If you decide to try your hand at calibrating I would certainly like to hear how you get on.
I will definitely post it here if I manage to get it work.
Hmm, the processor has a 32kHz crystal, so it should be able to automatically calibrate. I think there exists an application note that describes this process but I seem to be able to find it with Google right now.
I think the application note is AVR053 but it doesn't need a 32kHz crystal for this calibration process. The programmer (AVRISP/JTAG/STK500) will supply a 32.768kHz clock to the MCU for calibration. I found that we can write the OSCCAL register directly in AVR Studio but we have to point where the value is in EEPROM or flash using one of the programmer. So, in this case we don't have to load this value in the sketch.
It includes interchangeable power shields for various sources (solar/battery, piezo, peltier, RF), but nothing explicitly for charging LiIon yet.
I made something similar for my company usage where we have different power shields, RF shields, and sensor interface shields that fits into different applications. But, we were using ATmega128 as our board was based on Crossbow's Micaz at that time. It provides flexibility but it gets annoying when your stacking goes higher and higher although we were using low profile board to board connector (1.00mm pitch).
Did you get a chance to measure the sleep current on this design?
I have yet to measure yet because I went straight to the 2nd revision as I changed the buck-boost converter (LTC3530) to TPS63001 instead as it uses so much less external components and cheaper too! But if I recall correctly, using ATmega128 and similar setup, it was around 40 uA. I didn't go for very low LDO design like MCP1700 as I think some other shields might need more than the 250 mA for the 3.3 V rail. For example an XBee Pro module. Once, I get the 2nd revision up running, I will post the current during power save mode here.
Do you need the 22pF caps on the watch xtal, or is that in case you can't find a 6pF xtal.
The crystal load capacitance is 12.5pF. I estimate the stray to be around 5pF and using C=2CL-CS it comes out to 20pF actually. But I have a reel of 22pF lying around, so I think might as well give it a try.