A Multiple Seed Linear Feedback Shift Register

Very frequently in built-in self-test (BIST) applications the random pattern test length is much shorter than the cycle length of the linear feedback shift register (LFSR) that generates those patterns. Starting with a given seed in the LFSR, the generated random patterns only constitute a short walk through the sequence of patterns producible by it. It is very often advantageous to uniformly draw patterns from the entire space producible by the LFSR. One way of achieving this is by changing seeds every so often. The change of seeds can be viewed as a jump from one place in the autonomous sequence produced by the LFSR, to another place in the same sequence. In this way the random patterns generated are not contiguous in nature, but do cover several intervals in the LFSR pattern-space. The object of this correspondence is to describe a design of an LFSR that can easily accommodate a change-of-seeds feature. This new LFSR is controlled by two separate clocks one for the normal LFSR operation and one for the change of seeds option.