64 VLSI Test Principles and Architectures As mentioned in Section 2.4.3, the operation of a polarity-hold SRL is race-free if clocks C and B as well as A and B are nonoverlapping. This characteristic is used to implement LSSD circuits that are guaranteed to have race-free operation in normal mode as well as in test mode. The required design rules [Eichelberger 1977] [Eichelberger 1978] are briefly summarized below: ± All storage elements must be polarity-hold latches. ± The latches are controlled by two or more nonoverlapping clocks such that any two latches where one feeds the other cannot have the same clock. ± A set of clock primary inputs must exist from which the clock ports of all SRLs are controlled either through a single clock tree or through logic that is gated by SRLs and/or non-clock primary inputs. In addition, the following three conditions should be satisfied: (1) all clock inputs to SRLs must be inactive when clock PIs are inactive, (2) the clock input to any SRL must be controlled
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