I worked on a project that included a PCF8563 RTC connected to a microcontroller. The equipment was powered by a 9V Alkaline battery that was user supplied and replaceable. In addition a small lithium coin cell was soldered in place on the circuit board to provide for backup of the RTC. Within the design there was a switching type power converter to efficiently downconvert the 9V battery to +5V for the microcontroller and other peripheral circuits.

The following circuit was used to supply the power to the RTC chip. The 9V battery typically supplied the backup power to the RTC then the equipment was turned off. If the 9V battery was delpeted or removed then the Lithium battery took over and would supply the RTC backup. When the equipment was turned on then the RTC derived its power from the +5V supply.


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Michael Karas