Your mileage may vary (YMMV) because the ATmega1284p is only spec-ed to power-down at 150nA from 1.8v in the datasheet, but here's the proof some of their chips can operate lower:
In More Detail
In the photo above the Pro Mini XL (v2) has been fused to run at 1 MHz with it's Brown-out Detection (BoD) turned off - allowing it to operate, seemingly happily from a fresh 1.5v AAA battery.
The video below shows it first powering up, doing a bit of low-power housekeeping and then powering-down to less than 100nA:
Board pin 2 (INT0) had been configured as an active-low wake-up interrupt and so when I ground it, you can see that the board springs to life again, before powering back down.
The code (available here) would actually light the on-board LED if the forward voltage was high enough, sadly 1.5v won't cut it so it appears off.
But the photos and video below shows the same board, same code just with a regulated 1.8v supply drawing roughly 1nA more whilst in power-down and the "proof-of-life" LED being lit (bottom corner) when it's awake.
It's awake - honest!
And by way of comparison at 3v, running directly from those two AA batteries, it just peaks over 100nA:
Again, same board, same code - just varying the supply voltage:
Thanks to various awesome Internet folk and resources including Nick Gammon, Jean-Claude Wippler (JeeLabs), Charles Hallard (ch2i), Pascal Moreau (MySensors) and others, I've also been developing something that enables any Pro Mini to draw these few nA from an automatically topped-up "storage" - a sort of "just-in-time" regulator - cutting the power-down battery draw to basically zero.
I'll write more about it soon, but here's a photo and video (to follow) to whet your appetite:
Believe it or not, in the above photo, the board is actually in power-down and can happily remain there for a configurable amount of time.
Again, same board, same code, but watch what happens when I trigger the interrupt...
See the LED light up at the bottom corner of the board - it's alive!
I'll update this log with more details soon!