Improving Path Delay Testability of Sequential Circuits

We analyze the causes of low path delay fault coverage in synchronous sequential circuits and propose a method to improve testability. The three main reasons for low path delay fault coverage are found to be: A) Combinationally false (nonactivatable) paths; B) Sequentially nonactivatable paths; and C) Unobservable fault effects. Accordingly, we classify undetected faults in Groups A, B and C. Combinationally false paths can be made testable by modifying the circuit or resynthesizing the combinational logic as discussed by other researchers. Majority of the untestable faults are, however, found in Group B, where a signal transition can not be functionally propagated through a combinational path. A test requires two successive states necessary to create a signal transition and propagate it through the target path embedded in the sequential circuit. We study a partial scan technique in which flip-flops are scanned to break cycles and show that a substantial increase in the coverage of path delay faults is possible.