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Test Generation 235 a b c On-input ± FIGURE 4.50 Sensitization criteria. ± When the corresponding on-input of P has a noncontrolling to controlling transition, the values for the off-input must be a steady noncontrolling value for both vectors. Because a robust test for a path P can detect P irrespective of any other delay faults in the circuit, they are the most desirable tests. For most circuits, however, the number of robustly testable paths is small. Thus, for those robustly untestable paths , less restrictive tests must be sought. Consider the AND gate shown in Figure 4.50. Suppose signal a is the on-input and b is the off-input along some path. In the robust sensitization criterion, because the on-input is going from a noncontrolling to a controlling value, the off-input must be at steady noncontrolling value. As discussed before, such a restriction will ensure that the path going through a , if testable, will be tested irrespective of any other delay faults in the circuit. However, if such a test is not possible, one may wish to relax the condition such that the target path is the only faulty path in the
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Unformatted text preview: circuit. In other words, if the off-input b is not late, then it may be possible to relax the steady noncontrolling value somewhat. In this example, if the transition on the target path through a is late, and if the transition on the off-input is on time, then the output c will still have a faulty value. Therefore, it may be sufficient to require the values of X 1 for b instead of a steady 1. This sensitization condition is called the nonrobust sensitization condition. Figure 4.51 illustrates an example of a path, ↑ bcdf , that is robustly untestable but is nonrobustly testable. Note that in a robustly testable path, a transition is present at every gate along the path. On the other hand, in a nonrobustly testable path, some transitions may be lost along the path. In the example shown in Figure 4.51, the transition is lost at f . f a e d c b S 1 ± FIGURE 4.51 A nonrobust path....
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