Hazards, Critical Races, and Metastability

  • Authors:
  • Stephen H. Unger

  • Affiliations:
  • -

  • Venue:
  • IEEE Transactions on Computers
  • Year:
  • 1995

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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.