Well, the clamping diodes that are present in all I/O pins are supposed to be used only for ESD protection. I think they were refering to the fact that some peo

Well, the clamping diodes that are present in all I/O pins are supposed to be used only for ESD protection. I think they were refering to the fact that some people have used them (and a quite big series resistor) as a "cheap" way to connect them to AC directly. There's even a application note (from 1997!) mentioning this: http://ww1.microchip.com/downloads/en/AppNotes/00521c.pdf http://ww1.microchip.com/downloads/en/AppNotes/00521c.pdf Obviously the device loses its galvanic isolation and should be kept in an enclosure that can't be touched. This can also be used for "accepting" 5v inputs in 3.3v devices, provided that the clamping current doesn't support the maximum allowed. Other microcontrollers don't always define maximum values for those currents, so it might be harder to use them like that.