Fault Modeling
          Faults are the manifestations of physical defects.
          Logical fault modeling lead to a number of benefits.
                 Reducing complexity
                 Technology-independent modeling
                 Achieving abstraction
          Logical faults- 
                 Explicit – may be enumerated
                 Implicit – can not be enumerated
                 Structural – modify the interconnections among components
                 Functional – modify the functions performed by components
                                                     
                                    Modeling levels
        Modeling can be done at different levels
               Geometrical to functional levels
                                        
                Combinational circuits
     Consider an n input fault free combinational 
       circuit C whose output is F.
     Presence of fault f transforms C into differenct 
       circuit Cf with output Ff(x)
     Let t represents specific input vector called test 
       vector.
     F(t) response of C
     Test vector t is said to detect fault f iff F(t)!=Ff(t)
                        
                   Sequential circuit
     Circuit SC
     Has initial state q
     Output F
     Fault f and output Scf
     Need for a sequence of tests
                       
                    Sensitized path
     In a given circuit , a line whose value under test 
      t changes in the presence of fault f is said to be 
      sensitized to the fault f by test t.
     A path composed of sensitized lines is called a 
      sensitized path.
     The test t propagates the fault effect along a 
      sensitized path.