Hierarchical functional timing analysis

We propose a hierarchical timing analysis technique for combinational circuits under the tightest known sensitization criterion, the XBDO delay model. Given a hierarchical combinational circuit, a generalized delay model of each leaf module is characterized first. Since this timing characterization step takes into account false paths in each module, the delay model is more accurate than the one obtained by topological analysis. Then topological delay analysis is performed on the circuit composed of generalized gates replacing the leaf modules, where the "gate" delay model is the derived one. As far as the authors know, this is the first result that shows that hierarchical analysis is possible under state-of-the-art tight sensitization criteria. We demonstrate by experimental results that loss of accuracy in using the hierarchical approach is very minimal in practice. The theory developed in this paper also provides a foundation for incremental timing analysis under accurate sensitization criteria.