Browse Prior Art Database

Improved LSSD Scan Test

IP.com Disclosure Number: IPCOM000120008D
Original Publication Date: 1991-Mar-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 2 page(s) / 49K

Publishing Venue

IBM

Related People

McAnney, WH: AUTHOR [+3]

Abstract

Disclosed is a method that avoids the necessity of computing an expected signature when testing an LSSD shift register string using a linear feedback shift register (LFSR) to drive the string input and a single- input signature register (SISR) to compress the string output pattern.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 84% of the total text.

Improved LSSD Scan Test

      Disclosed is a method that avoids the necessity of
computing an expected signature when testing an LSSD shift register
string using a linear feedback shift register (LFSR) to drive the
string input and a single- input signature register (SISR) to
compress the string output pattern.

      The LFSR feedback must implement a primitive polynomial and the
SISR feedback must implement its reciprocal.

      The LFSR is initialized with some nonzero seed, and the shift
register string and the SISR are loaded with all 0's (assuming no
inversion down the string).  The shifting clocks are pulsed 2N - 1 +
K times, where N is the number of shift register latches in the LFSR
(and the SISR) and K is the number of latches in the shift register
string.  This loads into the SISR the maximum length sequence (2N - 1
bits) of the LFSR.  Under these circumstances, if the string, LFSR,
and SISR are fault-free, the final signature in the SISR is exactly
zero.

      However, when the seed used in the LFSR is improperly chosen,
there are other all-zero states of the SISR, and thus the possibility
of aliasing a fault.  A short list of the lexicographically-least
preferred seeds is given in the figure.  Seeding the LFSR with a
preferred seed ensures that only the first and last states of the
SISR are all zeros.

      Since the SISR is initially seeded with zeros and since the
final good signature is also all zeros, it is suggested that the
clo...