Hardware Reference
In-Depth Information
This modeling technique is very similar to pattern faults or the notation in
Kundu
et al. (
2006
). Again, any CLF with a Boolean function can be noted with fault tuples,
more complex conditions cannot be expressed.
5.3.1.2
A Taxonomy of Static Bridging Faults
As already described in the second chapter, bridges are an important fault class.
They usually involve two signal lines which interact in a certain manner. Depending
on the type of bridge and the current values of the signal lines, one or both signals
may change their logic value. The types of bridges are described by two CLFs at
most.
Static bridges provide a good example of how the CLF calculus can be used to
express a class of fault models. There are many different fault models available for
static bridges (e.g. wired-logic, dominant-driver).
Roussetetal.
(
2007
)
presentsa
taxonomy for the most common models. Common to all these fault models is the
fact that they do not model timing related behavior. The conditions can therefore
be expressed using Boolean functions which depend on the current values of the
involved signals.
Another basic property of static bridge fault models is the fact that errors only
occur, if the two involved signal lines carry different values. This necessary precon-
dition is described by an XOR-term in the conditions. If this precondition is true,
the actual behavior of the two signals is determined by two Boolean functions f
a
and f
b
. The function f
a
depends only on signal b, because the value of signal a
is already determined by the precondition. Similarly, function f
b
depends only on
signal a. This leads to the following generalized CLF formulation of an arbitrary
bridge between two signal lines a and b:
a
˚
Œf
a
.b/
.a
˚
b/;
b
˚
Œf
b
.a/
.a
˚
b/
There are exactly four basic expressions for f
a
and f
b
, respectively. An expression
may be constant 0, constant 1 or may use the positive or the inverted value of the
other signal in the bridge:
f
a
.b/
2f
0; 1; b; b
g
;
b
.a/
2f
0; 1; a; a
g
Any more complex Boolean formula can be simplified by using the precondition and
Boolean identities. The formulas given above therefore model every possible static
bridge configuration. There are 4
2
= 16 possible configurations that are derived by
choosing one of the four possible expressions for f
a
and f
b
. From these 16 config-
urations, there are six, that are actually derived from other bridges by interchanging
the roles of the signals a and b. This leads to ten unique bridge types including the
