The following figure shows a multiplexer data-hold CDC example. In this structure, the
multiplexer select signal is synchronized to the destination clock domain,
clk_b, and data_out feeds back to the multiplexer
input.
Figure 1. Mux Data Hold CDC Example
As with clock enable controlled CDC, there is no restriction on how the multiplexer select
signal is synchronized. The signal must be reported as a safe CDC path individually. You
are responsible for constraining the crossing delay on FF2_c, typically
using a set_max_delay -datapath_only constraint.