Synchronizers are the special structures that are put on the clock domain crossings to prevent metastability arising due to the non-deterministic nature of the capturing event.
The most important design parameters which govern the synchronizer design are the robustness, area, latency and the power. The robustness of synchronizers is typically measured by the MTBF (Mean Time Before Failure), which is the average time between two successive failure events due to metastability. A carefully designed synchronizer may be MTBF in the order of few years to a few million years. Two-flop synchronizers are the most commonly used structures to avoid metastability, however FIFO based synchronizers are most robust.
