The reliability of approximate testability measures

Techniques for gauging the accuracy of approximate testability measures that estimate the random-pattern testability of gate-level faults in designs with combinational logic are considered. The measures examined are overall fault-exposure distribution, high coverage, and fault grading. Sampling techniques are compared with the Stafan and Protest approximate testability measures. For random-pattern testing, it is clear that state-of-the-art testability measures like Stafan and Protest do provide some information about the testability of single faults or complete designs, but this information is not accurate; in many areas of use they cannot compete with carefully chosen sampling techniques. The three techniques described here are applicable to testing strategies other than the random-pattern testing of stuck-at faults; they are equally useful in a weighted random-pattern testing environment, for example.<>