When an input is high, Attiny85 starts working with no Vcc

On the ATtiny (and most digital chips) there are diodes that connect each input to Vcc and Vss; they are for protection against electrostatic discharge (ESD) and normally they don’t conduct. In this case, the input is higher than Vcc (since power isn’t connected) so it conducts and feeds power to Vcc. They can’t handle lots of current, but are great for causing surprises like this :slight_smile:

Note that the reset pin doesn’t have a protect diode to Vcc since it can be driven to 12V during high-voltage programming and you wouldn’t want that back-feeding Vcc.

/mike