Posting here, since Nordic DevZone is broken for me (Made a long post that didn't upload, just kept loading).
I am currently approaching this from HW point of view (I personally not developing firmware).
Before nRF goes into low power mode, the nRF_COIN_EN is pulled high to attach the coin cell. Every other GPIO is pretty much disconnected (not to consume anything else). When 3.3V disappears, the Vcoin is the only thing keeping nRF alive. nRF comes back up when 3.3V is applied (detecting this via nRF's LCOMP). RTC keeps track of time, when Vcoin is above 2.5V (nRF sees ~2.4V). Under that RTC data is gone.
Shouldn't it be that the nRF is able to run even down to 1.8V...? but I can't seem to get it so.
The current draw is ~80uA peaks (I think when RTC is counting), 11uA average. The voltage levels are indistinguishable when RTC data is preserved or when it's not. I have noticed that powering on 3.3V when Vcoin is below a certain level it always loses RTC data and when above a level, RTC data remains intact.
CH1 - yellow - VnRF CH2 - blue - Vcoin
Vcoin (blue line) was about 2.51V The drop during current peaks is about 80 - 100mV. This remains the same whetver the coin cell is full or even lower than 2.3V (when RTC data for sure is gone).
MATH measurement. Measured the difference btw Vcoin (blue) and VnRF (yellow). I don't see anything dramatic here to suggest that RTC has lost it's data.
Any ideas what else should I check ? Maybe there are some firmware things we need to check to allow nRF to run down to 1.8V ?