CPE/EE 428, CPE 528 Testing Combinational Logic (2)

1
CPE/EE 428, CPE 528 Testing Combinational Logic
(2)

• Department of Electrical and Computer Engineering
  University of Alabama in Huntsville

2
Testing Digital Systems

• Combinational vs Sequential Systems
• We shall cover combinational first
• Sequential circuits can be tested using
  combinational test generation and scan chains
• The state FFs are connected in a shift register.
  Any value can be shifted in (setting an
  arbitrary state), the next state loaded, and then
  shifted out. Thus tests can be directly applied
  to the combinational logic.

3
Testing Digital Systems Detection
A good circuit N produces function Z(x)
A circuit with fault f produces a different
function Z f (x)
x
Z f (x)
N f

• Detecting a fault
• A test vector t is an assignment of input values.
  It detects a fault f iff Z(t) ? Z f (t)
• The set of all tests T that detects f is found
  by Z(x) ? Z f (x) 1

4
Testing Digital Systems Detection

• Assume x4 s-a-0 (stuck-at 0, sa0)

x2
x3
x1
Good Circuit
Z
x4
x2
x3
x1
Faulty Circuit
Zf
5
Testing Digital Systems Detection
test for x4 s-a-0
x2
(ab) ? a ? (ab) a (ab) a aa ab
(a b) a ab ((x2x3)x1) x1x4
(cx1) x1x4 (c x1) x1x4 cx1x4
x1x4 x1x4
x3
x1
Z
x4
Z(x) ? Z f (x) 1 Z (x2 x3) x1 x1x4 Z f
(x2 x3) x1
? gt x1x4 1

• This says that any input vector with x1 0 and
  x4 1 is a test vector for x4 s-a-0. x2 and x3
  are dont-cares.

6
Testing Digital Systems Detection
x2
x3
0
0
test for x4 s-a-0
x1
0
1/0
Z
1
1
x4
1/0
1/0
s-a-0

• The combined good/bad circuit can be drawn
• values shown are for v/vf
• that is, values in the good circuit / values in
  the faulty circuit
• v/vf shows a discrepancy between good/faulty
  circuit values

7
Fault Activation and Propagation
x2
x3
0
0
x1
0
1/0
test for x4 s-a-0
Z
1
1
x4
1/0
1/0
s-a-0

• Two basic concepts in fault detection illustrated
• A test must activate the fault by creating
  different v/vf values at the fault site
• thus x4 is assigned to be 1. If it really is
  stuck at zero, we know there will be a change in
  circuit values.
• A test must propagate the error to a primary
  output
• other circuit values must be selected to allow
  the good/faulty value to be seen at an output.

8
Path Sensitization
x2
x3
0
0
x1
0
1/0
test for x4 s-a-0
Z
1
1
x4
1/0
1/0
Sensitized path
s-a-0

• Path sensitization
• A line whose value (with the test t) changes in
  the presence of fault f is said to be sensitized
  to fault f by test t
• these lines are indicated by having different
  v/vf values
• A path composed of sensitized lines is a
  sensitized path

9
Another example

• Test x1 s-a-0

x2
x3
x1
Z
x4
s-a-0
10
Another example

• x1 s-a-0

sensitized path
1
1
x2
1/0

x3
1/0
x1
1
1/0
Z
0/1
0
x4
0
s-a-0
Assign 1 to x1 to activate the fault
Assign other inputs to enable propagation
11
An Alternate Test Vector

• Alternate sensitized path for x1 s-a-0

x2
x3
x1
Z
x4
s-a-0
12
An Alternate Test Vector

• Alternate sensitized path for x1 s-a-0

0
0
x2
x3
0
0
1/0
x1
1
0/1
Z
0/1
0/1
x4
1
sensitized path
s-a-0
13
Stem vs. Branch Faults

• Actually x1 could be treated as three different
  fault sites.
• A stem fault. The fault is on the common stem.
• A branch fault. The fault is on one of the
  branches.
• i.e. the input to the AND and NOT gates could be
  different
• need to know the actual circuit topology

stem
x2
x3
1
x1
0/1
0/1
1/0
Z
x4
0/1
branch s-a-0
14
Controlling and Inverting Values

• Aside
• Primitive logic gates (AND, OR, NAND, NOR) can be
  characterized by two parameters
• controlling value c
• inversion i
• Controlling value
• the value when on any one input will determine
  the gates output regardless of the other inputs
• (e.g. 0 on any AND gate input)
• If one input has the controlling value, the
  gates output will be
• c ? i, where c and i come from the following
  table

15
Controlling and Inverting Values

• Along the sensitized path
• any input sensitized to the fault will have a
  value, call it d
• all other inputs will have c (complement of
  controlling value)
• a non-controlling, or enabling, value
• the output will have value d ? i

d
d
1
d
d
d
could be 1/0 or 0/1
16
Controlling and Inverting Values
x2
x3
0
0
x1
0
1/0
Z
1
1
x4
1/0
1/0
s-a-0
Which are controlling, which are enabling?
17
Testing Digital Circuits

• Another test for another fault
• think in terms of controlling and inverting values

s-a-1
x2
x3
x1
Z
x4
18
Testing Digital Circuits Redundancy

• Fault f is detectable if there exists a test t
  that detects it i.e. Z(t) ? Z f (t)
• However, f is undetectable if Z(x) Z f (x)
  for all x
• Cool! There are some circuits where even if
  there is a fault in certain places, they still
  work!
• A circuit that contains an undetectable fault is
  a redundant circuit.
• The fault site obviously has no effect on the
  circuit function
• The circuit can be simplified you can remove
  something!

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Testing Digital Circuits Redundancy
X
a
b
Y
F
c
Z

• Example
• F ab bc ac
• Is the fault Y s-a-0 detectable?
• Activate and propagate Y s-a-0

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Redundancy

• Y s-a-0 is undetectable
• F ab bc ac ab ac
• The term bc is a redundant cover in the Kmap
• its not an essential implicant of the function
  the other two are
• Change to circuit
• Gate Y can be removed from the circuit without
  affecting the logic function.
• Or you can keep it and have some fault tolerance

21
Redundancy Pro and Con

• Pro and Con
• The fault is undetectable.
• This can be good! The circuit still works even
  if there are certain faults in it.
• Are others undetectable too? or harder to
  detect
• The redundant circuit requires extra hardware
  extra area on the IC

22
Redundancy Pro and Con

• The circuit is hazard free on transition 111 gt
  011
• Hazard the value of the function takes on an
  intermediate value different from the final value
• With a non-redundant circuit, there is a chance
  of a 1 - 0 - 1 hazard
• With the redundant circuit, if the inputs change
  from 111 gt 011, the output will not go to zero.
  Are there other such transitions?

X
a
b
Y
c
Z
23
Testing Digital Circuits Redundancy

• Removing redundant covers
• Other redundancies
• Triple modular redundancy a method for
  achieving fault tolerance.
• faults are correct by additional logic
• many faults would be untestable theyd be
  automatically corrected
• and logic synthesis would optimize the
  redundancy away!
• Need a test mode to disable correction

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Testing is it so simple?

• Test engineers Sherlock Holmes of the industry
• Methods for automatically generating tests were
  necessary
• Collectively known as ATPG gtAutomatic Test
  Pattern Generation

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D-Calculus
D (Detect) 1/0 - represents a logic 1 in the
good circuit and a logic 0 in the bad circuit
Five-valued logic 0, 1, D, Dbar, X (dont care)
26
D-Calculus
Truth tables for AND, OR, NAND, and NOR gates
27
Definitions

• Test generation algorithms work in terms of
• Primary inputs (PI) a controllable input to a
  circuit. E.g., a pin on an IC, or an output of
  an FF in a scan system
• Primary outputs (PO) an observable output of
  the circuit. E.g., a pin on an IC, or a D input
  to an FF in a scan system
• Justify, justification the process of selecting
  PIs to force a certain line to have a specific
  value
• Propagate, propagation the process of selecting
  appropriate PIs that allow a discrepancy D to
  be pushed to a PO
• Test generation algorithms are all about
• finding the appropriate PIs to control to
  activate a fault
• finding the appropriate PIs to control to
  propagate the fault to one of the POs.

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More Definitions

• Forward implication
• Def Knowing one or more gate inputs, imply the
  output value.
• Assume all gate inputs are the same value
  either all c or all c
• Then the output is
• output value ? i
• We can refine this if we know the controlling
  value
• i.e. only one of the inputs needs to have c to
  know output
• Backward implication
• Def Knowing the output and possibly some inputs,
  imply one or more of the inputs
• Assume all gate inputs are the same either all
  c or c
• Then the inputs are
• inputs output ? i
• We can refine this if we know the controlling
  value
• If the input needed to produce the output is c,
  then only one input needs to have it.

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Justify Algorithm

• Justify (l , v) Recursive algorithm to justify
  line l to value v
• l v
• if l is a primary input return youre done on
  this path
• set c and i to controlling/inversion values of
  gate driving l
• inval v ? i
• if (inval c)
• select one input j of gate l
• Justify (j, inval)
• else
• for every input j of gate l
• Justify (j, inval)

30
An example of justification

• l v
• if l is a primary input return youre done on
  this path
• set c and i to controlling/inversion values of
  gate driving l
• inval v ? i
• if (inval c)
• select one input j of gate l
• Justify (j, inval)
• else
• for every input j of gate l
• Justify (j, inval)

31
Test Generation Propagate Algorithm

• Prop (l , err) Propagate value err from line l
• l err
• if line l is a primary output return youre
  home
• k fanout gate of line l
• c,i controlling/inversion value of gate k
• for every input j of k other than l
• Justify (j, c)
• Propagate (k, err ? i)

32
Testing Digital Circuits

• What you know
• Fault models what can go wrong and how we model
  it
• physical and logical
• Basic idea of detection activate fault and
  propagate to output
• What you dont know
• how to figure out, systematically, whether the
  whole thing works
• how to reduce the number of faults to consider
  when generating tests
• Today
• Review equivalence and fault collapsing
• Begin test generation algorithms

33
Detection

• Basic approach seen so far
• Select a line and a fault line l s-a-v
• Activate the fault
• Drive line l to v selecting the inputs needed
  to set an internal line to a known value is
  known as line justification
• Activation creates a discrepancy D
• Propagate the fault
• Propagate the discrepancy D along a sensitized
  path to any primary output

discrepancy
s-a-0
0
1/0
Notation good value/bad value
x
0/1
1
34
Fault Dominance

• Equivalence vs. Dominance
• Dominance is a special case of fault equivalence
• Fault equivalence, if Z f (x) Z g (x) for
  all xthen the faults are functionally
  equivalent.
• If this is true for a subset of x, then there is
  a dominance relation
• Dominance
• Let Tg be the set of all tests that detect a
  fault g.
• A fault f dominates the fault g iff f and g are
  functionally equivalent under Tg.
• Z f (t) Z g (t) for all t in Tg
• Tg is a subset of Tf

35
Equivalence and Dominance Summary

• What are the equivalence classes?

s-a-0 s-a-1
Equivalence A0, B0, Z1
s-a-0 s-a-1
s-a-0 s-a-1
Dominance Z0 dominates A1, B1
11, 01, 10
36
Aside Fault Location

• Detection got us down to three tests
• Were left with three tests for this gate if
  were interested in fault detection.
• If were interested in fault location, we need
  more
• To isolate y s-a-1
• Need to apply both 10 and 01
• 10, alone, detects the equivalent faults y s-a-1
  and z s-a-0
• 01, alone, detects the equivalent faults x s-a-1
  and z s-a-0
• Together, they can isolate the three faults
  (assuming only one fault active).

x
sa1
z
sa0
y
sa1
37
Overall process
set of faults for circuit
38
Test Generation

• Toward an algorithmic means to generate test
  vectors
• What do we want in a test vector?
• fault activation and propagation
• if the discrepancy D wiggles (i.e. from good to
  bad), then so does the output
• how do we determine if a function changes with
  respect to a variable
• Use Automatic Test Generation algorithms (ATG)

39
Primary inputs and outputs

• Test generation algorithms work in terms of
• Primary inputs (PI) a controllable input to a
  circuit. E.g. A pin on an IC, or an output of
  an FF in a scan system
• Primary outputs (PO) an observable output of
  the circuit. E.g. A pin on an IC, or a D input
  to an FF in a scan system
• They all operate in terms of
• finding the appropriate PIs to control to
  activate a fault
• finding the appropriate PIs to control to
  propagate a discrepancy to one of the POs.

40
Propagate, Justify

• A few definitions
• justify, justification the process of selecting
  PIs to force a certain line to have a specific
  value
• the verb justify a 0 on the input a of gate B
• the noun justification is the process of
  justifying
• propagate, propagation the process of selecting
  appropriate PIs that allow a discrepancy D to
  be pushed to a PO
• propagate the D to any output
• propagation is the process
• involves justification

41
Imply all you can

• Forward implication
• Def Knowing one or more gate inputs, imply the
  output value.
• Assume all gate inputs are the same value
  either all c or all c
• Then the output is
• output value ? i
• We can refine this if we know the controlling
  value
• i.e. only one of the inputs needs to have c to
  know output

42
Look behind yourself too

• Backward implication
• Def Knowing the output and possibly some inputs,
  imply one or more of the inputs
• Assume all gate inputs are the same either all
  c or c
• Then the inputs are
• inputs output ? i
• We can refine this if we know the controlling
  value
• If the input needed to produce the output is c,
  then only one input needs to have it.

43
Justify Algorithm

• Justify (l , v) Recursive algorithm to justify
  line l to value v

l v if l is a primary input return youre
done on this path set c and i to
controlling/inversion values of gate driving l
inval v ? i if (inval c) select one input
j of gate l Justify (j, inval) else for
every input j of gate l Justify (j, inval)
44
An example of justification
l v if l is a primary input return youre
done on this path set c and i to
controlling/inversion values of gate driving l
inval v ? i if (inval c) select one input
j of gate l Justify (j, inval) else for
every input j of gate l Justify (j, inval)
45
Test Generation Propagate Algorithm

• Prop (l , err) Propagate value err from line l

l err if line l is a primary output return
youre home k fanout gate of line l c,i
controlling/inversion value of gate k for every
input j of k other than l Justify (j,
c) Propagate (k, err ? i)
46
Will this always work?

• Will justify and propagate always work?
• Circuits without reconvergent fanout
• select one and justify are each independent
  of any previous justification
• youre guaranteed that propagation and justify
  will not interfere

47
Test Generation Basic Algorithm

• Algorithm to test line l s-a-v

begin set all values to x (unknown) Justify line
l to value v if (v 0) Propagate D on
line l else Propagate D on line l end
48
Automatic Test-Pattern Generation (ATPG)

• Test U2.ZN for s-a-1
• 1) Activate (excite) fault gtU2.ZN 0
• 2) Work backward gt A 0
• 3) Work forward (sensitize the path to PO)
  gtU3.A2 1, U5.A2 1
• 4) Work backward (justify outputs) gtABC 110

49
Reconvergent Fanout
Fault B s-a-1?
Fault U4.A1 s-a-1?
We create two sensitized paths that prevent fault
from propagating to the PO. The problem can be
solved by changing A to 0, but this breaks rules
of the ATPG! The PODEM algorithm solves the
problem.
Signal B branches and then reconverges at logic
gate U5. ATPG works.
50
Test Generation example

• With reconvergent fanout
• Fanout paths from a gate reconverge at some later
  gate
• Inputs needed for propagation may be inconsistent
  with ones needed for justification

Procedure justify G1 to 0 gt abc1 propagate
to G4 gt requires G2 1 but a1 makes
G20 Inconsistency crash and burn
Kaboom!
51
Test generation example, contd

• Need to backtrack propagate on other path

52
Backtracking

• Backtracking requires that a decision tree be
  maintained
• Each node describes a designs state
• values previously justified on lines
• implications, forward and backward
• Each arc describes a new decision
• justify a line, activate a fault
• Need to be able to go back
• to former state

53
Maintaining the decision tree
Procedure justify G1 to 0 gt abc1 propagate
to G4 gt requires G2 1 but a1 makes
G20 Inconsistency propagate to G5 gt justify
G3 to 1 this works with e0
State 1 all xs
justify G1 to 0
State 1A
abc1
Prop. to G5
Prop. to G4
G3 1 e 0 win
G21, a1 inconsistency fail
State 1A1
State 1A2
Backtrack, G21 no longer part of design state.
Revert to previous state.
54
Observations on approach

• Enumeration used
• justify algorithm was recursive
• When gate has controlling value on input, one
  path selected
• may need to backtrack and follow another
• eventually, may need to follow all
• Propagate algorithm was recursive
• When there is a fanout at a propagation point,
  one path selected toward output
• may need to backtrack and follow another
• eventually, may need to follow all
• The backtracking, again, is due to reconvergent
  fanouts and previous values justified on them
• No solution?  redundant wrt the fault
• As it turns out
• The natural state maintenance in recursive
  programs can keep track of the decision tree

55
More terminology

• When propagating a discrepancy
• Often, due to fanout, there are several options
• Propagate needs to pick one for the sensitized
  path
• D - frontier
• The D-frontier is the set of all gates with D or
  D on one or more inputs and an x on its output
  (no other inputs are controlling)
• This is the set from which you select a
  propagation (sensitization) path

56
D-Frontier

• Back to our example
• After the activation of the fault, and forward
  implication, the D- frontier is ?
• If D-frontier Ø, then no path to primary output
  
• failure, backtrack
• previous justifications have made this path
  impossible

57
J-Frontier

• In line justification
• The J-frontier is the set of all gates whose
  output values are known, but the outputs are not
  implied (yet) by the inputs
• Some inputs may be known, but the current output
  value is not implied
• Similar to D-frontier, but looking backward

58
J-Frontier

• Back to the Example
• The fault is activated and forward implication is
  done
• A gate is selected from the D-frontier for
  propagation
• In this case, G5 is the only choice
• The J-frontier is then ?

59
Implication Revisited

• Implication Process
• Compute all values uniquely determined by
  implication
• 1, 0, D, D, x looking forward and backward
• more aggressive than previous implication
• maintain the D and J frontier

60
Backward Implication
new implication front
implication front
After
Before
lt
1
x
lt1
1
x
lt
1
x
lt
0
lt0
0
1
1
x
x
0
lt0
J-frontier
J-frontier , a
a
x
a
x
lt1
1
lt
1
x
1
lt
1 gt
1
x
61
Forward Implication
After
Before
0gt
0
0gt
x
x
x
1
1gt
1gt
x
1
1
0gt
0 a
0
0 a
J-front
J-front, a
x
x
1gt
0 a
1
0 a
J-front
J-front, a
lt0
x
D
D
x a
Dgt a
D-front, a
D-front
1gt
1
D
x a
D
0gt
D-front
D-front, a
0gt
0
62
Where are we now?

• Pieces of test generation algorithms seen
• justify, propagate
• problems with reconvergent fanout
• need to backtrack makes for a messier algorithm
• need to keep track of state, and what
  combinations have been tried before.
• heuristics to guess at best next path to follow
• To come
• D algorithm
• and eventually Podem

63
Implication Process Revisited

• Unique D-drive
• If there is only one gate on the D frontier,
  then implication propagates D through the gate.
• Its the only direction D could propagate

after
before
D
D
x
D gt
a
x
lt 1
D-frontier a
D-frontier
64
All Pieces in Place

• Pieces
• Controlling and inverting values
• Fault activation
• Justification
• Propagation
• Forward/backward implication
• D and J frontiers
• Decision tree maintenance
• Discussion of the D algorithm
• note that this is a version of the D algorithm
• a number of situations have been left open, e.g.
• select an input , select a gate
• which one?

65
D-Algorithm

• Initialization
• set all line values to X
• activate the target fault by assigning logic
  value to that line
• 1. Propagate D to PO
• 2. Justify all values
• Imply_and_check() does only necessary
  implications,no choices
• if D-alg() SUCCESS then return SUCCESS
• else undo assignments and its implications

66
Test Generation The D Algorithm
Decorate design with all known values. Check for
inconsistencies.

• if (imply_and_check() FAIL) return FAIL
• if (error not at primary output)
• if (D-frontier Ø) return FAIL
• repeat
• select an untried gate (G) from D-frontier
• c controlling value of G
• assign c to every input of G with value x
• if (D-Alg() SUCCESS) return SUCCESS
• until all gates from D-frontier tried
• return FAIL
• if (J-frontier Ø) return SUCCESS
• select a gate G from the J-frontier
• c controlling value of G
• repeat
• select an input (j) of G with value x, assign c
  to j
• if (D-Alg() SUCCESS) return SUCCESS
• assign c to j / reverse decision/
• until all inputs of G are specified
• return FAIL

Push D to a primary output
Once at primary output, justify all values needed
to have D on the primary output
67
A circuit and fault to test
68
Tracing through an example
1
all xs
a 0, b c 1
D
0
1
1
Decisions Implications Comments a
0 Activate the fault h 1 b
1 Unique D-drive through g c 1 (the
unique path for D) g D D-frontier becomes
i,k,m
69
Tracing through an example
1
all xs
1
0
D
a 0, b c 1
D
0
1
1
d 1
Decisions Implications Comments d1
Propagate through i i D d
0 D-frontier becomes k, m, n
70
Tracing through an example
all xs
1
1
0
D
a 0, b c 1
1
D
0
1
D
1
0
1
d 1
1
1
1
jklm1 Bang
Decisions Implications Comments
jk1 Propagate through n lm1 nD
e0, e1 kD But k 1 Contradiction!
D-frontier remains k, m, n
71
Tracing through an example
all xs
1
1
0
D
a 0, b c 1
1
0
1
D
D
0
1
d 1
1
e 1
jklm1 Bang
Decisions Implications Comments e
1 Propagate through k kD e0
j1 D-frontier becomes m, n
72
Tracing through an example
all xs
1
1
0
D
a 0, b c 1
1
0
1
D
D
0
d 1
1
1
1
0
1
1
e 1
jklm1 Bang
Decisions Implications Comments
lm1 propagate through n n D f
0 f 1 m D But m 1,
contradiction! D-frontier remains m, n
lm1 Bang
73
Tracing through an example
all xs
1
1
0
D
a 0, b c 1
1
D
0
1
D
D
0
d 1
1
1
1
0
1
D
e 1
jklm1 Bang
Decisions Implications Comments f
1 Propagate through m m D f 0
l 1 n D J-frontier is Null
lm1 Bang
f 1, n D party!
74
What about the J-frontier?

• In this example, all inputs were easily justified
  through implication
• essentially, d, e, and f were primary inputs
• if these were driven by other gates, the earlier
  inputs might not have been implied. e.g.

a
a
x
x
lt 0
0
x
x
J-frontier
J-frontier , a
75
What about the J-frontier?

• The D-algorithm
• picks a gate from the J-frontier
• and then tries to set each input to a controlling
  value
• If that value fails due to imply_and_check, it
  is inverted and a new input is tried
• how does it handle the case where none of the
  inputs should be controlling?
• if (J-frontier Ø) return SUCCESS
• select a gate G from the J-frontier
• c controlling value of G
• repeat
• select an input (j) of G with value x, assign c
  to j
• if (D-Alg() SUCCESS) return SUCCESS
• assign c to j / reverse decision/
• until all inputs of G are specified
• return FAIL

exit here if output of gate G is justified,
possibly before setting all inputs
76
J-Frontier

• Assume a change to the example circuit
• Then we would be left with elements in the
  J-frontier set
• J-frontier is f
• If the xs are primary inputs, this is easy
• If theyre not primary inputs,
• more gates begin to show up in J-frontier
• you may not be able to set the input you select
  to the controlling value
• If there is a redundancy, the whole process might
  fail.

1
1
0
D
1
D
0
1
D
D
0
1
1
1
x
0
1
D
77
Another example
all xs
Decision Tree
c-sa0
decisions implications comments
78
Another example
all xs
c1, b1
0
0
Decision Tree
D
h1
1
1
c-sa0
D
D
j0
D
0
a1
1
d0
decisions implications comments
activate fault, unique D drive
c1,b1,gfD
h1 iD prop through i. j,kDf
hJf a1 jD, k1 prop through j. Df null.
backtrack (a x) j0 a0, kD prop
through i, fault at output d0 justify h
79
Summary D algorithm

• How does it work
• Conceptually
• Activate fault and propagate
• Then justify the remaining gates
• When propagating
• assign c to other inputs of the gates on the
  sensitized path
• do forward and backward implication
• when going backward, specify gate inputs if they
  are all c
• if one input should be c, put gate into J-frontier

80
Summary D algorithm

• Oh, by the way
• justify the rest of these inputs
• That is, the D-frontier is pursued with only
  partial regard to whether the c values selected
  are self consistent
• In the process, the J-frontier grew large
• 5 gates shown highlighted
• plus the gates that drive them
• and theres lots of reconvergent fanout to
  cause justification problems.

81
Summary D Algorithm

• Depth-first push toward primary output
• do justification and consistency afterward as
  needed
• backward implication can cause problems
• use backtracking as necessary
• Exhaustive, exponential
• The number of operations performed is an
  exponential function of the number of gates
• This is worst case, typically only seen when a
  fault turns out to be undetectable
• But you dont know its undetectable until you
  exhaustively try everything
• Heuristics for selecting one of help reduce
  search time of successful searches
• Test generators are often limited in their search
  depth, thus some detectable faults dont have
  tests.