Hardware Reference
In-Depth Information
fault may be propagated through the shorter path a-f-k. In this case the delay de-
fect size needs to be larger for it to be detected. However a defect of a smaller size
than detectable by the test will affect circuit operation when the transition on a is
propagated through the longer path under normal operation. For this reason meth-
ods to activate and propagate TDFs through longest delay paths have been proposed.
We review some of the recent works on generating TDF tests to detect small delay
defects.
3.2.1
Functional Broadside Tests
There are two reasons for non-functional operation in scan based tests. One is the
very fact that tests are shifted in to scan chains which is not a functional opera-
tion and the states of the circuit under tests go through many states that are not
functional. The other is during launch and capture cycles of the application of two
pattern tests non-functional operation may cause excessive switching activity that
may cause supply voltage droops and higher heat dissipation. Voltage droops cause
increase in circuit delays which may fail good chips ( Saxena et al. 2003 ). Addition-
ally tests using non-functional operation may propagate faults along non-functional
paths potentially failing good chips even if the switching activity during test is not
excessive. In this section we discuss recently developed methods to address the issue
of non-functional operation during launch and capture cycles.
An LOC or broadside test can be represented by <s1,a,b>, where s1 is the state
scanned in and a and b are the primary input values. The state part of the second
pattern of the two pattern test is obtained through the functional logic. Hence if s1 is
a state that can be reached during normal functional operation then the circuit will
only operate within normal functional operation during test also. Observing this,
Functional Broadside Tests were proposed in Pomeranz et al. ( 2006 ). In a functional
broadside test the shifted in state s1 is a reachable state. A reachable state is a state
that can be reached from the state of the circuit after it is synchronized. Any state
reached after synchronization is a state that can occur during the normal operation
of the circuit. Functional broadside tests insure that switching activity and supply
current demands during launch and capture cycles are within those during normal
operation. Additionally no non-functional paths will be activated. In Table 3.2 the
numbers of TDFs detected by functional broadside tests ( Lee et al. 2008 )arecom-
pared with the numbers of faults detected using arbitrary broadside tests in full scan
ISCAS-89 circuits. In Table 3.2 , after the circuit name the numbers of collapsed
TDFs are given followed by the numbers of faults detected by functional broad-
side and arbitrary broadside tests. From this data one can observe that numbers of
faults detected by the functional broadside tests are sometimes smaller as can be
expected. However, overall the numbers of detected faults are similar in most cir-
cuits. Expanding the functional operation to include the state transitions encountered
during the application of a synchronizing sequence permits additional tests called
Synchronization Broadside Tests ( Pomeranz et al. 2009 a). These tests may shift in
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