On identifying functionally untestable transition faults

This paper presents a new approach on identifying functionally untestable transition faults in nonscan sequential circuits. We formulate a new dominance relationship for transition faults and use it to identify more sequentially untestable transition faults. The proposed method consists of two phases: first, a large number of functionally untestable transition faults is identified by a fault-independent sequential logic implications implicitly crossing multiple time-frames, and the identified untestable faults are classified into three conflict categories. Next, additional functionally untestable transition faults are identified by dominance relationships from the previous identified untestable transition faults. The experimental results for ISCAS89 sequential benchmark circuits showed that our approach can quickly identify many more functionally untestable transition faults than previously reported.