Clock domain crossing

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.^[1] 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.^[2]

References

1. ^ Parker, Roy H., Caution: Clock Crossing A prescription for uncontaminated data across clock domains, Chip Design Magazine, Issue 5, Article 32, July 2004.
2. ^ Stein, Mike, Crossing the abyss: asynchronous signals in a synchronous world, Paradigm Works, EDN Magazine, 24 July 2003.

Clock Domain Crossing Tools

These are some tools that support clock domain crossing.

• Encounter Conformal Equivalence Checker with Clock Domain Checks
• SpyGlass-CDC
• 0-In Clock-Domain Crossing (CDC)

See also

• Technical Paper on Clock Domain Crossing - A Comparison of Metastability Modeling Methods
• White Paper on Clock Domain Crossing
• Crosstalk (electronics)
• Metastability in electronics
• Clock Domain Crossing Article - Simulation Falls Short with Asynchronous Clocks