Last time we looked at how metastability is unavoidable and the nature of the clock domain crossing (CDC) problem. This time we will look at design principles.
CDC design principles
Because metastability is unavoidable in CDC designs, the robust design of CDC interfaces is required to follow some strict design principles.
Metastability can be contained with “synchronizers” that prevent metastability effects from propagating into the design. Figure 9 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: