I am trying to find out what these voltages are for the Photon 2 (VIH = minimum input voltage construed as high; VIL = maximum input voltage construed as low; etc.). I found this table, but I'm not sure how to read it. Related to this, what happens if a voltage between VIH and VIL is applied? What would digitalRead() return? I'm thinking that it's indeterminate, but I just wanted an expert opinion ...
Those numbers are for the Photon 1 which is a completely different MCU (STM32F205).
The P2 and Photon 2 (RTL8721DM) have a minimum input high voltage of 2.0V and a maximum input low voltage of 0.8V.
The space between 0.8V and 2.0V is indeterminate. If you expect to have voltages in that range, you should either measure it as an analog input (A pin, using analogRead), or insert a Schmitt trigger IC in front of the GPIO digital input.
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