RTC LSE not working on power off with Vbat

[OldSTMan]

I'm using the stm23F103C8 and I would use the RTC internal cloak/calendar. The program is working fine and I have used the LSE osc (32KHz..) as clock source. I have connected a 3V battery to the VBAT pin. My problem is that the switching off the main supply (3.3V) the LSE and RTC stops (vrified with scope).

Attached the main.c.

Can you help me??

[Techn · Accepted solution]

Since the seconds register is 32 bits, it takes a long time to overflow, so if the rtc works in battery mode till the overflow condition, the calendar data will be accurate. If you go through the rtc.c file you will get to know how the

backup domain is used to get it done smoothly.

Please check the link to get an idea.. RTC_F103/Core/Src/rtc.c at master · lamik/RTC_F103 · GitHub

I have done programming of RTC for this processor, I used STM32cubeMX  tool to set the parameters. 

please refer to the HAL and LL documentation as follows
35.2.3
WARNING: Drivers Restrictions
RTC version used on STM32F1 families is version V1. All the features supported by V2
(other families) will be not supported on F1.
As on V2, main RTC features are managed by HW. But on F1, date feature is completely
managed by SW.
Then, there are some restrictions compared to other families:
Only format 24 hours supported in HAL (format 12 hours not supported)
Date is saved in SRAM. Then, when MCU is in STOP or STANDBY mode, date will be
lost. User should implement a way to save date before entering in low power mode
(an example is provided with firmware package based on backup registers)
Date is automatically updated each time a HAL_RTC_GetTime or HAL_RTC_GetDate
is called.
Alarm detection is limited to 1 day. It will expire only 1 time (no alarm repetition, need
to program a new alarm)