Hierarchical Defect-Oriented Fault Simulation for Digital Circuits

A new fault model is developed for estimation of the coverage of physical defects in digital circuits for given test sets. Based on this model, a new hierarchical defect oriented fault simulation method is proposed. At the higher level simulation we use the functional fault model, at the lower level we use the defect/fault relationships in the form of defect coverage table and the conditional defect probabilities. A description and the experimental data are given about probabilistic analysis of a complex CMOS gate. Analysis of the quality of 100% stuck-at fault test sets for two benchmark circuits in covering physical defects like internal shorts, stuck-open’s and stuck-on’s. It has been shown that in the worse case a test with 100% stuck-at fault coverage may have only 50% coverage for internal shorts in complex CMOS gates. It has been shown also that classical test coverage calculation based on counting of defects detected without taking into account the defect probabilities may lead to considerable overestimation of results.