TOPIC : Types of fault simulation

UNIT 2 : Fault modeling

Module 2.4 Fault Simulation

Fault Simulation Set-up

Fault Simulation types

In large circuits,◦The number of faults are large◦Time taken to test the circuit will also

be large.◦So we do different types of type of

simulation to save time and memory.◦We will analyze different types of

fault simulation techniques for stuck at fault.

Different types of fault Simulation

Serial fault simulationParallel fault simulationDeductive fault simulationConcurrent fault simulation

Serial fault SimulationIt consists of transforming the

model of the fault-free circuit N so that it models the circuit Nf created by the fault f.

Then Nf is simulated. The entire process is repeated for

each fault of interest. Thus faults are simulated one at a time.

Advantage of Serial fault simulationNo special fault simulator is

required, as Nf is simulated by a fault-free simulator.

It can handle any type of fault, provided that the model of Nf is known.

the serial method is impractical for simulating a large number of faults because it would consume an excessive amount of CPU time.

Parallel fault SimulationIn this the good circuit and a fixed

number(say W), of faulty circuits are simultaneously simulated.

The values of a signal in the good circuit and the values of the corresponding signals in the W faulty circuits are placed in a register.

Then these values are all compared with the value of good circuit to check for fault using some logic(e.g. XOR).

Parallel fault Simulation ExampleFor example, if we use 2-valued

logic and a 32-bit word, then W=31

Advantages and disadvantages of Parallel fault Simulation

In parallel fault simulation several faulty circuits can be simulated in parallel.

These types of evaluations are convenient for evaluating functional elements, such as memories and counters.

An event, a value change, of a single fault or fault-free circuit leads to the computation of the entire word.

Deductive fault simulationThe deductive technique simulates the good

circuit and deduces the behavior of all faulty circuits.

This method utilizes a dynamic data structure.It comprises of two steps:

a) Fault-free circuit simulation is performed for the given vector.

b) The value implied by the vector at every line in each faulty circuit is deduced (using set theoretic rules).

Fault lists can be stored as linked lists, sequential tables, or characteristic vectors.

Deductive fault simulation ExampleIn parallel simulation the word

associated with a line i stores the value of i in every faulty circuit..

During simulation, however, the value of i in most faulty circuits is the same as in the good circuit.

This waste is avoided in deductive simulation by keeping only the bit positions (used as fault names) that are different from the good value.

Here line 4 and 7 are faulty, so in deductive fault simulation a list of Li={4,7} is made while, in parallel fault simulation a register is filled with all the faulty circuits

Concurrent fault simulationMost of the time during simulation, most

of the values in most of the faulty circuits agree with the corresponding values in the good circuit.

The fault-free version of the circuit, and each of its faulty versions, are concurrently simulated for a given vector.• Simulates the good circuit N.• For every faulty circuit Nf, simulate only

those elements in Nf that are different from the corresponding ones in N.

Concurrent fault simulation ExampleData structure used:– Concurrent fault list, in which

entries are of the form: < fault, input_values,

output_value >

Critical Path Tracing

A line l is critical w.r.t. a pattern t iff t detects the fault l/v.

Paths of critical lines are critical paths.A gate input i is sensitive if complementing

the value of I changes the value of the gate output.

A gate input i is critical w.r.t. a pattern t if the gate output is critical and i is sensitive w.r.t. t.

Backtracking : In a fanout-free circuit, the criticality of all lines can be determined by backward traversing the successive sensitive gate inputs from POs, in linear time.