void IdleMode(void)
{
  SCB->SCR &= ~SCR_SLEEPDEEP;
  
  //  например прерывание происходит вот здесь

  PMC->PMC_FSMR &= ~PMC_FSMR_LPM;
  __WFE();
}

volatile bool lfrcoReady = false
void CMU_IRQHandler(void)
{
/* Clear interrupt flag */
CMU_IntClear(CMU_IF_LFRCORDY);

/* Indicate that LFRCO is ready */
lfrcoReady = true;
}


/***************************************************************************//**
* @brief Main function. Enables LFRCO and waits in EM1 until it is ready
******************************************************************************/
int main(void)
{
/* Chip revision alignment and errata fixes */
CHIP_Init();

/* Enable CMU IRQ when LFRCO is ready */
CMU_IntEnable(CMU_IF_LFRCORDY);

/* Enable CMU interrupt vector in NVIC */
NVIC_EnableIRQ(CMU_IRQn);

/* Enable LFRCO but do not wait until it is ready */
CMU_OscillatorEnable(cmuOsc_LFRCO, true, false);

/* Wait in EM1 until LFRCO is ready.
* Disable interrupts first to avoid interrupt executing between lfrcoReady
* check and _WFI(); This would have caused the program to get stuck! */
__disable_irq();
while(!lfrcoReady)
{
__WFI(); /* Pending and enabled IRQs will wake up the CPU, but not go to ISR */
__enable_irq(); /* ISR for any pending and enabled IRQs will be executed after this */
}

/* Wait here at the end */
while(1);
}

Table 14.2 WFI and WFE Wakeup Behavior
WFI Behavior                                       Wake Up               IRQ Execution
IRQ with BASEPRI
IRQ priority > BASEPRIv                            Y                           Y
IRQ priority =< BASEPRI                           N                          N
IRQ with BASEPRI and PRIMASK
IRQ priority > BASEPRI                              Y                           N
IRQ priority =< BASEPRI                            N                          N
WFE Behavior
IRQ with BASEPRI, SEVONPEND = 0
IRQ priority > BASEPRI                               Y                           Y
IRQ priority =< BASEPRI                             N                           N
IRQ with BASEPRI, SEVONPEND = 1
IRQ priority > BASEPRI                               Y                           Y
IRQ priority =< BASEPRI                             Y                           N
IRQ with BASEPRI and PRIMASK, SEVONPEND = 0
IRQ priority > BASEPRI                               N                           N
IRQ priority =< BASEPRI                             N                           N
IRQ with BASEPRI & PRIMASK, SEVONPEND = 1
IRQ priority > BASEPRI                                Y                          N
IRQ priority =< BASEPRI                              Y                          N