Question

What is path sensitization? Why do we do it or why it is important, explain with an example?

Answer 1

Path sensitization :

In a complex circuit there are several number of wires. Too many wires too many faults. Now, creating all tests for all fault sites  is time consuming and not useful at all. So, it is a good approach to deal with several wires that form a path. In path sensitization technique, we choose some paths from the origin of the fault to the circuit output. A path is sensitized if the inputs to the gates along the path are assigned values such that the effect of the fault can be propagated to the output. There are three main steps of the path sensitization procedure.

1. Activation (Fault activation)
2. Propagation (Fault propagation by path sensitization)
3. Justification (Line justification)

Why do we do it or why it is important ?

• It help us to identify a signal path from a fault site to an observable output, where the logical state of any gate output along that path is sensitized to logic value at fault site.
• This technique is also help us to construct a test (when one exists) for a particular fault site via one path.
• Using this technique we can test pattern generation of combinational circuits easily and fast.

Example :

To sensitize a path through an input of an AND or NAND gate, all other inputs must be set to 1 and for an OR or NOR gate, all other inputs must be set to 0. In the below example, there is a path p1-->p2-->p3-->p4. Now, if we want to test the path we need to put the value of i1, i2 and i3 as 1. Then only the output f will depend on the input a.