Generalized Feed Forward Shift Registers and Their Application to Secure Scan Design

Abstract

In this paper, we introduce generalized feed-forward shift registers (GF²SR) to apply them to secure and testable scan design. Previously, we introduced SR-equivalents and SR-quasi-equivalents which can be used in secure and testable scan design, and showed that inversion-inserted linear feed-forward shift registers (I²LF²SR) are useful circuits for the secure and testable scan design. GF²SR is an extension of I²LF²SR and the class is much wider than that of I²LF²SR. Since the cardinality of the class of GF²SR is much larger than that of I²LF²SR, the security level of scan design with GF²SR is much higher than that of I²LF²SR. We consider how to control/observe GF²SR to guarantee easy scan-in/out operations, i.e., state-justification and state-identification problems are considered. Both scan-in and scan-out operations can be overlapped in the same way as the conventional scan testing, and hence the test sequence for the proposed scan design is of the same length as the conventional scan design. A program called WAGSR (Web Application for Generalized feed-forward Shift Registers) is presented to solve those problems.