SCB->SCR |=SCB_SCR_SLEEPDEEP;  
PWR->CR &=~ PWR_CR_PDDS;
PWR->CSR &=~ PWR_CSR_WUF;
PWR->CR |=  PWR_CR_LPSDSR;  
__dsb(15);
__WFI();
__isb(15);
PWR->CR &= ~PWR_CR_LPSDSR;

The•
•
wakeup timer clock input can be:
RTC clock (RTCCLK) divided by 2, 4, 8, or 16.
When RTCCLK is LSE(32.768kHz), this allows to configure the wakeup interrupt period
from 122 μs to 32 s, with a resolution down to 61μs.
ck_spre (usually 1 Hz internal clock)
When ck_spre frequency is 1Hz, this allows to achieve a wakeup time from 1 s to
around 36 hours with one-second resolution. This large programmable time range is
divided in 2 parts:
–
from 1s to 18 hours when WUCKSEL [2:1] = 10
–
and from around 18h to 36h when WUCKSEL[2:1] = 11. In this last case 216 is
added to the 16-bit counter current value.When the initialization sequence is
complete (see Programming the wakeup timer on page 506), the timer starts
counting down.When the wakeup function is enabled, the down-counting remains
active in low power modes. In addition, when it reaches 0, the WUTF flag is set in
the RTC_ISR register, and the wakeup counter is automatically reloaded with its
reload value (RTC_WUTR register value).