Models for Delay Faults - Models in Hardware Testing - page 97

a

LC

SC

Segment 1

SC CC

Segment 2

b

LC CC

SC

Segment 1

SC

Segment 2

c

CC

SC

Segment 1

LC CC

SC

Segment 2

d

LC CC

SC

Segment 1

LC

SC

Segment 2

e

CC

LC

SC

Segment 1

LC CC

SC

Segment 2

Fig. 3.27 Various ways to launch and capture in a segmented scan chain design

This is in general true independent of the number of segments. That is, the fault
coverage using any selected launch method is the same independent of the capture
scheme used as long as capturing in every segment is considered.
From Fig. 3.28 b through d it can be noted that using LOC tests that launch off a
single segment achieves better coverage than launching off both segments as done
using unsegmented design. It was found that this is true in most of the ISCAS-89
benchmark circuits ( Zhang et al. 2007 b). Additionally, it can be seen that if the nor-
mal LOC that launches all segments is used together with launching off the second
segment 100% TDF coverage can be obtained for the circuit in Fig. 3.28 . Thus, us-
ing a combination of launching schemes it is possible to achieve much higher TDF
coverage in segmented scan designs in addition to reduced switching activity dur-
ing test. Finally, even though the discussion above used LOC tests for TDFs similar
observations are valid for LOS tests and for other delay fault models.