Combinatorial Approach to Multiple Contact Faults Coverage in Programmable Logic Arrays

The increasing number of applications of programmable logic arrays (PLA´s) has evoked the development of test generation methods for these circuits. There are known methods for complete single contact fault detection test set generation. These test sets fail to detect all multiple faults in a PLA due to the phenomenon of masking. In this correspondence, we present a method to quantitively predict the multiple fault coverage capability of a single fault detection test set in a PLA. The method enables us to determine the coverage ratio, which is defined as the ratio of the number of multiple contact faults detected by a single fault test Tc to the total number of all multiple faults. It is shown that the multiple fault coverage ratio of Tc drops with an increasing size of faults, and most unexpectedly, the ratio increases with an increasing number of rows. The number of crosspoints in one product line has very little influence on the ratio.