Delay computation in combinational logic circuits: Theory and algorithms

The authors provide necessary and sufficient conditions for a path to be true in the floating mode of operation. Static cosensitization is introduced as a necessary condition, which allows one to avoid the problem of identifying false paths as responsible for delay. The results are extended to determine the truth or falsity of entire sets of paths simultaneously by expressing them in terms of the testability of a multifault in an ENF (equivalent normal form) expression. This result is applied directly to an unmodified multilevel circuit. Because the circuits that are most troublesome for false-path-eliminating static timing analyzers are those with millions of paths, and in particular millions of longest paths, the ability to handle entire sets of paths simultaneously results in a very efficient delay computation procedure. This is demonstrated by the results from a preliminary implementation of the algorithm.