Before delivering, the manufactured ICs are tested for defects. Diagnosis is the procedure to determine the sites where defects are likely to be present in a chip that failed a test set. For any diagnosis algorithm the accuracy of diagnosis depends on the test pattern set employed. More the test pattern set distinguishes fault pairs, more is the diagnosis quality. This paper addresses issue of assigning values to don't care bits (X bits) in the test pattern set so that the circuit can be reasonably diagnosed by analyzing the output response. For this, a Particle Swarm Optimization (PSO) based approach has been proposed to fill the don't-care bits. The proposed algorithm when run on full-scan ISCAS'89 benchmark circuits shows significant improvement over 0-fill and 1-fill based don't care filling and reasonably good improvement over random-fill based techniques.
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