Going-low Switch Input Circuit

[dontz125]

I've been playing here for many years, and I've recommended the standard going-low guard circuit shown in all the hardware manuals to many folks over that time, but for some reason I've never really questioned it before. The series diode blocks voltages higher than the processor pin and the pull-up resistor; the pull-up resistor provides a definitive voltagestep when the switch closes. What function does the ground-to-pin diode serve? It looks like a flyback diode, but 5v through a 1k pull-up is only 5mA; I can't see too much induction effect built up. I seem to recall someone mentioning it was for reverse protection, but the series diode should prevent any issues there.

Is there a benefit? Or is this just one of those "It's always been done this way!" artifacts that accumulates in every body of wisdom?

lowinput.jpg

[jsmcortina]

It protects against voltage levels below zero.

James

[dontz125]

Ok. Fair enough. I've been pondering the second half of the equation here - what about the current that flows through that reverse voltage? This is fine for a momentary surge, but what about an actual misconnection? If a voltage source is hooked up backwards, you have a dead short through those two diodes, and something is going to fail, and I can't decide where to put a limiting resistor that wouldn't interfere with the flyback function. Am I overthinking this?