Test compaction for synchronous sequential circuits by test sequence recycling

We introduce a new concept for test sequence compaction referred to as recycling. Recycling is based on the observation that easy-to-detect faults tend to be detected several times by a deterministic test sequence, whereas hard-to-detect faults are detected once towards the end of the test sequence. Thus, the suffix of a test sequence detects a large number of faults, including hard-to-detect faults. The recycling operation keeps a suffix S₁ of a test sequence T₁ and discards the rest of the sequence. The suffix S₁ is then used as a prefix of a new test sequence T₂ . In this process, S₁ is expected to detect the more difficult to detect faults as well as many of the easy-to-detect faults, resulting in a new sequence T₂ which is shorter than T₁ . Recycling is enhanced by a scheme where several faults are targeted simultaneously to generate the shortest possible test sequence that detects all of them