Physical Fault Models and Fault Tolerance - Models in Hardware Testing - page 238

X: reference fault locations — O: Observation locations

Fig. 8.14 Target system levels and fault pathology

8.3.3.2 Error Equivalences

For characterizing the behavior of a computer system in presence of faults, it is not
necessary a priori that the injected fault be “close” to the target (reference) fault. It is
sufficient that it induces similar behaviors. Similar errors can actually be induced by
different types of faults (e.g., a bit-flip in a register or memory cell can be provoked
by an heavy-ion or as the result of a glitch provoked by a software fault). What is
important is not to establish an equivalence in the fault domain, but rather in the
error domain (see expression 8.4 in Section 8.3.1.2 ) .

8.3.3.3 Distances

What matters is that the respective error propagation paths converge before the level
where the behaviors are observed. Two important parameters can be defined on these
various levels (Fig. 8.15 ) :
- the distance dr , separating the level where faults are injected from the reference
fault level(s);
- the distance do , separating the level where the faults are injected from the levels
their effects are observed .

The shorter dr and the longer do are, the more likely the injected faults will exhibit
behaviors similar to those provoked by the targeted reference faults.

8.3.3.4 Constraints on Error Propagation

In practice, the presence of a specific FTM may alter the error propagation paths.
This has a significant impact on the scope of (real) faults actually covered by the