A new procedure for weighted random built-in self-test

It is proposed that a pseudorandom sequence and a single weighted random sequence be used to implement built-in self-test (BIST) efficiently in a large integrated scan circuit which would otherwise need an excessive pseudorandom test length. A method of determining the weight set and the approximate pseudorandom and weighted random test lengths, based on fast fault simulation tools, is suggested. By modifying specific scan cells, the BIST hardware conditionally generates the weighted stream locally, at specific input sites. A weighted control signal is used to regulate the proportion of weighted and pseudorandom inputs. Apart from determining that, in the cases examined, one weight set was sufficient for a notable decrease in test time, it was also noticed that a very coarse weight set (i.e. restricting biases to 0, 0.25, 0.5, 0.75, and 1) provides acceptable results. Using finer resolution within the weight set usually results in a slightly higher coverage, but at the expense of a much higher area overhead.<
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