1. TOPIC : Introduction to Fault Simulation
UNIT 2 : Fault modeling
Module 2.4 Fault Simulation
TOPIC : Introduction to Fault Simulation

2. Introduction
Definition: Simulating a faulty circuit is known as fault simulation.
Aim of fault simulation is to arrive a test (set of vectors) which can detect any possible fault in a given circuit.
We analyze the operation of the circuit under various fault conditions.

3. Applications of Fault Simulation
Evaluate a test – How effective the test is?
Fault coverage
Defect level
Test generation – During test generation we simulate the circuit for the given test.
Used in constructing fault dictionaries
Analyzing the operation in the presence of faults

4. Fault Dictionary
The outputs of the simulated circuits are stored in the form of fault dictionaries.
It is a database generated by a fault simulator for diagnosing of faults.
It gives the relation between the fault and the corresponding output that results because of the fault.
This information can be used to identify the fault from the measured output of a faulty circuit.

5. Fault Simulation concept
To obtain information on different faults in a circuit, one simulates the healthy circuit and the faulty circuit.
If the outputs of both the circuits are different for the considered test pattern, then we say that the fault is detectable.

6. How to arrive at a test set?
A test set is to be developed that can detect any possible fault in the given circuit.
There are two approaches:
1) Random Test Generation (RTG)
2) Fault-targeted Test Generation (FTG)

7. Random Test Generation
Take a random vector as test vector.
Calculate the performance of the circuit with & without faults.
Identify all the faults that can effect the output for this test vector.
Remove the covered faults from the fault list.
If there are some more faults in the fault list, take another random vector and repeat the process until the fault list becomes empty.

8. Flow diagram of RTG

9. Fault-targeted Test Generation(FTG)
Here we start with a vector which can detect a specified fault.

10. RTG & FTG
The fault targeted simulation is a slow process initially compared to RTG.
If the number of faults is high, then a random test vector can detect more faults. If we use FTG then we need to generate a test for each single fault.
If the number of faults still left to be tested is less, then FTG is used.
Hence one adopts RTG in the initial stage when the number of faults to be tested are very high and when the number gets reduced, FTG is used.