Secure and Testable Scan Design Utilizing Shift Register Quasi-equivalents

Scan design makes digital circuits easily testable, however, it can also be exploited to be used for hacking the chip. We have reported a secure and testable scan design approach by using extended shift registers called "SR- equivalents" that are functionally equivalent but not structurally equivalent to shift registers(14), (15), (16), (17), (18). In this paper, to further extend the class of SR-equivalents we introduce a wider class of circuits called "SR-quasi- equivalents" which still satisfy the testability and security similar to SR-equivalents. To estimate the security level, we clarify the cardinality of each equivalent class in SR-quasi-equivalents for several linear structural circuits, and also present the actual number of SR-quasi-equivalents obtained by the enhanced program SREEP.