A clock domain crossing (CDC), or simply clock crossing, is when a signal crosses from one clock domain into another. If a signal does not assert long enough and is not registered, it may appear asynchronous on the incoming clock boundary.
Synchronizing a signal that crosses into a higher clocked domain can be accomplished by registering the signal through a flip-flop that is clocked by the source domain, thus holding the signal long enough to be detected by the higher clocked destination domain. Synchronizing a signal traversing into a slower clock domain is more cumbersome. This typically requires a register in each clock domain with a form of feedback from the destination domain to the source domain, indicating that the signal was detected.
