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

Discussions on the software and hardware for Atmel's STK standard.

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Remo
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Joined: Sun Jul 02, 2017 11:51 am

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

Post by Remo » Sun Jul 02, 2017 12:14 pm

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!

n1ist
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Joined: Wed Mar 22, 2006 11:02 am

Re: When an input is high, Attiny85 starts working with no V

Post by n1ist » Mon Jul 03, 2017 8:14 am

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

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