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178 VLSI Test Principles and Architectures D x x x D D -frontier x x (b) D -frontier contains two gates (a) D -frontier contains one gate D -frontier D x x x x x a x ± FIGURE 4.12 Illustrations of D -frontier. x D x 0 x J -frontier x x ± FIGURE 4.13 Illustration of J -frontier. Whenever there are paths to choose from in advancing the D -frontier, observ- ability values can be used to select the corresponding gates. However, this does not guarantee that the more observable path will definitely lead to a solution. When a D or a D has reached a primary output, all the gates in the
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Unformatted text preview: J-frontier must now be justified. This is done by advancing the J-frontier backward by placing pre-decessor gates in the J-frontier such that they justify the previous unjustified gates. Similar to propagation of the fault-effect, whenever a conflict occurs, a backtrack must be invoked. In addition, at each step, the D-frontier must be checked so the D (or D ) that has reached a primary output is still there. Otherwise, the search returns...
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