One of the nicest logic families ever – HC (and HCT) CMOS – has the marvellous characteristic that someone thought about interfacing its inputs with voltages higher than Vdd early – and created a really nice input protection structure. What is good, is that, by design rather than luck, up to +20mA can be pushed …
This story continues at Interfacing AVRs (and Arduinos) to voltages higher than Vdd
Or just read more coverage at Electronics Weekly