STMicroelectronics STM32F405 Reference Manual page 828

Real-time clock (RTC)
Refer to Section: Memory map for the register boundary addresses.
Caution: In Table
registers are not affected by a system reset. For more information, please refer to
Section 26.3.7: Resetting the
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120, the reset value is the value after a backup domain reset. The majority of the
RTC.
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