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172 VLSI Test Principles and Architectures it may not require all decision variables before the ATPG can determine that it is on the wrong path. For example, if a certain path already sets a value on the fault site such that the fault is not excited, then no value combination on the remaining decision variables can help to excite and propagate the fault. Using Figure 4.7 as an example again, suppose the path a = 0, c = 1, d = 1 cannot excite the target fault ± . Then, the rest of the decision variables, b²e²f²³ ³ ³ , cannot undo the effect rendered by a = 0, c = 1, d = 1. Backtracking Whenever a conflict is encountered ( i.e. , a path segment leading to no solution), the search must not continue searching along that path, but must go back to some earlier point and re-decide on a previous decision. If only two choices are possible for a decision variable, then some previous decision needs to be reversed, if the other branch has not been explored before. This reversal of decision is called a
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This note was uploaded on 05/16/2011 for the course ENGINEERIN mp108 taught by Professor Elbarki during the Spring '08 term at Alexandria University.

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