16
VLSI Test Principles and Architectures
The circuit in Figure 1.8 has a total of eight (2
×
4) possible single transistor faults;
however, there are equivalent faults at the transistor level, as stuckopen faults in
a group of series transistors (such as
P
1
and
P
2
) are indistinguishable. The same
holds true for stuckshort faults in a group of parallel transistors (such as
N
1
and
N
2
); therefore, fault collapsing can be applied to transistorlevel circuits [Stroud
2002]. The number of collapsed transistor faults in a circuit is given by:
Number of collapsed faults
=
2
×
T
−
T
S
+
G
S
−
T
P
+
G
P
where
T
is the total number of transistors,
T
S
is the total number of transistors in
series,
G
S
is the total number of groups of transistors in series,
T
P
is the total number
of transistors in parallel, and
G
P
is the total number of groups of transistors in
parallel. For the twoinput NOR gate of Figure 1.8, there are four transistors (
T
=
4),
two transistors (
P
1
and
P
2
) in the only group of series transistors (
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 Spring '08
 elbarki
 Transistor

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