Models for Bridging Defects Test and Diagnosis - Models in Hardware Testing - page 49

The defect is therefore redundant whatever the value of the bridge resistance. In the
second situation, it exists some vectors able to excite and propagate the defect.
The defect redundancy therefore depends on the value of its bridge resistance. If
the bridge resistance falls within the Global-ADI, the defect is detectable since it ex-
ists at least one input vector to propagate the defective value to a primary output. In
contrast, if the bridge resistance falls out the Global-ADI, the defect is redundant
since a defect-free value is propagated to the primary outputs whatever the input
vector. It is clear that in both situations, redundant defects cannot be detected and
thus are not in the optimization focus.
The actual objective of the optimization process is detectable defects, i.e. defects
with unpredictable parameters falling into the Global-ADI. In other words, the ob-
jective of the optimization is to cover the Global-ADI. A given test sequence may
cover or not the Global-ADI. Therefore the concept of ' Covered-ADI ' related to a
test sequence can be introduced in the following way:

Definition 2.2. Given a circuit under test and the list of Analogue Detectability
Intervals ADI V associated to each vector V for a considered defect, the Covered
Analogue Detectability Interval C-ADI related to a test sequence is given by the
union of the ADIs associated with all the vectors of the test sequence V T

W

ADI D [ ADI V T C

As an illustration, let us assume a test sequence including 3 vectors: #2, #3, and #13
of Table 2.4 . The Covered-ADI for this sequence is:

ADI D 0; R 1 C [ R 1 C ;R 3 C [;D 0; R 3 C

C

It can be observed that this test sequence does not cover the Global-ADI [0; R 4 C ].
Three possible situations exist according to the unpredictable value of the bridge
defect resistance:

The unpredictable value of the bridge defect resistance falls into C-ADI; the se-
quence will detect the defect.
The unpredictable value of the bridge defect resistance falls into G-ADI but out
of C-ADI; the sequence will not detect the defect.
The unpredictable value of the bridge defect resistance falls out of G-ADI; the
sequence will not detect the defect.

The third situation corresponds to the case of a redundant defect for which it exists
no sequence able to detect the defect. Consequently, nothing can be done to optimize
the detection of this defect for the test technique under consideration. In contrast,
the second situation does not correspond to the case of a redundant defect, implying
that it exists one or several input vectors able to detect the defect. Consequently, the
considered test sequence is not the most favourable and better vectors could be used
to ensure an optimal detection range of the bridge defect resistance value.