Strange current spikes in nRF52 with RTC only

[Mateusz Gancarczyk]

Hi, currently I'm working on ultra low power device. This device need to be wake up in 333 ms interval. I use RTC for this. My question is about strange current spikes with i observed on Nordic Power Profiler Kir ( nRF6707 ). What is a source for this spikes?

This spikes have interval ~25ms. Average current looks quite OK: System on ( 1.2uA)+ RC ( 1uA ) + RTC ( 0.1 uA ) = 2.3 uA I'm not use any soft device. I use internal RC clock source for LFC.

My code is very simple: main:

int main(void)
{	
  NRF_POWER->DCDCEN = 1;
  lfclk_config();               
  rtc_config();
  
  while (1)
  {
		power_manage();	
  }
}

rtc_config:

static void rtc_config(void)
{
	uint32_t err_code;

	nrf_drv_rtc_config_t config = NRF_DRV_RTC_DEFAULT_CONFIG;
	config.prescaler = 64; // 32 768 Hz / 38 = 1024 Hz
	err_code = nrf_drv_rtc_init(&rtc, &config, rtc_handler);
	APP_ERROR_CHECK(err_code);
	nrf_drv_rtc_cc_set(&rtc,0, MEASURMENT_TIME_MS, true);
	nrf_drv_rtc_counter_clear(&rtc); 
	
	nrf_drv_rtc_enable(&rtc);
}

RTC handler, power manage and lfck config:

#define MEASURMENT_TIME_MS 333 
const nrf_drv_rtc_t rtc = NRF_DRV_RTC_INSTANCE(2); 

static void rtc_handler(nrf_drv_rtc_int_type_t int_type)
{
  if (int_type == NRF_DRV_RTC_INT_COMPARE0)
  {					
    nrf_drv_rtc_cc_set(&rtc,0, MEASURMENT_TIME_MS, true);		
    nrf_drv_rtc_counter_clear(&rtc); 
  }
}

static void power_manage( void )
{
	// WFE - SEV - WFE sequence to wait until a radio event require further actions.
	__WFE();
	__SEV();
	__WFE();
}

static void lfclk_config(void)
{
    ret_code_t err_code = nrf_drv_clock_init();
    APP_ERROR_CHECK(err_code);

    nrf_drv_clock_lfclk_request(NULL);
}

Does anyone know what could be the source of this spikes? RC temperature corecction? Nordic power profiler kit bug?

[Shahrear]

Need more details.. this spikes across which pin?

[Mateusz Gancarczyk]

It's from power supply directly for nRF52. I've use BL652-SA-01 module, so I measure current between pin VCC and GND.

[Shahrear]

Run a while loop with 1ms delay and test the power supply again. change the loop delay time and looks for spike. if spike vary depends on loop time it will be power supply's fault.

[MartinBL]

Hi,

Those short spikes are probably due to the nRF52's power supply working in ref refresh mode. Here is a quote from the current measurement guide:

The nRF52 power supply uses auto-controlled refresh modes to maximize efficiency. What this basically means is that a capacitor is recharged regularly to provide power to the chip. The recharge off the capacitor happens during a very short time interval, and results in a high frequency peak in current. This peak is measured to be about 10 - 15 us long. In order to get the correct average current during these peaks, a sampling frequency corresponding to half the peak length is required. This results in a minimum frequency of 200k samples per second (1 sample every 5 us).

BTW, aren't your RTC intervals closer to ~650ms? With a prescaler of 64 and a compare register value of MEASURMENT_TIME_MS = 333, you will get a f_RTC 32.768 / (64 + 1) = ~504 and hence, an interval of 333/504 = ~660ms.

[Mateusz Gancarczyk]

It looks like you right. It's from power supply working in ref refresh mode. The spikes interval are 22ms, amplitude 35uA and rising edge ~25uS:

BTW: yes, my mistakes. In RTC prescaler shoudl be 32.