Just before you finish calculating:

You can set the PIC to sleep and wake up to check the keys (or keypress
wakeup), and use a LDO regulator that works all the time. This leads to a
minimum parts solution.

Peter

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads