turn off PIC after few minutes ??

hello
i want to turn off, or power off(dissconnect it from VCC) the PIC after a few minutes and when
i press any key (keypad for example) or even power ON pic. the pic will gets on.

does that need any logic gate between Vcc and pic ?? i put the question because i note
some calculators turn off after 10 minutes, and i want to do something like that

any idea for that

This has been discussed many times before. Do a forum search for "auto-off" for some examples.