The multi-bit synchronizer topology that is detected is equivalent to multiple single-bit synchronizers grouped together based on the startpoint-endpoint names and matching CDC rules. In this context, a bus is defined by the startpoint and endpoint cell names and not by the net names. The expected bus name format is
baseName[index]. Also the startpoint and endpoint indexes must match. The following figure shows an example of a multi-bit synchronizer that is 2 bits wide.
If all bits of a CDC bus do not match the same CDC rule, the bus is reported as single bits or bus segments with continuous indexes that match a same CDC rule.
It is essential to understand that having a register-based synchronizer on a bus does not make the domain crossing safe for the bus. This is the reason the CDC rule CDC-6 is a Warning, as the tool cannot decide whether or not the topology is adequate for the design. It is up to the designer to confirm the safety of the CDC.
If the bus is Gray coded, it is safe to use a register-based synchronizer on all the bits of the bus as long as the adequate timing constraints have been set on the bus to make sure that no more than one data at a time can be captured by the receiving domain.
If the bus is not Gray coded, other synchronizer topologies should be used instead, such as CE Controlled CDC or MUX Controlled CDC.