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CDC design principles

Making CDC interfaces predictable


Metastability is unavoidable in CDC designs and so some strict design principles are necessary for robust CDC interfaces.

Metastability can be contained with “synchronizers” that prevent metastability effects from propagating into the design. The diagram shows the configuration of a double-flop synchronizer which minimizes the load on the metastable flop. The single fan-out protects against loss of correlation because the metastable signal does not fan out to multiple flops.

The probability that metastability will last longer than time t is governed by the following equation:

wheretauis the resolution time constant dependent upon the latch characteristics and ambient noise. This configuration resolves metastability with a very high probability, leading to a very large mean time between failures as governed by the equation:

where P is the probability that metastability is not resolved within one clock cycle. Triple or higher flop configurations may be used for very fast designs.

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