46
VLSI Test Principles and Architectures
Many methods have been developed to calculate the probabilitybased testability
measures. A simple method is given below, whose basic procedure is similar to the
one used for calculating combinational testability measures in SCOAP except that
different calculation rules are used. The rules for probabilitybased controllability
and observability calculation are summarized in Tables 2.3 and 2.4, respectively.
In Table 2.3,
p
0
is the initial 0controllability chosen for a primary input, where
0
< p
0
<
1.
Compared to SCOAP testability measures, where nonnegative integers are used,
probabilitybased testability measures range between 0 and 1. The smaller a
probabilitybased testability measure of a signal, the more difficult it is to control or
observe the signal. Figure 2.3 illustrates the difference between SCOAP testability
TABLE 2.3
ProbabilityBased Controllability Calculation Rules
0Controllability (Primary Input,
Output, Branch)
1Controllability (Primary Input,
Output, Branch)
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 Spring '08
 elbarki
 Calculation, Input/output

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