When using the hardware RTC and a separate supply (backup feature is being used):
The VRTC-BACKUP pin can be tied directly to the external backup supply. If the backup supply is a replaceable battery, the addition of ESD and reverse-polarity protection is highly recommended.
When using the hardware RTC and VRTC-BACKUP is not connected to a separate supply (backup feature is not used):
The VRTC-BACKUP pin should be tied directly to GND. If VRTC-BACKUP is left floating, the automatic supply switch-over logic in the RTC may inadvertently switch over from VDD.
When not using the hardware RTC:
The VRTC-BACKUP pin should be left floating.