So I was making a modification to the Standalone Temperature Logger (SATL) to make it log voltage instead of temperature and happen to discover an issue with the Brown-out Detector which is set to 2.7V. Download the SATL v2.1 or visit the SATL project page.
It turns out that I had a 3V coin cell which was a bit discharged as it was sitting at 2.95 volts and when I connected it to the circuit it would drop to 2.7 volts and draw about 0.5mA of current which was very odd. At 2.95 volts I thought it should still have some life left. After trying to program very simple programs to the ATtiny85 like a blinking LED, making it sleep right away, etc I had no success. Eventually I reset the fuse bits and it all started working well again which lead me to discovering that BOD was the issue.
I tried adding 0.1uF, 4.7uF and 470uF capacitors which sometimes worked but it wasn’t reliable. I’m now recommending that BOD be disabled for the SATL unless you know the 3V coin cell is brand new or near new condition. The BOD issue brought my attention to other places within the SATL that could be improved upon.
Increase the button pull down resistor
Connected to the button we have a standard 10k pull down resistor and each time the button is pressed or held 0.3mA is consumed, seems sort of wasteful.