Suggestions on HP21 restoration

[EdS2]

I think you're right - it's in the milliamp range, at one volt, and that's a lot higher than other microcontrollers as used in calculators. 100 hours of standby isn't enough, although 100 hours of operation might be fine for a lot of people.

So, such a build might need an actual power switch, not a soft power switch, or need a power controller chip.

It might be that using one of the zero-power (bistable) LCDs (which continue to display even without power) would allow the microcontroller to power off, waking on any keypress, in a way which is not user detectable.

https://ghubcoder.github.io/posts/waking...l-trigger/