Defect Diagnosis of Digital Circuits Using Surrogate Faults - VLSI Design and Test - page 380

Fig. 4. Simulation effort comparison with dictionary method applied to C432
3.1 Fault Ranking
There is a small probability that the diagnosis procedure comes up with no
result, i.e., SET1 and SET2 are both empty after fault simulation. That can
happen in a situation like multiple faults with masking and interference such
that they together produce faulty output responses that will allow a few of the
test patterns detecting them to pass and other test patterns detecting them to
fail. For example, consider the case where every fault detectable by failing patters
is also detectable by at least one passing pattern. This is a rare phenomenon and
only in such cases a ranking procedure is used.
Phase 1 short lists faults in sus flts . For ranking, while carrying out Phase 1
we keep a count of the number of failing patterns that detect each fault. This
number is called the weight of the corresponding fault, e.g., if fault F1 is detected
by three failing patterns, then the weight of F1 at the end of Phase 1 will be
three. Similarly, in Phase 2, which simulates only the faults found detectable in
Phase 1, we keep a count of the number of passing patterns for each fault. This
number is subtracted from the weight of the fault found in Phase 1. At the end
of Phase 2 we get the final weight of every fault. The faults with the highest
weight are reported to be prime suspect (SET1) faults and the faults with the
second highest weight are reported to be surrogate (SET2) faults.
Note that the final weights can also be negative. This will happen when a
fault is detectable by more passing patterns and by fewer failing patterns. Even
in this case, the top two highest weights are considered to be suspects. Also,
there can be cases where the final weight is zero. This will happen when the
fault is detected by the same number of passing and failing patterns.
The fault simulation in ranking is more expensive because it is done with-
out fault dropping. In practice, however, the four phase diagnostic procedure