Models in Memory Testing - Models in Hardware Testing - page 165

represents the content of the f-cells after applying the operations contained in the
SOS , while

r f cell 1 r f cell 2 :::r f cell k with r

R D

2f 0; 1; g

(6.5)

represents a set of erroneous outputs of the memory obtained when reading the
f-cells . This element of the FB is required to represent classes of faults where a read
operation returns an erroneous value while the content of the memory cell is correct.
The symbol ' ' is used whenever a write operation, and not a read operation, is
responsible for sensitizing the fault.
Definition : A memory FFM is a non-empty set of fault primitives.

6.4.2 Generic Space of Faults

As stated in Section 6.4.1 , the definition of new memory FFMs requires a precise
identification and exploration of the generic space of possible faults. As we define
FFMs in terms of collections of FPs, this problem is actually reduced to the defini-
tion of the space of possible FPs.
FPs can be classified into different classes based on the characteristic of the re-
lated SOS . In particular, let j f j be the number of f-cell s involved in the SOS ,andm
(Eq. 6.2 ) the number of operations composing SOS.
Depending on j f j , FPs can be categorized into single-cell FPs when j f j D 1,
and n-coupling FPs when j f j D n. In case of n-coupling FPs, the set of f-cells is
split into victims (v) and aggressors (a) . In a FP the faulty behavior always manifests
into the victim cells, while the aggressors contribute to the sensitization of the fault.
Depending on the number m of operations composing the SOS , an FP can be
classified as static when m 1, i.e., a single memory operation is sufficient to sen-
sitize the target faulty behavior, or dynamic when m>1;i.e., n memory operations
are required to sensitize the target faulty behavior.
Figure 6.11 shows a graphical representation of the taxonomy of the FP space.
The two classifications criteria introduce two different hierarchies in the FP space.
Since FFMs have been defined as a non-empty set of FPs, we can inherit the same
type of classification and hierarchy when considering FFMs. For example, a FFM
composed of a collection of static FPs will be referred to as a static FFM, etc.

6.4.3 Static Fault Models

Static faults consist of groups of static FPs (m 1) sensitized by at most a sin-
gle memory operation. They were historically the first set of faults observed into
memory devices. In the following the most important classes of static faults will be
presented.