I am currently designing a very small "stamp" board to fit any of the 64-pins QFP (and maybe also lesser pin-count chips via a superposed footprint) Mega chips that will contain the basic support circuitry and have all IO's routed to headers. This is part of a bigger project in which I will need to be able to swap out chips in a cartridge fashion. Part of this circuitry will be a 32KHz crystal and CR1025 10mm coin battery for backup.
I have done so before on ARM chips, but these had a dedicated VBAT pin for the RTC that used very little current. My question is in regards with the way Atmel recommends to connect the battery in such a setup. It goes straight to VCC through a schottky diode. I am wondering how to manage this. Won't the chip by default try to pull off the full current it needs to run its main program? How can I insure the module goes into low-power shutdown mode if no proper power source other than the battery is connected? For example, if I wanted to have the module keep it's time through shelf time?