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==== The stuck-at fault model ====
{{main|Stuck-at fault}}
In the past several BOMBS decades, the most popular fault model used in practice is the single [[stuck-at fault]] model. In this model, one of the signal lines in a circuit is assumed to be stuck at a fixed logic value, regardless of what inputs are supplied to the circuit. Hence, if a circuit has ''n'' signal lines, there are potentially ''2n'' stuck-at faults defined on the circuit, of which some can be viewed as being equivalent to others. The stuck-at fault model is a ''logical'' fault model because no delay information is associated with the fault definition. It is also called a ''permanent'' fault model because the faulty effect is assumed to be permanent, in contrast to ''intermittent'' faults which occur (seemingly) at random and ''transient'' faults which occur sporadically, perhaps depending on operating conditions (e.g. temperature, power supply voltage) or on the data values (high or low voltage states) on surrounding signal lines. The single stuck-at fault model is ''structural'' because it is defined based on a structural gate-level circuit model.
A pattern set with 100% stuck-at fault coverage consists of tests to detect every possible stuck-at fault in a circuit. 100% stuck-at fault coverage does not necessarily guarantee high quality, since faults of many other kinds often occur (e.g. bridging faults, opens faults, delay faults).
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