How to find out by software that main power lost?

Arduino use 12V for main power from power supply. To USB port just hookup +5V backup battery. So wheck when power down Arduino still working without interuption. Work fine. But how to find out in my program when 12V lost (220V down)?

I know I can use any pin hookup to 12V( using resitors) and messure, but may be more easy way to do that ?

I hope it should be like Interupt that happend when Arduino switch from one power to backup and back

I know I can use any pin hookup to 12V( using resitors) and messure, but may be more easy way to do that ?

I hope it should be like Interupt that happend when Arduino switch from one power to backup and back

Well you could wire your voltage divided 12vdc to pins 2 or 3 and write an interrupt handler?

Thanks! I know that as I post. I'm asking may be there is a simplest way, like read some register which source is used... Or may be I'm wrong - may be proccessor don't know which source it is used.

I think this question is a common for using Arduino as a monitoring controller

Thanks!

This is the simplest way to detect the power change.

The Arduino does not have built-in monitoring.

Thanks. I'll use pin2.