Delay Characterization of Combinational Modules1

We address three related issues on timing characterization of combinational modules. We first introduce a new notion called timing safe-replaceability as a way of comparing the timing characteristics of two combinational modules formally. This notion allows us to determine whether a new module is a safe replacement of an original module in terms of timing under any surrounding environment. Second, we consider false path detection of combinational modules. Although false path detection is essential to accurate delay modeling, we argue that the conventional definition of false paths is not appropriate for defining the falsity of a path for a combinationalmodule since the falsity is relative to an arrival time condition. We introduce a new definition of false paths to resolve this issue. Finally, we propose a new algorithm that removes these false paths from a combinational module by a circuit transformation. We prove that the resulting circuit is a timing safe-replacement of the original.