Browse Prior Art Database

Improved Method for Weighted Random Pattern Weight Generation

IP.com Disclosure Number: IPCOM000108308D
Original Publication Date: 1992-May-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 5 page(s) / 220K

Publishing Venue

IBM

Related People

Forlenza, OP: AUTHOR [+4]

Abstract

Disclosed is a method that will more efficiently generate logic tests, thus minimizing the number of patterns required for DC fault coverage. The result of this technique is realized in the reduction of test application time, hence improving tester thruput.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 34% of the total text.

Improved Method for Weighted Random Pattern Weight Generation

       Disclosed is a method that will more efficiently generate
logic tests, thus minimizing the number of patterns required for DC
fault coverage.  The result of this technique is realized in the
reduction of test application time, hence improving tester thruput.

      Test patterns may be divided into three categories:
Deterministic, Flat Random (also called Pseudo Random) and Weighted
Random. Deterministic patterns are explicitly generated test patterns
used to detect particular faults. The Random test patterns cause a
Primary Input (PI) or Shift Register Latch (SRL) to have an equal
probability to be a '0' or a '1'.  Weighted Random is similar to Flat
Random except that the probability of a PI's state is controlled by
the application of predetermined weights.

      Existing methods for Weighted Random Pattern (WRP) method for
weight generation is based upon the assumption that it takes more
non-controlling values than controlling values to test a logic gate.
Starting from the Primary Outputs and looking back into the internal
logic blocks, the weights for each PI and SRL are calculated.

      This approach will be described in three parts.
           1.  A New WRP method for weight generation.
           2.  A multiple global weight methodology.
           3.  A hardware implementation for more precise weight
generation.
A New WRP Method for Weight Generation:

      This new method does not depend upon the internal logic blocks,
but on the accumulation of the test patterns available for fault
detection. A weight is defined as the ratio of the number of times an
input (both PI and SRL) has been stimulated to a logic '1' state,
divided by the total number of times the input has been stimulated.
A weight of 0.75 implies that an input has three times the
probability of being stimmed to a '1' than it has to a '0'.

      For example, to test all of the stuck faults of a four-input
AND gate, five test patterns would be required as shown in Fig. 1.
PIA has been stimulated to a '1' four times in five patterns;
therefore, its weight is calculated to be 4/5 (or 0.8).  The same is
true also for PIs B, C and D.

      There are two sources of data that this New WRP method can use
for weight calculation.  The first source is the deterministic test
patterns that are generated using a CAD system.  The deterministic
patterns are parsed and the various stim and load operations are
updated in their respective PI and SRL tables.  From these tables,
the weights are calculated.

      The results from an experiment comparing the new method of
weight generation to that of the existing methods has shown the
testability has remained the same while the number of patterns have
been reduced.  The New Method shows an average pattern saving of 15%
over the existing WRP weight generation methods and a 20% saving over
the Flat Ran...