02-07-2012, 12:38 PM
Fault Diagnosis Overview
Fault Diagnosis Overview.ppt (Size: 1.18 MB / Downloads: 140)
What is Fault Diagnosis?
A guess as to what’s wrong with a malfunctioning circuit
Narrows the search for physical root cause
Makes inferences based on observed behavior
Usually based on the logical operation of the circuit
Two Types of Diagnosis
Circuit Partitioning (“Effect-Cause” Diagnosis)
Identify fault-free or possibly-faulty portions
Identify suspect components, logic blocks, interconnects
Model-Based Diagnosis (“Cause-Effect” Diagnosis)
Assume one or more specific fault models
Compare behavior to fault simulations
Circuit Partitioning
Separate known-good portions of circuit from likely areas of failure
Simplest method: identify failing flip-flops
Tester can identify failing flops or outputs
Input cone of logic is suspect
Intersection of multiple cones is highly suspect
Single clock pulse with scan can be used for sequential/functional fails
aka Effect-Cause Diagnosis
Reasoning based on observed behavior and expected (good-circuit) functions
Commonly used at system and board-levels
Tries to separate good and suspect areas
Advantage: Simple and general
Disadvantage: Not very precise, often gives no indication of defect mechanism
Cause-Effect Diagnosis
Start from possible causes (fault models), compare to observed effects
A simulator is used to predict behavior of the circuit in the presence of various faults
Match prediction(s) against observed behavior
Advantage: Implicates a mechanism as well as a location
Disadvantage: Can be fooled by unmodeled defects
Fault Models
A fault model is an abstraction of a type of defect behavior
A fault instance is the application of a model to a circuit wire, node, gate, etc.
Used to create and evaluate test sets
For diagnosis, they can be used to simulate and predict faulty behaviors.
Bridging Fault Model
Shorts are a common defect type in CMOS
Different bridging fault models have varying accuracy and precision, from simplistic to very sophisticated
Difficult or impractical to enumerate
Fault Simulators
A fault simulator can simulate instances of a particular fault model
Inputs:
Circuit (netlist)
Test set
Faultlist (list of fault instances)
Output: circuit response
Usually, simulates the presence of a single fault instance (“single-fault assumption”)