.

[Jarmo]

Thanks for comment. How do I enable scan mode?

[Jarmo]

If scan mode is on, is there more than those two buffers in use? Or does the easy-DMA put the samples of enabled channels in a row into the same single buffer? I mean like this way:

ADC0[0],ADC1[0],ADC2[0],ADC3[0] .. ADC0[1],ADC1[1],ADC2[1],ADC3[1] .. ADC0[2],ADC1[2],ADC2[2],ADC3[2] ..

So, the memory layout is "interleaved" with all the enabled channels?

[Jarmo]

To answer my own question above: No, there is only those two buffers. And yes, it looks like the channels are interleaved into those buffers.