Not working standby mode in the stm32l100

I'm tuning stm32l100 for a STANDBY mode. The MCU should woke up 2 times per second. For this I use RTC wakeup timer. But after how MCU has entered to the STANDBY mode it immediately wakes up.

If instead the STANDBY mode I use a simple sleep-mode (__WFI) all works fine.

My code is here

After the MCU is starting

a. saves flags CSR_SBF and CSR_WUF, after that clears them.
```
if (PWR->CSR & PWR_CSR_SBF_BIT) {
   // ...
}

if (PWR->CSR & PWR_CSR_WUF_BIT) {
    // ...
}

PWR->CR |= PWR_CR_CSBF_BIT|PWR_CR_CWUF_BIT;

//while (PWR->CSR & PWR_CSR_SBF_BIT);
while (PWR->CSR & PWR_CSR_WUF_BIT);
```
I noted that CSR_SBF never clears. If I uncomment string with while then MCU stall on it place. I don't understand why.

b. saves reset source flags, after that clears them.
```
volatile uint32_t csr;
csr = RCC->CSR;

// .... saving

// clear flags
RCC->CSR |= RCC_CSR_RMVF_BIT;
```
After how MCU woke up from the STANDBY all reset source flags are cleared. It seems how reset sources are absent but code execute from 0x0.

Tuning RTC wakeup-timer

void rtc_set_wakeup_mode(const uint32_t wakeup_counter)
{
    RTC_WRITE_PROTECT_DISABLE();
    RTC->CR &= ~RTC_CR_WUTE_BIT;
    while (!(RTC->ISR & RTC_ISR_WUTWF_BIT));

    RTC->WUTR = wakeup_counter;
    RTC->CR &= 0xfffffff7;
    RTC->CR |= RTC_WAKEUPCLOCK_RTCCLK_DIV2;

    __HAL_RTC_WAKEUPTIMER_EXTI_ENABLE_IT();
    __HAL_RTC_WAKEUPTIMER_EXTI_ENABLE_RISING_EDGE();

    RTC->CR |= RTC_CR_WUTIE_BIT|RTC_CR_WUTE_BIT;
    RTC_WRITE_PROTECT_ENABLE();
}

Enter in the STANDBY mode

void pwdm_enter_standby_mode(void)
{
    PWR->CR |= PWR_CR_PDDS_BIT;
    SCB->SCR |= SCB_SCR_SLEEPDEEP_BIT;

    __WFI();
}

Solution

So, finally I got it.

It was my own fail with startup initialization. The key was in this:

I noted that CSR_SBF never clears. If I uncomment string with while then MCU stall on it place. I don't understand why.

I placed the code working with wakeup/standby flags at the begining of main. But this code didn't do this operation __HAL_RCC_PWR_CLK_ENABLE(); because that operation will executed in a oscill initialization routine. So, the PWR->CSR was in undefined state and the PWR_CSR_WUF flag is never clearing.