Looking at the data sheet for the AT91SAM9261, it says the back up supply voltage range is 1.08 to 1.32 volts with a current of 2.5 microAmps. This could be supplied by a watch battery except I have no idea how to regulate the voltage to this range without draining the battery from the regulator quiessent current. I don't recall seeing any circuits that could provide a regulated voltage and not draw at least 10 times this much current.
What was Atmel thinking when they drew up these plans for a battery backed RTC?
Or am I missing something obvious, like a 1.2 volt, low self drain battery that is perfect for this job?