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206
VLSI Test Principles and Architectures
often, and the computational overhead due to a bigger set of impossible value
combinations remains acceptable.
Before proceeding to the algorithm, the following terms are first defined:
Definition 1
Nonterminating necessary condition set
(NTC): NTC for an assignment
x
=
v
is
defined as the set of value assignments
±a
i
=
w
i
±
w
i
∈
±
0
²
1
³³
that are necessary to
achieve
x
=
v
. However, there may exist other assignments that are necessary to
achieve some or all conditions in NTC.
For example, in Figure 4.37,
h
=
1 and
g
=
1 are necessary for
z
=
1. However, there
exist assignments (
f
=
1,
c
=
1,
d
=
1, and
e
=
1) that are necessary to achieve
h
=
1
and
g
=
1. Thus,
h
=
1,
g
=
1 forms the NTC for
z
=
1.
Definition 2
Terminating necessary condition set
(TNC): TNC for an assignment
x
=
v
is the
set of value assignments
±a
i
=
w
i
±
w
i
∈
±
0
²
1
³³
necessary to achieve
x
=
v
such that
there exist no additional assignments that are necessary to achieve any conditions
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 Spring '08
 elbarki

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