In this application, a 2-input XNOR gate is used as a phase difference detector as shown in Figure 8-1. The remaining three gates can be used for other applications in the system, or the inputs can be grounded and the channels left unused.

The device is used to identify phase difference between a reference clock and another input clock. Whenever the clock states are different, the XNOR output will pulse HIGH until the clocks return to the same state. The output is fed into a low-pass filter to obtain a DC representation of the phase difference.