nRF52 SAADC burst sampling

[Andreas52]

I am evaluating the SAADC options and I'm looking for sampling one input at fastest speed. Easy DMA looks promising, but my function for SAADC communication hangs, if I set

  NRF_SAADC->RESULT.MAXCNT = 2;

With =1 everything works fine and I can get sample after sample. That is good for about 60kHz. When reading the specs it seems that you can set the pointer to an array of 16Bit integer and set MAXCNT to the desired amount of samples, but this does not work in my setup. 

int16_t anaVal() // int16_t *values
{
  // Serial.println("Inside hwCPUVoltage function.");
#if defined(NRF_SAADC)
  // NRF52:
  // Settings: 8Bit, AnalogInput4, 3us Time, internal ref., 1/6 gain, no oversample
  // Serial.println("This is an NRF_SAADC.");
  int32_t sample[5];
  NRF_SAADC->ENABLE = SAADC_ENABLE_ENABLE_Enabled << SAADC_ENABLE_ENABLE_Pos;
  NRF_SAADC->RESOLUTION = SAADC_RESOLUTION_VAL_8bit << SAADC_RESOLUTION_VAL_Pos;
  NRF_SAADC->CH[0].PSELP = SAADC_CH_PSELP_PSELP_AnalogInput4 << SAADC_CH_PSELP_PSELP_Pos;
  NRF_SAADC->CH[0].PSELN = SAADC_CH_PSELN_PSELN_NC << SAADC_CH_PSELN_PSELN_Pos;
  NRF_SAADC->CH[0].CONFIG = (SAADC_CH_CONFIG_BURST_Disabled << SAADC_CH_CONFIG_BURST_Pos) |
                            (SAADC_CH_CONFIG_MODE_SE << SAADC_CH_CONFIG_MODE_Pos) |
                            (SAADC_CH_CONFIG_TACQ_3us << SAADC_CH_CONFIG_TACQ_Pos) |
                            (SAADC_CH_CONFIG_REFSEL_Internal << SAADC_CH_CONFIG_REFSEL_Pos) |
                            (SAADC_CH_CONFIG_GAIN_Gain1_6 << SAADC_CH_CONFIG_GAIN_Pos) |
                            (SAADC_CH_CONFIG_RESN_Bypass << SAADC_CH_CONFIG_RESN_Pos) |
                            (SAADC_CH_CONFIG_RESP_Bypass << SAADC_CH_CONFIG_RESP_Pos);
  NRF_SAADC->OVERSAMPLE = SAADC_OVERSAMPLE_OVERSAMPLE_Bypass << SAADC_OVERSAMPLE_OVERSAMPLE_Pos;
  NRF_SAADC->SAMPLERATE = SAADC_SAMPLERATE_MODE_Task << SAADC_SAMPLERATE_MODE_Pos;

  NRF_SAADC->RESULT.MAXCNT = 1;
  NRF_SAADC->RESULT.PTR = (int32_t)&sample[0];  

  NRF_SAADC->EVENTS_STARTED = 0;
  NRF_SAADC->EVENTS_END = 0;
  NRF_SAADC->TASKS_START = 1;
  while (!NRF_SAADC->EVENTS_STARTED);
  NRF_SAADC->TASKS_SAMPLE = 1;
  while (!NRF_SAADC->EVENTS_END);

  return sample[0];

#else
  Serial.println("Unknown MCU!!");
  // unknown MCU
  return 0;
#endif
}

[Kenneth]

Hi,

If you set MAXCNT to 2, then you need to call SAMPLE task two times before END event will trigger. You may get DONE events for each sample.

Kenneth

[Andreas52]

Thanks for clarification. So, there is no automatic retrigger of a next sample task with a DONE event. Is there an example sketch available to see the maximum sample rate for one input channel? Do I need to wait for the DONE event before calling SAMPLE task again?

[Kenneth]

You can read of the different modes of operation here:
https://infocenter.nordicsemi.com/topic/com.nordic.infocenter.nrf52832.ps.v1.1/saadc.html#saadc_operationmodes

There is no automatic retrigger no, but I assume you can setup a timer to trigger the sample task for instance every 100us using ppi from a timer compare event -> sample task, and then if the saadc buffer is for instance 100bytes, you only need to handle the data every 100us * 50 = 5ms in the saadc callback handler (every sample take up 2bytes of the buffer). You should get 100kHz this way.

Figure 4 should show how this can be done, only thing missing from that figure is the ppi channel is triggering the sample task from a timer. The end event can trigger the saadc callback so you may handle the data and prepare a new pointer to next buffer that should be used by the saadc.

Kenneth