Understandin SAADC example

Question

Hello,

I am having trouble understanding the SAADC example in the description. Can anybody tell me what do the following instructions in the code do?

1. NRF_SAADC->SAMPLERATE = SAADC_SAMPLERATE_MODE_Task << SAADC_SAMPLERATE_MODE_Pos; /* does this mean that the sampling of the input voltage to the analog pin will only happen when I set a certain bit?*/

2. NRF_SAADC->TASKS_SAMPLE = 1; // does this instruction make the sampler sample the input voltage?
while (NRF_SAADC->EVENTS_END == 0); // what is this for?
NRF_SAADC->EVENTS_END = 0; // what is this for?

3. NRF_SAADC->TASKS_START = 1; //why do i need these lines of code since i already enabled the SAADC? do I need to call them every time I make a sample?
while (NRF_SAADC->EVENTS_STARTED == 0);
NRF_SAADC->EVENTS_STARTED = 0;

4. NRF_SAADC->TASKS_STOP = 1; //do i need to call these lines of code every time I make a sample?
while (NRF_SAADC->EVENTS_STOPPED == 0);
NRF_SAADC->EVENTS_STOPPED = 0;

5. __WFE(); //what does the waiting for event macro do in this example?

6. NRF_SAADC->RESULT.MAXCNT = 1; //what does this line do?
NRF_SAADC->RESULT.PTR = (uint32_t)&result; //why doesn't the result change its value, since I tried to print the value of it when I connect P0.02 to GND and then to VDD?

#include <nrf.h>

int main(void)
{
  volatile int16_t result = 0;
  volatile float precise_result = 0;

  // Start HFCLK from crystal oscillator, this will give the SAADC higher accuracy
  NRF_CLOCK->TASKS_HFCLKSTART = 1;
  while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0);
  NRF_CLOCK->EVENTS_HFCLKSTARTED = 0;

  // Configure SAADC singled-ended channel, Internal reference (0.6V) and 1/6 gain.
  NRF_SAADC->CH[0].CONFIG = (SAADC_CH_CONFIG_GAIN_Gain1_6    << SAADC_CH_CONFIG_GAIN_Pos) |
                            (SAADC_CH_CONFIG_MODE_SE         << SAADC_CH_CONFIG_MODE_Pos) |
                            (SAADC_CH_CONFIG_REFSEL_Internal << SAADC_CH_CONFIG_REFSEL_Pos) |
                            (SAADC_CH_CONFIG_RESN_Bypass     << SAADC_CH_CONFIG_RESN_Pos) |
                            (SAADC_CH_CONFIG_RESP_Bypass     << SAADC_CH_CONFIG_RESP_Pos) |
                            (SAADC_CH_CONFIG_TACQ_3us        << SAADC_CH_CONFIG_TACQ_Pos);

  // Configure the SAADC channel with VDD as positive input, no negative input(single ended).
  NRF_SAADC->CH[0].PSELP = SAADC_CH_PSELP_PSELP_VDD << SAADC_CH_PSELP_PSELP_Pos;
  NRF_SAADC->CH[0].PSELN = SAADC_CH_PSELN_PSELN_NC << SAADC_CH_PSELN_PSELN_Pos;

  // Configure the SAADC resolution.
  NRF_SAADC->RESOLUTION = SAADC_RESOLUTION_VAL_14bit << SAADC_RESOLUTION_VAL_Pos;

  // Configure result to be put in RAM at the location of "result" variable.
  NRF_SAADC->RESULT.MAXCNT = 1;
  NRF_SAADC->RESULT.PTR = (uint32_t)&result;

  // No automatic sampling, will trigger with TASKS_SAMPLE.
  NRF_SAADC->SAMPLERATE = SAADC_SAMPLERATE_MODE_Task << SAADC_SAMPLERATE_MODE_Pos;

  // Enable SAADC (would capture analog pins if they were used in CH[0].PSELP)
  NRF_SAADC->ENABLE = SAADC_ENABLE_ENABLE_Enabled << SAADC_ENABLE_ENABLE_Pos;

  // Calibrate the SAADC (only needs to be done once in a while)
  NRF_SAADC->TASKS_CALIBRATEOFFSET = 1;
  while (NRF_SAADC->EVENTS_CALIBRATEDONE == 0);
  NRF_SAADC->EVENTS_CALIBRATEDONE = 0;
  while (NRF_SAADC->STATUS == (SAADC_STATUS_STATUS_Busy <<SAADC_STATUS_STATUS_Pos));

  // Start the SAADC and wait for the started event.
  NRF_SAADC->TASKS_START = 1;
  while (NRF_SAADC->EVENTS_STARTED == 0);
  NRF_SAADC->EVENTS_STARTED = 0;

  // Do a SAADC sample, will put the result in the configured RAM buffer.
  NRF_SAADC->TASKS_SAMPLE = 1;
  while (NRF_SAADC->EVENTS_END == 0);
  NRF_SAADC->EVENTS_END = 0;

  // Convert the result to voltage
  // Result = [V(p) - V(n)] * GAIN/REFERENCE * 2^(RESOLUTION)
  // Result = (VDD - 0) * ((1/6) / 0.6) * 2^14
  // VDD = Result / 4551.1
  precise_result = (float)result / 4551.1f;
  precise_result; // to get rid of set but not used warning

  // Stop the SAADC, since it's not used anymore.
  NRF_SAADC->TASKS_STOP = 1;
  while (NRF_SAADC->EVENTS_STOPPED == 0);
  NRF_SAADC->EVENTS_STOPPED = 0;


  while (1)
  {
    __WFE();
  }
}