Battery level measurement without SAADC

Use the following code to measure the battery voltage on the nRF52820 without any external components

It essentially implements a successive approximation of Vin using the COMP device with the internal VDDH/5 source and modifying the hysteresis threshold:

void comp_handler1(nrf_comp_event_t event)
{
}

uint32_t get_battery_voltage()
{
      uint32_t ladder = 0; //approximate from below
      uint32_t current = 32;
      nrfx_comp_config_t config = {
          .reference = NRF_COMP_REF_Int1V2, // use 1.2 V reference
          .ext_ref = NRF_COMP_EXT_REF_0,
          .main_mode = NRF_COMP_MAIN_MODE_SE,
          .threshold = {.th_up = ladder+1, .th_down = ladder},
          .speed_mode = NRF_COMP_SP_MODE_Normal,
          .hyst = NRF_COMP_HYST_NoHyst,
          .input = NRF_COMP_VDDH_DIV5, // this mode is unique to the 52820
          .interrupt_priority = NRFX_COMP_CONFIG_IRQ_PRIORITY};
      nrfx_comp_init(&config, comp_handler1); // no handler is used but the API requires a blank handler

      while (current > 0) {   
          ladder = ladder | current;         // successive approximation
          nrf_comp_th_t th = {
            .th_up = ladder,
            .th_down = ladder 
          };
          nrfx_comp_stop();   // update the COMP
          nrf_comp_disable();
          nrf_comp_th_set(th);
          nrf_comp_enable();
          nrf_comp_event_clear(NRF_COMP_EVENT_READY);
          nrfx_comp_start(0, 0);
          while(nrf_comp_event_check(NRF_COMP_EVENT_READY) == 0);
          uint32_t sample = nrfx_comp_sample();  // sample 
          if (!sample) { // VDDH/5 is less than VUP
              ladder = ladder & (~current);  // reset the MSB and continue with lower bits
          }
          current = current >> 1;
      }
      nrfx_comp_uninit();
      uint32_t volt = (3000 * (ladder+3))/32;       // the hysteresis ladder is (th_up+1)*VREF (1.2)
                                                    // theoretically this should be latter+1 - +3 is a fudge factor adjustment
      return volt;
}

逐次逼近法是否需要修改？因为我用了之后不准确，请问uint32_t volt = (3000 * (ladder+3))/32有什么解释吗？

逐次逼近法是否需要修改？因为我用了之后不准确，请问uint32_t volt = (3000 * (ladder+3))/32有什么解释吗？

How accurate do you need it? The resolution of this ladder is only 64 bit so it will be rather coarse.

The formula is explained like this:

the input is VDDH/5. so 3 V VDDH will be 0.6 V. the comparison value is 1.2 V so at 3 V the approximation should stop yield 32 (as 1.2 V will be 63). so (3000 * ladder) /32 should give the approximate VDDH in mV

As we are approximating from the bottom up, one should add +1 do the value. I added +3. as empirically this gave me the closest match to the 3V reference power. Adjusting the VDD up and down from there will definitely change the output of the approximation. I cannot say that it is particularly linear or accurate. But in my application it is enough to give a reasonable battery gauge.

Thank you for your prompt reply, but I have some problems, is the battery you need to measure connected to P0.07?

No, you cannot measure an external pin that way. Only the supply voltage VDDH

So your battery is connected to VDDH?

Yes of course. That is how the battery supplies power to the SoC