Hardware Reference
In-Depth Information
Tabl e 8. 4
Characterization of defects/faults according error classes and error types
Error
Permanent
Transient
Single
- Short or open in
switch-like network
- Open of connection
between gates
- Open of load transistor
- Threshold voltage drift of
one or more transistors
inside one gate
- Clean short between
two gates
Unidirectional
- Open of supply line
- Threshold voltage drift of
one or more transistors
inside one gate
- Short between more
than two gates
Multiple
- Resistive short
between two gates
leading to an
undefined logic level
- Degradation of
propagation time
- Degradation of
propagation time
8.2.4.5
Synthesis
Summing up, the faults can be divided into three classes depending whether they
induce single, unidirectional or multiple errors. Each of these classes can also be
further divided into two groups: permanent and transient errors. Table
8.4
charac-
terizes the defects/faults according to these error classes and types.
8.2.5
Constraints for Designing Self-Checking Circuits
Among the different properties that a self-checking (SC) IC can possess, we study
here the constraints related to the
fail-safe
property. This property, like the others, is
based on the use of detecting codes.
We say that a circuit using a detecting code C is fail-safe if, for all the faults
belonging to the class considered and whatever the input pattern applied, the output
is correct or the output does not belong to the code C .
The failure of the circuit is signaled by the means of a checker verifying that the
output belongs to the code C . This checker must also be fail-safe itself with a coded
output; in general the used code is a two-rail 1-out-of-2 code (two variables taking
complementary values during normal operation). The use of an error detection code
enables an immediate on-line testing and signaling of any incorrect operation of a
circuit independently of the permanent or transient nature of the error. Thus, in the
following, we no longer make any distinction between the temporal characteristics
(permanent or transient) of the errors induced. We describe in turn hereafter the im-
plementation rules related to the detection techniques of the three classes of errors:
defects inducing single, unidirectional and multiple errors.
