What is a real-time clock (RTC) battery backup switching circuit?

Real Time ClockFunction

These RTCs have a backup battery switch-over circuit which detects power failures and automatically switches to the battery supply when a power failure occurs. They are also equipped with two or three power supply pins so there is no need of adding a diode.

Notes: R2262 has the SW2 switch and the BAT pin instead of the VSB pin. The SW2 switch is constantly turned on unless it is turned off by a register setting.