46VLSI Test Principles and ArchitecturesMany methods have been developed to calculate the probability-based testabilitymeasures. A simple method is given below, whose basic procedure is similar to theone used for calculating combinational testability measures in SCOAP except thatdifferent calculation rules are used. The rules for probability-based controllabilityand observability calculation are summarized in Tables 2.3 and 2.4, respectively.In Table 2.3,p0is the initial 0-controllability chosen for a primary input, where0< p0<1.Compared to SCOAP testability measures, where non-negative integers are used,probability-based testability measures range between 0 and 1. The smaller aprobability-based testability measure of a signal, the more difficult it is to control orobserve the signal. Figure 2.3 illustrates the difference between SCOAP testabilityTABLE 2.3Probability-Based Controllability Calculation Rules0-Controllability (Primary Input,Output, Branch)1-Controllability (Primary Input,Output, Branch)
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