An Evaluation of a Testability Measure for State Assignment to Estimate Transition Fault Coverage for Controllers

With shrinking feature sizes, growing clock frequencies, and decreasing power supply voltage, modern VLSIs are increasingly suffering from the impact of timing related defects. Therefore, transition fault testing is necessary. However, test generation for transition faults generally identifies many untestable faults due to the circuit structures and functions of VLSIs. When a target fault is untestable, a test that would have covered its site is missing from the test set. As a result, the test set might not detect defects around the site, even if the defects are detectable. Therefore, design for testability to improve transition fault coverage is very important. This paper proposes PS2R value which is a measure to represent the number of sensitizable paths between status registers in controllers after assigning state codes. Experimental results for the MCNC'91 benchmark circuits showed that there was a strong correlation between PS2R value and transition fault coverage.