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 S-1 of a test sequence T-1 and discards the rest of the sequence. The suffix S-1 is then used as a prefix of a new test sequence T-2. In this process, S-1 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 T-2 which is shorter than T-1. Recycling is enhanced by a scheme where several faults are targeted simultaneously to generate the shortest possible test sequence that detects all of them.