MarkT:
The protection diodes are rated at 0.5V max forward bias - that's rather less that 20mA I think - or have I missed the right part of the specs?Vf and Imax are two different specification for the internal clamping diodes. The first tells when it starts to conduct, the second how much current can conduct before damage.
Actually specifying the voltage is a rather poor way of doing this as forward voltage varies significantly across the operating temperature range.
Yea, the Vf spec is of just passing interest, we know that a 5 volt signal will fully turn on the positive clamping protection diode on a input digital pin on a 3.3 volt powered board, what we want to know is how much continous current the diode can withstand, that is the datasheet spec of interest.
Lefty