I want to use a spare tiny25 and watch crystal as a real time clock over I2C for a data logging application.
The master processor is a mega168 connected to an SD card, the clock is for keeping track of the file creation time.
Another processor also connects to the I2C and handles the User interface display switches and IR remote input.
I would want the clock portion to be battery backed up which wakes up once a second and increments a counter.
Question is, that if I power the tiny25 from a coin cell, how do I keep current from leaking into the mega168 and powering it and the User interface controller?