Please advise how to transit from main power(3.3VDC) to battery power when main power on, vice versa. as computer does.
I would like to rtc clock such as personal computer.

Thanking you in advance.

 

Use a Schottky diode such as 1N5817 between the main power and the battery powered circuit.

 

you mean switch from main to battery power when main power is off
why would you want to run on battery when main power is present.