That’s very helpful, thanks. I’ll look into both.
I’ll change R1/R2 from 33k/8k2 to 330k/82k first, just to see the practical result. In theory it should lower the parasitic drain (@4V) about 0.09mA, pushing it from 0.25mA to 0.16mA. I might try another x10, which would save another 0.009mA, and see if voltage measuring will still work.
Turning off ADC rings a bell. Found it again in post #2 of Toykeepers Firmware repository. She mentions @Crux:
Investigate, measure, and document ultra-low-power sleep modes. Crux’s posts here and here, ideas here (link is external) and here (link is external) and here (link is external)
The firmware I just use has an ADC_off routine, but it’s only called when there is no voltage monitoring. Calling it before starting sleep mode looks easy, but I’ll have to find where to put the ADC_on command to reenable it after a switch press.
But that looks very promising and together might bring the drain down by x100. 