Design for Testability for Path Delay Faults in Sequential Circuits

We experimentally study the reasons for low coverage of path delay faults in several sequential benchmark circuits. Causes for undetected faults are classified into three categories: (A) Combinationally nonactivated paths, (B) Sequentially nonactivated paths, and (C) Unobservable fault effect. The type A faults can only be made detectable by modifying or resynthesizing the combinational logic as has been discussed by others. We find that almost 80% of sequentially untested faults are in category B. Most are not activated because the two successive states necessary to create a transition and to propagate it through the path cannot be produced in the sequential circuit. We study the partial scan technique in which flipflops are scanned to break cycles and show that a substantial increase in the coverage of path delay faults is possible.