I have tested to see how long the attiny can keep sleeping and retain its RAM after hitting power down mode when voltage hits 2v.

I turned on both LED allowing device to drain to 2v (which takes about 45 min), then let it go in power saving mode over the evening and night.

• After 5 hours, voltage was 1.78 v
• After 13 hours, voltage was 1.48 v

In normal running mode I have tested the device to be functional down to 1.5 volts. But this is with BOD fuse turned off. At 1mhz I think its rated for 1.8v.

At 30-40 uA, I imagine it could retain RAM at a much lower voltage. But I think its save to say after VCC hits 2 volts and it goes into deepsleep, theres a 10-12 hour window to charge it and save or keep working.

I tested program that was loaded in RAM after I recharged in the morning and it worked fine, so the RAM was not lost.

I suppose operating out of the datasheets specs could result in inconsistant behavior in this regard, but until theres an issue, there is no issue.

If we had a program that was in deepsleep 90% of the time, we could theoretically run this device sitting on a window sill indefinitively. It only takes 3-6 minutes of direct sun to fully charge, and on a cloudy day maybe 10-12 minutes.

The next thing I am going to work on is add an interrupt  to STOP the running program. This interrupt will also wake the device from deepsleep if it was put in in that mode by user (not from low voltage).