I have just answered a question here, and it has spurred me to ask a question:
How fast will this type of protection work? Would it work with, say, a 500kHz signal? And is there an easy way to work this out fro the zener datasheet? (Using zener and resistive divider to protect from overvoltage)
I would expect, at some point, that oscillations would occur where the zener doesn't break down fast enough?
The previous question: 12V input on microcontroller pin
Schematic example given by Michael on that thread: