I've not seen any official figures for clamping diodes on AVR pins, but have seen a post on this forum from someone saying Atmel told them 0.5V, 1mA.
I'm working on a test rig where several ATmega48 boards need to be placed on an I2C bus (each board is an I2C slave). The problem is, each board has its own power supply which can be anything from 1.8V to 5V.
If my I2C master device pulls SDA/SCL up to +3.3 via 2K2 resistors, does anyone think I'm likely to encounter problems?
Are external clamping diodes likely to be of any use?