RTC_A/B/C的比较，可以查看时钟精度是否满足你的要求
The switch is controlled by the high-side SVS that supervises the primary supply, DVCC.
By default (for example, at power on) the backup subsystem is supplied from the backup battery (VBAT).
As soon as primary supply is available and the high-side SVS releases the device the backup supply is
switch to the primary supply. When the primary supply falls below the configured high-side SVS level the
supply is switch back to the battery voltage VBAT. This means the automatic switch control requires that
the high-side SVS is always enabled (also in low power modes). If the high-side SVS is disabled the
switch also selects the battery voltage VBAT to supply the backup subsystem.
The LOCKBAK bit can only be reset when the supplies in the back-up domain have settled. If for example
a discharged capacitor is connected to VBAT this might take a couple of milliseconds after DVCC is
supplied. The recommended flow is to check if LOCKBAK reads 0 after having reset it by software.
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