Positive and Negative Extra Clocking of LFSR Seeds for Reduced Numbers of Stored Tests

When test data compression uses a linear-feedback shift-register (LFSR) for on-chip decompression, stored tests consist of seeds for the LFSR. Extra clocking of an LFSR seed, bringing the LFSR to one of its next-states, was shown to allow several different tests to be applied based on every stored test, thus reducing the number of seeds that need to be stored, or increasing the fault coverage. Extra clocking that brings the LFSR to its next-states is referred to as positive extra clocking. Only this type of clocking was used earlier. This article suggests to replace a seed by a previous state of the LFSR as a way to increase the effectiveness of this approach without changing the test application process. The computation of previous states is referred to as negative extra clocking. The procedure described in this article uses negative extra clocking to replace seeds with previous states while adjusting the positive extra clocking of the seeds. Experimental results are presented for benchmark circuits to demonstrate the importance of negative extra clocking in reducing the number of seeds that need to be stored.