202_pdfsam_VLSI TEST PRINCIPLES & ARCHITECTURES

202_pdfsam_VLSI TEST PRINCIPLES & ARCHITECTURES -...

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Test Generation 171 In certain cases, it may not be possible for the ATPG to deduce that all vectors with some primary input set to a given logic value definitely do not qualify to be solution vectors. However, it may be able to make an intelligent guess that input vectors with primary input # i set to some specific logic value are more likely to lead to a solution. In such a case, the ATPG would make a decision on primary input # i . Because the decision may actually be wrong, the ATPG may eventually have to alter its decision, trying the vectors that have the opposite Boolean value on primary input # i . The process of making decisions and reversing decisions will result in a decision tree . Each node in the decision tree represents a decision variable. If only two choices are possible for each decision variable, then the decision tree is a binary tree. However, there may be cases where multiple choices are possible in a general search tree. Figure 4.7 shows an example decision tree. While this figure only allows decisions
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