Clock domain crossing
A clock domain crossing (CDC), or simply clock crossing, is a crossing by a signal 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.
See also 
- Crosstalk (electronics)
- Metastability in electronics
- globally asynchronous locally synchronous
- The topic is duplicated in Flip-flop_(electronics)#Timing_considerations
- Parker, Roy H., Caution: Clock Crossing A prescription for uncontaminated data across clock domains, Chip Design Magazine, Issue 5, Article 32, July 2004.
- Stein, Mike, Crossing the abyss: asynchronous signals in a synchronous world, Paradigm Works, EDN Magazine, 24 July 2003.
- Understanding Clock Domain Crossing Issues
- White Paper on Clock Domain Crossing
- Clock Domain Crossing Article - Simulation Falls Short with Asynchronous Clocks
- Technical Paper on Clock Domain Crossing - A Comparison of Metastability Modeling Methods
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