High power consumption when starting external LFCLK

Question

Just starting the external LFCLK needs a considerable amount of energy. With my evaluation board it takes about 340ms for the XTAL to get started, which is well within the specifications. However, during all this time, the system draws about 820µA at least. I guess this current is not needed for the XTAL itself but for the system to permanently check whether the frequency is stable enough already. It would be much more efficient if this check was done just once in a while, but the user does not have enough control over the clock management in order to change the implementation in this direction.
Is there any way to avoid this high power consumption?

Here is a small demo program which allows to switch the external LFCLK on with BUTTON_0 and off with BUTTON_1. When interpreting the current measurements, keep in mind that the buttons draw some current via the pullups while being pressed (~200µA).

#include "nrf_gpio.h"
#include "boards.h"

int main(void)
{
	NRF_POWER->RESET = POWER_RESET_RESET_Msk;	
	nrf_gpio_cfg_sense_input(BUTTON_0, NRF_GPIO_PIN_PULLUP, NRF_GPIO_PIN_SENSE_LOW);			
	nrf_gpio_cfg_sense_input(BUTTON_1, NRF_GPIO_PIN_PULLUP, NRF_GPIO_PIN_SENSE_LOW);			
	SCB->SCR |= SCB_SCR_SEVONPEND_Msk;
	NRF_GPIOTE->INTENSET = GPIOTE_INTENSET_PORT_Msk;
	for (int clkEnab=0;;)
	{
		__wfe();
		NRF_GPIOTE->EVENTS_PORT = 0;
		NVIC_ClearPendingIRQ(GPIOTE_IRQn);
		if (!clkEnab && nrf_gpio_pin_read(BUTTON_0)==0)
		{
			NRF_CLOCK->LFCLKSRC = CLOCK_LFCLKSRC_SRC_Xtal<<CLOCK_LFCLKSRC_SRC_Pos;
			NRF_CLOCK->TASKS_LFCLKSTART  = 1;
			clkEnab = 1;
		}
		else if (clkEnab && nrf_gpio_pin_read(BUTTON_1)==0)
		{
			NRF_CLOCK->TASKS_LFCLKSTOP  = 1;
			clkEnab = 0;
		}
	}
} ////

Stefan Birnir Sverrisson · Accepted Answer

Hi 
 I think there is nothing that can be configured to avoid this current consumption. It is automatically handled internally and can not be configured. Usually, you only draw this current once for each reset, so it should usually not be of that big concern. 
 Update 27.1.2014 
There is also the possibility of using the internal LFCLK RC with the 
 NRF_CLOCK_LFCLKSRC_RC_250_PPM_TEMP_4000MS_CALIBRATION
 
 option , which will add 2-3 uA compared to a 20ppm crystal. I guess it depends on how frequently you need to start the crystal if it would pay off to use the RC instead, see this thread