Hi, I’m a beginner. I try to program an Attiny85 by Arduino Uno as ISP. When I turn the power of the Attiny85 off, by disconnecting the Vcc, it remains on as long as the input is high (the Vcc and the input have two different power source grounded together).
I wonder if it’s because of the inernal pullup of the Attiny85. If so, how can I disable it? I defined the pin as an INPUT not as an INPUT_PULLUP.
Thanks!
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 
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