Power Profiler Kit Offset/Miscalibrated?

[Oliver]

Hi there,

I'm using the PPK (v1.1.0) to measure my nRF52 DK (v1.1.1). My sample programm is very simple: set up the LFXO, RTC with 8s CC-periode and the BSP_LEDs:

const nrf_drv_rtc_t rtc = NRF_DRV_RTC_INSTANCE(0);
static void rtc_handler(nrf_drv_rtc_int_type_t int_type)
{
  nrf_gpio_pin_toggle(BSP_LED_3);
  nrf_drv_rtc_cc_set(&rtc, 0, RTC0_PERIODE * 8, true);
  runFFT = true;
}

static void rtc_config(void)
{
  nrf_drv_rtc_config_t config = NRF_DRV_RTC_DEFAULT_CONFIG;
  config.prescaler = 4095;
  
  nrf_drv_rtc_init(&rtc, &config, rtc_handler);
  nrf_drv_rtc_cc_set(&rtc, 0, RTC0_PERIODE * 8, true);
  nrf_drv_rtc_enable(&rtc);

  // enable auto-reset of RTC0
  nrf_ppi_channel_t ppi_channel;
  nrf_drv_ppi_channel_alloc(&ppi_channel);
  nrf_drv_ppi_channel_assign(ppi_channel, (uint32_t)&NRF_RTC0->EVENTS_COMPARE[0], (uint32_t)&NRF_RTC0->TASKS_CLEAR);
  nrf_drv_ppi_channel_enable(ppi_channel);
}

static void FPU_enable(void)
{
  SCB->CPACR |= (3UL << 20) | (3UL << 22);
  __DSB();
  __ISB();
}

static void FPU_disable(void)
{
  SCB->CPACR = 0; 
  __DSB();
  __ISB();
}



int main(void)
{
  NRF_CLOCK->LFCLKSRC = (CLOCK_LFCLKSRC_SRC_Xtal << CLOCK_LFCLKSRC_SRC_Pos);;
  nrf_drv_clock_init();
  nrf_drv_clock_lfclk_request(NULL);
  
  rtc_config();
  bsp_board_leds_init();
  bsp_board_led_on(BSP_BOARD_LED_0);
  
#ifdef FPU_INTERRUPT_MODE
  NVIC_SetPriority(FPU_IRQn, APP_IRQ_PRIORITY_LOWEST);
  NVIC_ClearPendingIRQ(FPU_IRQn);
  NVIC_EnableIRQ(FPU_IRQn); 
#endif

  while (true)
  {
    if (runFFT)
	{
      nrf_power_dcdcen_set(true);
      FPU_enable();
      sine_freq = ((uint32_t)rand()) %((uint32_t)(SINE_WAVE_FREQ_MAX * FFT_TEST_SAMPLE_FREQ_HZ / 2.0f));
      testFFT_f32(sine_freq, 2048, false);
      FPU_disable();
      nrf_power_dcdcen_set(false);
	  
	  runFFT = false;
    }
	
    __SEV();
    __WFE();
	__WFE();
  }
}

With two different Amperemeters (Agilent and Fluke), the sleep current is around 2.4uA (while performing the FFT 4.11mA), but the PPK using Otti v1.2.0 measures 13.9uA (FFT: 4.02mA).

Is this in the range of measurement accuracy? The PPK manual states it's down to 0.2uA in the lowest range. Unfortunately I don't have an accurate µA-mA current sink to re-calibrate the PPK.

Do you have any ideas to improve my measurements and/or code optimizations?

Thanks in advance, Oliver

PS: Does anybody know, what causes the peaks in Otti? They are near to 45 Hz (our mains is at 50 Hz)?

[Sigurd]

But you are measuring on your nRF52-DK? The "Reg" position is used when performing measurements on external/custom hardware via the “DUT” connector. When doing measurement on a DK, the power select should be on "DK".

See page 12 in the PPK User guide.

[Sigurd]

If you put the chip in system ON sleep mode, what current are you seeing with the different instruments?

You should see about 1.2 µA power consumption.

You can put the chip in system ON sleep mode like this:

int main(void)
{

    while (true)
    {
        // Enter System ON sleep mode
        __WFE();
        __SEV();
        __WFE();
    }
}

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Regarding Otii, you should turn on the power switches...: