Battery level measurement without SAADC

Question

Hi! The nRF52820 chip does not have the SAADC module used to estimate the battery life in its larger counterparts (nrf52840 and nrf52833). Is there a different way to estimate battery life without an SAADC? I'm hoping for a method which does not include adding external components, and is measurement based rather then estimation based. We had considered an estimation method which is a summation of the current estimate per 'time used', 'advertisement cycles' passed, 'connection cycles passed' etc. The problem here is that a. I would need to estimate all possible parameters (TX power for example, time until advertisement is caught etc) b. I would not be able to catch leaking parts, as the leak is not part of my estimation. 
 Adding an external ADC is a possibility, but would require us to change our PCB which is not something we want. 
 we're using the VDD tied to VDDH and expect the Vin to be 1.8-3.3V. 
 Any other ideas?

Jørgen Holmefjord · Accepted Answer

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.

Johannes · Answer

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;
}

hmolesworth · Answer

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

Battery level measurement without SAADC

Top Replies