What is max input pin voltage when esp32 is powered down?

[mchahn]

An esp32 died during my hardware development. Everything worked fine for a day or two and then the chip quit responding to any uart RX input from my ftdi usb converter. I have a suspicion that the problem might be that the ftdi kept putting out its TX signal at 3.3v with low impedance while my chip was powered down (the chip uses a different supply, not the usb power).

If the esp used normal protection diodes then I understand how this might have happened. The ftdi would be trying to power up the esp through the protection diode. I have seen this kind of behavior in other chips. If the esp drew a lot of current from the ftdi I could see how the protection diode, if not the input itself, might be damaged from the current.

But I have heard that the esp32 uses snapback circuits instead of diodes. If I understand them correctly the snapbacks are triggered at a specific voltage, like 6V, not by 0.7V above the vdd pin as diodes are.

The reason I am concerned is that I'm worried something else in my circuitry is at fault. I would appreciate any opinions as to my theory about the different kinds of protection work. To ask a specific question, what voltage is allowed on an input pin when the chip is powered down?

[username]

Its good practice to never have power going to any GPIO pin when the processor is powered down. Backfeed issues can kill a pin.
If they are powered from different sources you should some form of isolation to prevent this like and opto, or other means.