18.104.22.168 1.8V and 3.3V Signal Transition Levels
All input and output timing parameters are referenced to Vref for both "0" and "1" logic levels. Vref = (VDD I/O)/2.
Figure 7-2 Input and Output Voltage Reference Levels for AC Timing Measurements
All rise and fall transition timing parameters are referenced to VIL MAX and VIH MIN for input clocks, VOL MAX and VOH MIN for output clocks.
Figure 7-3 Rise and Fall Transition Time Voltage Reference Levels