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Linear Feedback Shift Register Test Pattern Generator With Nonlinear "Spoiler"

IP.com Disclosure Number: IPCOM000101019D
Original Publication Date: 1990-Jun-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 2 page(s) / 48K

Publishing Venue

IBM

Related People

Eichelberger, EB: AUTHOR [+2]

Abstract

Linear feedback shift registers (LFSRs) are used as sources of pseudo-random test patterns. An LFSR of length n creates strings of bits m long, where m equals the number of primary inputs (PIs) and shift register latches (SRLs) in a device to be tested. However, if m>n, some test patterns cannot be created. Certain patterns are impossible because of linear dependencies between bits in the pseudo-random sequence.

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Linear Feedback Shift Register Test Pattern Generator With Nonlinear "Spoiler"

       Linear feedback shift registers (LFSRs) are used as
sources of pseudo-random test patterns.  An LFSR of length n creates
strings of bits m long, where m equals the number of primary inputs
(PIs) and shift register latches (SRLs) in a device to be tested.
However, if m>n, some test patterns cannot be created.  Certain
patterns are impossible because of linear dependencies between bits
in the pseudo-random sequence.

      An earlier solution to this problem ("True/Complement Linear
Feedback Shift Register Test Pattern Generator," IBM Technical
Disclosure Bulletin 27, 10A, 5729-5731 (March 1985)) reduced the size
of the problem, but is recognized as an incomplete solution.  It
eliminated all odd-bit linear dependencies, but was ineffective with
even-bit linear dependencies.

      The problem can be overcome by adding a non-linear "spoiler"
circuit to a normal LFSR pattern generator.  A simple realization of
the spoiler circuitry, labeled A in the figure, is, for instance, a
two-way AND circuit.  In addition, a single two-way EXOR circuit,
labeled B, is added after the final stage of the LFSR.  This EXOR
circuit will complement the LFSR output bit given a logical "1"
signal from the output of the spoiler circuit.  The result of
implementing the spoiler circuit with a two-way AND is that 1/4 of
the bits generated, by the underlying conventional LFSR, will be
complemented b...