Models for Delay Faults - Models in Hardware Testing - page 95

the scan cells connected to the scan enable signals that are at 1 shift during launch
and capture cycles and hence do not capture test responses and the achievable fault
coverage with this method can be expected to strongly depend on the grouping of
scan cells into subsets driven by different scan enable signals. In Devtaprasanna
et al. ( 2005 ) two different methods to group scan cells were investigated. It is im-
portant to note that when multiple independent scan enable signals are used standard
LOC tests can still be applied with all scan enables set to 0 during launch and capture
cycles. Thus the fault coverage using multiple scan enables is not lower than that by
LOC test method using a single scan enable signal. Additionally, it was shown that
multiple scan enable lines facilitate generation and application of tests with reduced
switching activity during scan shift and capture cycles ( Wang et al. 2007 ) . Thus use
of multiple independent scan enable signals facilitates simultaneous achievement of
higher delay fault coverage and reduced switching activity during test.

3.3.3 Higher Delay Fault Coverage Using Segmented

Scan Designs

Segmented scan design was proposed to reduce switching activity during loading of
scan based tests ( Whetsel 2000 ) . Segmented scan design is illustrated in Fig. 3.26
taken from Zhang et al. ( 2007 b) which shows a scan chain divided in to three

a

SIN

1 2 3

SOUT

CLK
SEN

A Single Scan Chain

b
SIN

Segment 1

SEN

CLK1
SO1

S
C
A
N

Segment 2

SOUT

CLK2
SO2

C
O
N
T
R
O
L

Segment 3

CLK3
SO2

Fig. 3.26 A segmented scan

A Three Segment Scan Chain

design