Storage-Based Logic Built-In Self-Test with Variable-Length Test Data

Storage-based logic built-in self-test (LBIST) approaches store deterministic test data on-chip, and use them for test application. The applied tests are closer to deterministic tests than the pseudo-random tests that are typically produced by LBIST. In earlier approaches, the stored test data consisted of scan vectors of equal length. This article describes a storagebased LBIST approach where the stored test data have variable length. Instead of storing a scan vector directly, the approach described in this article stores a sequence that, when repeated, produces a scan vector. The use of variable-length sequences that are shorter than scan vectors reduces the storage requirements. The article describes a software procedure for computing a set of variable-length sequences for a set of target faults. Experimental results are presented for single stuck-at and single-cycle gate-exhaustive faults in benchmark circuits to demonstrate the discussion.