In a synchronous system, we can avoid putting our flip-flops in illegal or metastable states by always obeying the setup- and hold-time constraints. When sampling asynchronous signals or crossing between different clock domains, however, we cannot guarantee that these constraints will be met. In these cases, we design a synchronizer that, through a combination of waiting for metastable states to decay and isolation, reduces the probability of synchronization failure.

A brute-force synchronizer consisting of two back-to-back flip-flops is commonly used to synchronize single-bit signals. The first flip-flop samples the asynchronous signal and the second flip-flop isolates the possibly bad output of the first flip-flop until any illegal states are likely to have decayed. Such a brute-force synchronizer cannot be used on multi-bit signals unless they are encoded with a Gray code. If multiple bits are in transition when sampled by the synchronizer, they are independently resolved, possibly resulting in incorrect codes, with some bits sampled before the transition and some after the transition. We can safely synchronize multi-bit signals with a FIFO (first-in first-out) synchronizer. A FIFO serves both to synchronize the signals and to provide flow control, ensuring that each datum produced by a transmitter in one clock domain is sampled exactly once by a receiver in another clock domain – even when the clocks have different frequencies.