Battery level measurement without SAADC

RE: Battery level measurement without SAADC

LuoXF — Thu, 10 Feb 2022 01:47:31 GMT

Thanks Johannes it works fine so far

RE: Battery level measurement without SAADC

Johannes — Tue, 25 Jan 2022 08:23:27 GMT

This is how accurate it will be. remember the resolution is only 6V/64 = 93.75 mV per bit

RE: Battery level measurement without SAADC

LuoXF — Tue, 25 Jan 2022 08:18:53 GMT

The positive pole of my battery is directly connected to VDDH, and the measured value of my battery with a multimeter is 3.97, using the algorithm you provided：

is it right or not?

RE: Battery level measurement without SAADC

Johannes — Tue, 25 Jan 2022 07:54:15 GMT

is your battery fed directly into VDDH or do you have a voltage regulator. Can't design the whole thing for you. the algorithm works perfectly as long as VDDH is connected directly with the battery. Otherwise you need a voltage divider to bring VBAT/5 to one of the AIN pins

RE: Battery level measurement without SAADC

LuoXF — Tue, 25 Jan 2022 07:49:21 GMT

Does the formula require no modification? What I collected seems to be a fixed value

RE: Battery level measurement without SAADC

Johannes — Tue, 25 Jan 2022 07:47:07 GMT

The formula should work up to 1.2*5V = 6 V. Maximum VDDH for 52820 is 5.5V, so it should work

RE: Battery level measurement without SAADC

LuoXF — Tue, 25 Jan 2022 06:28:14 GMT

I'm modifying my power supply and one more question, my lithium battery is rated at 4.2v, so, vddh/5, should be approximately 0.84, should the formula be changed accordingly? can you help me?

RE: Battery level measurement without SAADC

Johannes — Tue, 25 Jan 2022 05:04:19 GMT

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

RE: Battery level measurement without SAADC

LuoXF — Tue, 25 Jan 2022 05:01:32 GMT

So your battery is connected to VDDH?

RE: Battery level measurement without SAADC

Johannes — Tue, 25 Jan 2022 04:34:05 GMT

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

RE: Battery level measurement without SAADC

LuoXF — Tue, 25 Jan 2022 04:32:40 GMT

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

RE: Battery level measurement without SAADC

Johannes — Tue, 25 Jan 2022 03:45:29 GMT

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.

RE: Battery level measurement without SAADC

LuoXF — Tue, 25 Jan 2022 03:36:22 GMT

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？

RE: Battery level measurement without SAADC

A.P — Wed, 17 Nov 2021 07:45:30 GMT

Thank you Johannes!

I'll have a look and see if I can make these suggestions work.

RE: Battery level measurement without SAADC

Johannes — Tue, 16 Nov 2021 17:34:01 GMT

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;
}

RE: Battery level measurement without SAADC

Johannes — Fri, 01 Oct 2021 18:19:11 GMT

Actually there is no external circuity required. One can do an approximation using the hysteresis ladder feature of the COMP peripheral. The nrf52820 can compare VDDH/5 against fractions of the internal 1.2 V reference per the crude unoptimized approach below

uint32_t get_battery_voltage1()
{
      uint32_t ladder = 1; //approximate from below
      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, .th_down = ladder-1},
          .speed_mode = NRF_COMP_SP_MODE_Normal,
          .hyst = NRF_COMP_HYST_NoHyst,
          .input = NRF_COMP_VDDH_DIV5, // 
          .interrupt_priority = NRFX_COMP_CONFIG_IRQ_PRIORITY};
      nrfx_comp_init(&config, comp_handler1); // no handler is used but the API requires a blank handler

      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();
      while (sample == 1) {   // VDDH/5 is higher than VUP
          ladder++;           // increase ladder until VDDH/5 is larger than VUP
          nrf_comp_th_t th = {
            .th_up = ladder,
            .th_down = ladder -1
          };
          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);
          sample = nrfx_comp_sample();  // sample again
      }
      nrfx_comp_uninit();
      uint32_t volt = (3000 * (ladder+1))/32;       // the hysteresis ladder is (th_up+1)*VREF (1.2)
      return volt;
}

With this you can approximate VDDH from 0 to 6V in0.09375V increments

RE: Battery level measurement without SAADC

Jørgen Holmefjord — Mon, 04 Jan 2021 15:05:15 GMT

Yes, you are understanding it correctly. The internal regulators can only regulate down the voltage from VDD, if VDD drops below the reference the chip will no longer be able to regulate it.

You will need to use an external voltage/resistor divider together with one of the lower internal references.

RE: Battery level measurement without SAADC

A.P — Tue, 22 Dec 2020 10:09:53 GMT

Thank you hmoleshworth!
That is indeed a good idea. I will have to see if we can spare a GPIO for this purpose, as well as check if we have any capacitors we can use for this.

Thank you for the idea!

RE: Battery level measurement without SAADC

A.P — Tue, 22 Dec 2020 10:05:58 GMT

Hi Jørgen!
Thank you for the answer.
I'm slightly confused. Lets say I want to use the comperator to know if my VDD voltage has fallen below 2.4V, the higher reference generated by the pin. That would mean I would be comparing VDD with VREF(2.4V), but VDD must be 0.3V higher than VREF, which means that as VDD drops below 2.7V, I can not use the reference voltage to say anything about the VDD voltage.
Am I misunderstanding something?

RE: Battery level measurement without SAADC

hmolesworth — Wed, 16 Dec 2020 19:46:36 GMT

You can be creative and measure the voltage by charging or discharging a capacitor and measuring the time taken to hit the comparator trigger voltage. Two pins can be used, one as the charge pin and one as the sense pin, or better use a single pin as I show here:

//                 +---------------------------------
//                 |                 nRF52832
//                 |
//                 |    VDD
//                 |   --#--
//                 |     |
//                 |     |
//                 |     +-|
//                 |       |<- High
//                 |     +-|
//                 |     |
//   #-------#-----O-----#--------------#--> P0.nn COMP
//   |       |     |     |              |    Ref = VDD/n
//  +++      |     |     +-|            |
//  | |    -----   |       |<- Low    -----
//  | |    -----   |     +-|          -----
//  | |      |     |     |              |
//  +++      |     |     |              |
//   |       |     |   =====          =====
// =====   =====   |    ===            ===
//  ===     ===    |     =              =
//   =       =     |
//                 +----------------------------------

Conceptual Steps:

Set the port pin to an output, drive high to charge the capacitor

start a timer

set up comparator to detect voltage falling below a threshold, which releases the pin drive

comparator trip captures timer value

Vo = Vi.e^(-t/CR) where Vi is your Vdd voltage and Vo the comparator trip voltage

Use a lookup table to map result

The above example is charge-measure discharge time, but you can also discharge-measure charge time. The comparator options will probably make one of these easier.

Maybe study the Capsense examples for help in driving the peripherals

RE: Battery level measurement without SAADC

Jørgen Holmefjord — Wed, 16 Dec 2020 15:00:00 GMT

There are internal references generated by the chip at 1.2, 1.8, and 2.4 V. Note that the specifications requires you to have supply voltage (VDD) of minimum VREF + 0.3V for the used reference. You can also provide an external reference, which may make the battery voltage measurements simpler, but it would require external components to have a stable reference voltage.

RE: Battery level measurement without SAADC

awneil — Wed, 16 Dec 2020 12:35:10 GMT

[quote userid="83878" url="~/f/nordic-q-a/69524/battery-level-measurement-without-saadc/285249#285249"]Where do the references voltages come from?[/quote]

that would be in the Product Specification

RE: Battery level measurement without SAADC

A.P — Wed, 16 Dec 2020 09:05:55 GMT

Where do the references voltages come from? can I control the output voltage on a GPIO, with sufficient accuracy, or will I need to receive the reference voltages from an external source?

RE: Battery level measurement without SAADC

awneil — Tue, 15 Dec 2020 15:23:45 GMT

You can use it to tell that the battery voltage is above or below a certain level - so you could use that to give a good/low indication ...

RE: Battery level measurement without SAADC

A.P — Tue, 15 Dec 2020 10:54:54 GMT

Hi Awneil!
Yes, the nRF52820 does have analog level comp, specified here.
How can I use this to measure the battery?