The real-time clock (RTC) and the RTC backup registers can be powered from the VBATvoltage when the main VDD supply is powered off. To retain the content of the RTC backupregisters and supply the RTC when VDD is turned off, VBAT pin can be connected to anoptional standby voltage supplied by a battery or by another source.To allow the RTC to operate even when the main digital supply (VDD) is turned off, theVBAT pin powers the following blocks:1. The RTC2. The LSE oscillator3. PC13 to PC15 I/Os (when available)