Models for Delay Faults - Models in Hardware Testing - page 77

Tabl e 3. 1 Sensitization conditions on off-path inputs

Non-robust

Robust Strong Weak Functional

AND
NAND

OR
NOR

AND
NAND

OR
NOR

AND
NAND

OR
NOR

AND
NAND

OR
NOR Transition

0 ! 1 X1 S0 X1 H0 X1 X0 X1 XX

1 ! 0 S1 X0 H1 X0 X1 X0 XX X0

0 0

b d 0 1

g

b

0 1
0 1

1 1

f 0 1

0 1

c

1 1

a

h

e

0 0

a 0 1

Fig. 3.7 Multipath test ( Ke et al. 1995 )

but may have a hazard/glitch. It may be necessary to consider subsets of paths or
multipaths to generate tests ( Ke et al. 1995 ) . In Fig. 3.7 we show an example circuit
from Ke et al. ( 1995 ), in which the logical paths c-d-g-f and c-e-h-f rising can only
be tested together and not separately. In general, it is necessary to consider what are
called primitive multipath faults ( Ke et al. 1995 ).
Tests for sequential circuits can be applied using slow and fast clocks ( Malaiya
et al. 1983 ) , or using a single fast clock ( Pomeranz et al. 1992 ; Cheng 1993 ). When
using slow and fast clocks the initialization pattern is obtained using a slow clock
followed by the second vector of a two pattern test. Response to the second vector is
captured using a fast clock whose period is the desired clock period during normal
operation. The captured error values in flip-flops are then propagated to primary
outputs again using a slow clock. It is assumed that when a slow clock is applied the
circuit operates as a fault-free circuit. When only the system clock is used for delay
faults one has to accommodate the fact that the effect of the delay fault may remain
for one or more cycles ( Pomeranz et al. 1992 ; Cheng 1993 ).

3.1.4 Delay Fault Tests for Scan Designs

Scan is universally used in large industrial designs to facilitate test generation, debug
and failure diagnosis of synchronous sequential circuits. In Fig. 3.8 a scan design
called MUX-Scan is illustrated. Single pattern tests are applied by shifting/scanning
in the state part of the test with scan enable (SE) at logic 1. After loading the scan
chain the primary input values for the test are applied. SE is changed to 0 and one