Partial BIST insertion to eliminate data correlation

A new partial built-in self-test (BIST) insertion approach based on eliminating data correlation to improve pseudorandom testability is presented. Data correlation causes the circuit to be in a subset of states more or less frequently, which leads to low fault coverage in pseudorandom test. One important cause of correlation is reconvergent fanout. Incorporating BIST test flipflops into reconvergent paths will break correlation, however, breaking all reconvergent fanout is unnecessary since some reconvergent fanout results in negligible correlation. We introduce a metric to determine the degree of correlation caused by a set of reconvergent fanout paths. We use this metric to identify problematic reconvergent fanout which must be broken through partial BIST insertion. Based on this metric, we provide an exact method and a heuristic method to measure the data correlation. We provide an algorithm to break high correlation reconvergent paths. Our algorithm provides high fault coverage while selecting fewer BIST flipflops than required using loop-breaking techniques. Experimental results produced using our exact algorithm rank on average among the top 11.6% of all possible solutions with the same number of flip-flops.