In CMOS circuits will operate on a supply voltage (VDD) between 3 V and 18 V,
consequently the logic thresholds are determined by the actual supply voltage that is
used. ‘0’ is represented by voltages between 0 V and 1/3 of VDD, and ‘1’ by voltages
between 2/3 of VDD and VDD as shown in Figure 1-1 (B). By allocating voltage thresholds
in this fashion very definite regions of the pulse relate to the two logic levels.