On the Unavoidability of Metastable Behavior in Digital Systems

Fault-free digital systems can fail as a result of metastable behavior when asynchronous inputs have critical timing combinations. The problem of metastable behavior is generally considered to be unavoidable in digital systems that synchronize asynchronous inputs. This correspondence extends previous results on the unavoidability of metastable behavior. The set of inputs to the digital system is generalized to cover a wide range of possibilities encountered in practical circuits whilst still maintaining the result of unavoidability of metastable behavior. A sufficient condition for the set of inputs to contain a range of inputs that excite metastable behavior is that the set of input functions is connected and contains two input functions which drive the system to different stable states.