No current flowing in GPIO pin?

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jun 15, 2016 by lpcware
Content originally posted in LPCWare by DodigI on Sun Jun 08 04:12:01 MST 2014
Hello, can someone please explain this phenomena.

A GPIO pin is set as input with pull-up resistor enabled. Power supply is 3.3V. When I short the pin to ground a current of 55uA (60kR calculated pull-up value) is flowing through the pull-up resistor.
However, if I apply a 5V to the pin no current is flowing into the pin?! I would expect a current of ((5V-3.3V)/60kR) to flow, but I measure only 1nA (measured with a very precise source measure unit).

What is the catch here? I'm trying to understand how the ESD protection and 5V tolerant inputs work.. When the pull-up is disabled no current is flowing also, which makes sense if the pins are really 5V tolerant.

Thank you.