An approach to functional level testability analysis

The authors present an approach to testability analysis applicable to circuits containing functional modules described behaviorally. They consider two types of modules-combinational modules described by binary decision diagrams and sequential modules defined by state tables. Controllability and observability measures for such modules are defined, and algorithms are developed for computing them. The method has been applied to a few small modules and circuits, and appears to be feasible. The combinational measures, applicable to both combinational and sequential modules, indicate the probability of setting a lead to a particular value or observing the effect of a signal change at an output. The sequential measures, also applicable to both types of modules, give estimates of sequence lengths needed for controlling and observing any lead in the circuit. These two measures together give an indication of not only the difficulty in deriving tests for a circuit but also the length of test sequences that may be needed