If a CDC bus uses gray-coding (for example, FIFO) or if latency needs to be
limited between the two asynchronous clocks on one or more signals, you must use the
set_max_delay constraint with the option
-datapath_only to ignore clock skew and jitter on these
paths, plus override the default path requirement by the latency requirement. It is
usually sufficient to use the source clock period for the max delay value, just to
ensure that no more than one data is present on the CDC path at any given time.
When the ratio between clock periods is high, choosing the minimum of the source and destination clock periods is also a good option to reduce the transfer latency. A clean asynchronous CDC path should not have any logic between the source and destination sequential cells, so the Max Delay Datapath Only constraint is normally easy to meet for the implementation tools.
Some asynchronous CDC paths require a skew control between the bits of the bus
instead of a constraint on the bus latency. Using a bus skew constraint prevents the
receiving clock domain from latching multiple states of the bus on the same clock edge.
You can set the bus skew constraint on the bus with
set_bus_skew command. For example, you can apply
set_bus_skew to a CDC bus that uses gray-coding instead of using the Max
Delay Datapath Only constraint. For more information, see this link in the
Vivado Design Suite User Guide: Using
For the paths that do not need latency control, you can define a point-to-point false path constraint.