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Hazards, Critical Races, and Metastability
June 1995 (vol. 44 no. 6)
pp. 754-768

Abstract—The various modes of failure of asynchronous sequential logic circuits due to timing problems are considered. These are hazards, critical races and metastable states. It is shown that there is a mechanism common to all forms of hazards and to metastable states. A similar mechanism, with added complications, is shown to characterize critical races. Means for defeating various types of hazards and critical races through the use of one-sided delay constraints are introduced. A method is described for determining from a flow table situations in which metastable states may be entered. A circuit technique is presented for extending a previously known technique for defeating metastability problems in self-timed systems. It is shown that the use of simulation for verifying the correctness of a circuit with given bounds on the branch delays cannot be relied upon to expose all timing problems. An example is presented that refutes a plausible conjecture that replacing pure delays with inertial delays can never introduce, but only eliminate glitches.

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Index Terms:
Asynchronous, critical race, delays, dynamic hazards, essential hazards, inertial delays, metastability, pure delays, sequential logic, timing problems, timing simulation.
Stephen H. Unger, "Hazards, Critical Races, and Metastability," IEEE Transactions on Computers, vol. 44, no. 6, pp. 754-768, June 1995, doi:10.1109/12.391185
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