Models for Bridging Defects Test and Diagnosis - Models in Hardware Testing - page 41

Fig. 2.8 Didactic defective Gate a

circuit

Gate c

n 1

I 1
I 2

n 3

Gate e

Rsh

O

Gate d

I 3

Gate b

n 2

n 4

I 4

zero-resistance bridge can no longer be used and a realistic analysis of the defect
behaviour is required. Besides, a realistic model of defect behaviour must incorpo-
rate the unpredictable parameters.

2.3.1 Impact of the Resistance on the Defect Behaviour

To illustrate the impact of the bridging resistance on the defect behaviour and sub-
sequently on its detecting conditions, let us consider as an illustrative example the
small defective circuit given in Fig. 2.8 . Note we prefer to use here the term defective
(in relation with defect) than faulty (in relation with fault). The circuit is composed
of five logic gates and comprises four primary inputs (I 1 to I 4 ) and one primary
output (O). The bridging defect under consideration corresponds to the logical node
n1 bridged to ground through an Rsh resistance. Of course, the value of the intrinsic
bridge resistance Rsh is not known a priori.
Considering a classic Boolean test technique, the detection of this bridge requires
both defect excitation and propagation of its effect to a primary output. Regarding
excitation, a bridge-to-ground is excited by any input vector trying to set the bridged
node to logic '1'. The bridged node n 1 depends on the primary inputs I 1 I 2 through
the NAND gate 'a'; the defect excitation is therefore guaranteed by any of the fol-
lowing 12 vectors: I 1 I 2 I 3 I 4 D 00XX;01XX or 10XX. The defective value due to
the bridge has then to be propagated through the succeeding logic gates. The bridged
node n 1 is connected to the input of the NAND gate 'c' and to the input of the NOR
gate 'd'; consequently, the effect can be propagated through the NAND gate 'c'
setting its side input to a logic '1' (I 1 I 2 I 3 I 4 D XX1X) or through the NOR gate
'd' setting its side input to a logic '0' (I 1 I 2 I 3 I 4 D XX11). Note that propagation
through gate 'd' necessarily implies propagation through gate 'c'. Table 2.3 sum-
marizes the defect excitation and propagation characteristics associated with each
one of the 2 4 possible input vectors. It appears that six vectors allow both defect
excitation and effect propagation: vectors #2, #3, #6, #7, #10 and #11 (grey arrays
in Table 2.3 ) .
When considering classical faults such as stuck-at or zero-resistance bridging
faults, the two conditions of fault excitation and fault propagation completely de-
termine fault detection. This is not the case when realistic bridging defects with