OBO: An Output-by-Output Scoring Algorithm for Fault Diagnosis

Some of the simplest and most effective fault diagnosis procedures for scan circuits use scoring algorithms for associating a number, called a score, with every modeled fault they consider. The score measures the likelihood that the fault is present in a chip given its observed output response. The faults with the highest scores are included in the set of candidate faults. However, scoring algorithms do not provide information about how many of the highest scores should be used for defining a set of candidate faults that will point accurately to the defect locations in the chip. This paper introduces a new type of scoring algorithm to address this issue. The new algorithm, referred to as output-by-output scoring, considers every output value separately, and defines a set of candidate faults with the very highest scores on an output-by-output basis. This allows the algorithm to adjust the scores it uses automatically on an output-by-output basis and compute an accurate set of candidate faults.