Logic Design for Array-Based Circuits
Faults and Fault Detection
Last Edit July 22, 2001
Faults may be indeterminate in value (suspended between logical 1 and logical 0), or determinate in value (exhibiting a 0 or a 1).
They may be transient (intermittent, time varying), in which case they are elusive and difficult to detect. Faults may be permanent, i.e., considered hard or solid, in which case they may be detected if they are not masked, i.e., are observable.
Multiple faults occur when more than one fault occurs at one time. The probability of multiple faults occurring in a circuit is relatively less than the probability of a single fault, but is increasing with the increase in circuit density. Single faults remain the most likely event.
Multiple faults can occur in such a manner that they can be degraded to an equivalent single fault. In this case, the input vectors that test for the occurrence of the single fault also test for the occurrence of the multiple fault condition.
There are several equivalencies that exist which are useful in fault detection and which make fault location potentially more difficult. Some of these equivalencies are shown in Table 9-2 and two cases are diagrammed in Figure 9-1.
Table 9-2 Fault Equivalencies
Figure 9.1 Fault Equivalencies