Generalized Fault Modeling for Logic Diagnosis - Models in Hardware Testing - page 137

Take for instance the single stuck-at fault model. A stuck-at fault ties a victim line
to a fixed logic value. The excitation of a fault only depends on the original value
on the victim line. If the line is driven to the logic value opposite to the value of the
fault, an erroneous value is passed to the downstream logic. If the line is driven to
the value of the fault, no effect can be observed. A stuck-at fault is therefore a CLF,
which has a Boolean condition that only depends on the victim line:

a ˚ Œa W stuck-at 0
a ˚ Œ a W stuck-at 1

This notation reflects the basic properties of stuck-at faults. A fault only affects one
line and can be expressed with a single CLF. The condition is a Boolean function;
therefore the faults are deterministic and only depend on the current values in the
combinational circuit.
Many defects also have temporal aspects. They may depend on the behavior of
aggressing lines over time. Such conditions are noted in CLFs by allowing the con-
dition to depend on past values of lines. These past values of a line are noted with
a subscript giving the difference to the current time. For instance, the last value of
line a is denoted as a 1 , and the value before is a 2 .
As seen in Chapter 1 , stuck-open faults receive new attention in recent tech-
nologies (see also Wadsack 1978 ; Soden et al. 1989 ; Li and McCluskey 2005 ;
Rodrıguez-Montanes et al. 2007 ; Flottes Landrault et al. 1991 ). Gates with a stuck-
open fault retain their previous output value for a certain input pattern. If, for
instance, the p-transistor of the input a in a NAND gate is disconnected, the out-
put y D a NAND b will be disconnected, too, and hold its charge for the input
pattern (a,b) D (0,1). This fault is described by the CLF

y ˚ . ab y 1 /

Another example is a crosstalk fault, where transitions on an aggressor line a cause
glitches on an influenced (victim) line b. A glitch is only produced, if line a changes
from the value equal to b towards the opposite value (see Fig. 5.3 ) . For now, we con-
sider the glitch itself to be long enough to fully propagate through the downstream
logic to the outputs. The CLF notation of such a fault is:

b ˚ Œ.a 1 ˚ a/ .a ˚ b/

1 1

a a

0 0

1 1

b b

0 0

fault exitation no exitation