Simple siwtching question

I want to make a dash clock in my car with an atmega328. I'd like to keep the RTC running off the cars 12v all the time but I don't want the atmega running at all when the cars 12v ignition line is low. If I'm using a voltage regulator on the constant 12v line can I use the 12v ignition line to switch the atmega without another regulator? Is there anything I should do specially because it's a car battery?

Thanks

Why do that? An RTC chip will run for years on just a coin sized battery.

I know, I just figure I can reduce the number of components since ivegot a constant power supply.