On identifying don't care inputs of test patterns for combinational circuits

Given a test set for stuck at faults, some of primary input values may be changed to opposite logic values without losing fault coverage. We can regard such input values as don't care (X). In this paper, we propose a method for identifying X inputs of test vectors in a given test set. While there are many combinations of X inputs in the test set generally, the proposed method finds one including X inputs as many as possible, by using fault simulation and procedures similar to implication and justification of ATPG algorithms. Experimental results for ISCAS benchmark circuits show that approximately 66% of inputs of un-compacted test sets could be X in average. Even for compacted test sets, the method found that approximately 47% of inputs are X. Finally, we discuss how logic values are reassigned to the identified X inputs where several applications exist to make test vectors more desirable.

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