Globally Functional Transparent-Scan Sequences

Abstract

Maintaining functional operation conditions during the application of scan-based tests is important for avoiding overtesting of delay faults. This issue has been considered for the functional capture cycles of scan-based tests, where the faults are typically detected. When scan shift cycles are used for fault detection, functional operation conditions are important during scan shift cycles as well. A transparent-scan sequence allows both types of clock cycles to be considered uniformly, allowing faults to be detected during functional capture as well as scan shift cycles. This article defines the concept of a globally functional transparent-scan sequence that maintains close-to-functional operation conditions during all its clock cycles. The procedure described in this article for computing globally functional transparent-scan sequences starts from a transparent-scan sequence that avoids scan shift cycles in order to maintain functional operation conditions. The procedure introduces scan shift cycles into the sequence in order to detect target faults. This process is guided by a transparent-scan sequence that is translated from a conventional scan-based test set and achieves the same fault coverage. Experimental results for benchmark circuits demonstrate the tradeoff between the proximity to functional operation conditions and the fault coverage achievable by the globally functional transparent-scan sequence.