SAADC oversample with burst configuration

[Gianlucamilani]

Dear DevZone,

I am trying to configure the SAADC on my custom nRF52840 board.

I would like to use it in blocking mode together with oversample 16x and burst enabled.

I started from the example provided in the SDK, and I firstly removed all the parts related to non-blocking mode.

Then I modified the parameters to set oversample and burst mode. Here the steps I performed:

• I modified the configuration struct to enable burst in this way:

#define NRFX_SAADC_DEFAULT_CHANNEL_CONFIG_SE(PIN_P) \
{ \
.resistor_p = NRF_SAADC_RESISTOR_DISABLED, \
.resistor_n = NRF_SAADC_RESISTOR_DISABLED, \
.gain = NRF_SAADC_GAIN1_6, \
.reference = NRF_SAADC_REFERENCE_INTERNAL, \
.acq_time = NRF_SAADC_ACQTIME_3US, \
.mode = NRF_SAADC_MODE_SINGLE_ENDED, \
.burst = NRF_SAADC_BURST_ENABLED, \ //enabled burst mode to automatically acquire 2^oversample samples with one request
.pin_p = (nrf_saadc_input_t)(PIN_P), \
.pin_n = NRF_SAADC_INPUT_DISABLED \
}

• I modified the following value in the sdk_config file:

// <0=> Disabled
// <1=> 2x
// <2=> 4x
// <3=> 8x
// <4=> 16x
// <5=> 32x
// <6=> 64x
// <7=> 128x
// <8=> 256x

#ifndef NRFX_SAADC_CONFIG_OVERSAMPLE
#define NRFX_SAADC_CONFIG_OVERSAMPLE 4
#endif

Is the procedure correct? Am I missing something? Do you think there could be any issues related to the buffer size if I use the blocking mode (nrfx_saadc_sample_convert function)?

Thank you very much in advance,

best regard,

Gianluca Milani

[Jared]

Hi,

It seems correct for blocking mode. However, what is your intention of using blocking mode instead of non-blocking? Are you using PPI to connect the SAMPLE task with the COMPARE event as in the example? That could be a bit problematic as it could lead to the buffer swap issue.

regards

Jared 

[Gianlucamilani]

Dear Jared, 

thank you very much for your answer.

I would use it without any PPI, I am initializing the saadc without any instructions in the callback handler:

void saadc_callback_handler(nrf_drv_saadc_evt_t const * p_event)
{
  //Empty
}


void saadc_init(void)
{

  ret_code_t err_code;

  // Create a config struct and assign it default values along with Pin number for ADC input
  //
  // Chosen paramters:
  //
  //    - Internal reference
  //    - Gain 1/6
  //    - Acquisition time 3us
  //    - Single ended
  //
  // The reasons for the choice of these parameters values can be found at the following link:
  //
  // https://devzone.nordicsemi.com/nordic/nordic-blog/b/blog/posts/measuring-lithium-battery-voltage-with-nrf52

  // Configure the input as Single Ended (One Pin reading)


  nrf_saadc_channel_config_t  channel_config = NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN5);


  // Initialize the saadc

  err_code = nrf_drv_saadc_init(NULL, saadc_callback_handler);
  APP_ERROR_CHECK(err_code);

  // Initialize the channel which will be connected to that specific Pin
  // The selected channel is 0

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




}

and I am triggering the saadc conversion only when I need it, using:

 nrfx_saadc_sample_convert(0, &adc_val);

Am I doing the wrong steps if I want to implement oversampling?

Thanks,

Best regards,

Gianluca