>Being a digital input on the PIC, it shouldn't make a difference
>whether it is a generic input pin or an I2C input. Therefore, you
>would just look at the electrical characteristics charts at the end of
>the datasheet to find the min/max voltage thresholds for a digital
>input pin.

But I believe the SMBus levels assume the use of 3V3 logic, so have lower 
threshold levels than 'normal' 5V I2C levels (and this would be borne out by 
the OPs quoted levels). I believe this is why the extra function is added. 
It may not appear on 3V3 only PICs, I haven't checked. 

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