On Using Twisted-Ring Counters for Test Set Embedding in BIST

We present a novel built-in self-test (BIST) architecture for high-performance circuits. The proposed approach is especially suitable for embedding precomputed test sets for core-based systems since it does not require a structural model of the circuit, either for fault simulation or for test generation. It utilizes a twisted-ring counter (TRC) for test-per-clock BIST and is appropriate for high-performance designs because it does not add any mapping logic to critical functional paths. Test patterns are generated on-chip by carefully reseeding the TRC. We show that a small number of seeds is adequate for generating test sequences that embed complete test sets for the ISCAS benchmark circuits. Instead of being stored on-chip, the seed patterns can also be scanned in using a low-cost, slower tester. The seeds can be viewed as an encoded version of the test set that is stored in tester memory. This requires almost 10X less memory than compacted test sets obtained from ATPG programs. This allows us to effectively combine highquality BIST with external testing using slow testers. As the cost of high-speed testers increases, methodologies that facilitate testing using slow testers become especially important. The proposed approach is a step in that direction.