The following figure shows a multiplexer controlled CDC example. In this structure, the
multiplexer select signal is synchronized to the destination clock domain,
clk_b.
Figure 1. Mux-Controlled CDC Example
As with clock enable controlled CDC, the CDC engine does not restrict how the select
signal is synchronized. The select signal must be reported as a safe CDC individually.
You are responsible for constraining the crossing delay on FF2_c,
typically with a set_max_delay -datapath_only constraint.