STM32F401 RTC ALARMA wake up CPU from StandBy with 16 seconds error. Why?

[Spider]

I'm configuring RTC AlarmA to wake up CPU from StandBy. I'am expecting to CPU became back at specified time, but it event occurs up to 16 seconds later.

uint32_t time;
uint32_t date;
uint32_t subsec;
 
do {		
	time = LL_RTC_TIME_Get(RTC);
	date = LL_RTC_DATE_Get(RTC);
	subsec = LL_RTC_TIME_GetSubSecond(RTC);
} while (subsec != LL_RTC_TIME_GetSubSecond(RTC));
 
sec = __LL_RTC_CONVERT_BCD2BIN(__LL_RTC_GET_SECOND(time));
min = __LL_RTC_CONVERT_BCD2BIN(__LL_RTC_GET_MINUTE(time));
hour= __LL_RTC_CONVERT_BCD2BIN(__LL_RTC_GET_HOUR(time));
 
sec+=10; //
 
LL_RTC_DisableWriteProtection(RTC);
CLEAR_BIT(PWR->CSR, PWR_CSR_EWUP);
LL_RTC_ALMA_Disable(RTC);
do {} while (!LL_RTC_IsActiveFlag_ALRAW(RTC));
 
LL_RTC_ClearFlag_ALRA(RTC);
 
LL_RTC_ALMA_ConfigTime(RTC, LL_RTC_ALMA_TIME_FORMAT_AM, __LL_RTC_CONVERT_BIN2BCD(hour), __LL_RTC_CONVERT_BIN2BCD(min), __LL_RTC_CONVERT_BIN2BCD(sec));
 LL_RTC_ALMA_SetMask(RTC, LL_RTC_ALMA_MASK_DATEWEEKDAY | LL_RTC_ALMA_MASK_HOURS | LL_RTC_ALMA_MASK_MINUTES);
LL_RTC_ALMA_SetSubSecondMask(RTC, 0);
LL_RTC_ALMA_Enable(RTC);
 
LL_RTC_EnableWriteProtection(RTC);
 
TRACE_INFO("RTC->ALRMAR: 0x%08X\r\n",RTC->ALRMAR);
 
/* Enable the wake up pin */
SET_BIT(PWR->CSR, PWR_CSR_EWUP);
 
SET_BIT(PWR->CR, PWR_CR_PDDS | PWR_CR_CWUF | PWR_CR_CSBF);
SET_BIT(SCB->SCR, ((uint32_t)SCB_SCR_SLEEPDEEP_Msk));
__WFI();

If set time to WakeUp for example 00:00:10 -> CPU will WakeUp at 00:00:26 and so on. Always up to 16 seconds later.

What I do wrong?

[Mohamed Aymen HZAMI]

Hello,

How you configure the RTC alarm ?

Try to use HAL library function.

Best Regards,

Mohamed Aymen.