I connected up a pot between ground and 3V3 so I could set the voltage on the wiper between 0 and 3V3. I then attached the wiper to a GPIO set as an input and displayed level changes.
On my Pi
- voltages above 1.55 were displayed as high.
- voltages below 1.26 were displayed as low.
- voltages between 1.27 and 1.54 continually changed state between high and low (every few microseconds).