Bài giảng Digital electronics - Analysis combinational circuit - Lê Dũng
Func2on
hazard
•
Func2on
hazards
are
non-‐solvable
hazards
which
occurs
when
more
than
one
input
variable
changes
at
the
same
2me.
•
Func2on
hazards
can
not
be
logically
eliminated
with
actual
specifica2on
of
the
circuit.
The
only
real
way
to
avoid
such
problems
is
to
restrict
the
changing
of
input
variables
so
that
only
one
input
should
change
at
any
given
2me.
Hazard-‐free
design
• Hazards
are
hard
to
detect
by
hand:
importance
of
simula2on
• The
danger
for
hazards
increases
when
rise
2mes
and
fall
2mes
are
not
equal
• Are
hazards
a
problem?
For
synchronous
circuits,
they
are
not
Unless
they
control
the
clock
of
a
memory
element
For
asynchronous
circuits,
they
always
are
a
problem
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9/23/13
1
Analysis Combinational Circuit
Dr.
Le
Dung
Hanoi University of Science and Technology
Dr. Le Dung Hanoi University of Science and Technology
From
a
given
combina2onal
circuit
to
analysis
•
Its
func2on
(Truth
table,
Expression
forms)
•
Timing
diagram
(Test
vectors,
Delay,
Hazard/Glitch)
9/23/13
2
Dr. Le Dung Hanoi University of Science and Technology
An
example
(1)
2
to
4
Decoder
A0
A1
m0
m1
m2
m3
4
to
1
MUX
S1
S0
D0
D1
D2
D3
Y
a
b
c
d
f(a,b,c,d)
?
Analysis
this
combina2onal
circuit
Dr. Le Dung Hanoi University of Science and Technology
An
example
(2)
4
to
1
MUX
S1
S0
D0
D1
D2
D3
Y
Modular
understanding
2
to
4
Decoder
A0
A1
m0
m1
m2
m3
A1
A0
m0
m1
m2
m3
0
0
1
0
0
0
0
1
0
1
0
0
1
0
0
0
1
0
1
1
0
0
0
1
S1
S0
Y
0
0
D0
0
1
D1
1
0
D2
1
1
D3
9/23/13
3
Dr. Le Dung Hanoi University of Science and Technology
An
example
(3)
Truth
table
Expression
forms
c
d
a
b
f
0
0
0
0
1
f=Y=D0
0
0
0
1
0
0
0
1
0
0
0
0
1
1
0
0
1
0
0
0
f=Y=D1
0
1
0
1
1
0
1
1
0
0
0
1
1
1
0
1
0
0
0
0
f=Y=D2
1
0
0
1
0
1
0
1
0
1
1
0
1
1
0
1
1
0
0
0
f=Y=D3
1
1
0
1
0
1
1
1
0
0
1
1
1
1
1
f
=
Σ
(0,
5,
10,
15)
=
Σ
(0,
5,
10,
15)
cdab
abcd
f
=
abcd
+
abcd
+
abcd
+
abcd
Dr. Le Dung Hanoi University of Science and Technology
An
example
(4)
Expression
forms
=
abcd
+
abcd
+
abcd
+
abcd
c
d
F
=
Y
=
0
0
D0
=
m0
=
a’b’
0
1
D1=
m1
=
a’b
1
0
D2=
m2
=
ab’
1
1
D3=
m3
=
ab
f
=
m0.cd
+
m1.cd
+
m2.cd
+
m3.cd
=
(a
c)(b
d)
=
(a
+
c)
+
(b
+
d)
9/23/13
4
Dr. Le Dung Hanoi University of Science and Technology
An
example
(5)
Timing
diagram
with
no
delay
a
b
d
c
f
f
=
(a
c)(b
d)
Test
vectors
:
abcd
=
000010001010111011110111010101000000
Dr. Le Dung Hanoi University of Science and Technology
An
example
(6)
Timing
diagram
with
delay
a
b
d
c
f
m1
=
D1
m0
=
D0
Test
vectors
:
abcd
=
0000
0100011001000101
9/23/13
5
Dr. Le Dung Hanoi University of Science and Technology
Sta2c
Hazard
/Glitch
•
Hazard
condi2on:
A
single
variable
change
causes
a
momentary
output
change
when
no
output
change
should
occur.
•
Glitch:
The
momentary
output
change
=
unwanted
transient
pulse
+
Sta2c
1-‐hazard
=
glitch
101
(in
SOP
)
+
Sta2c
0-‐hazard
=
glitch
010
(on
POS)
•
Cause:
different
delay
in
two
paths
(see
Example)
•
Solu2on:
Adding
redundant
terms
(product
terms
or
sum
terms)
Dr. Le Dung Hanoi University of Science and Technology
An
example
of
sta2c
1-‐hazard
a
c
b
A
B
A
B
Y
X1
Y
X2
A
B
Y
=
f
c’
AND
2
OR
2
AND
2
INV
a
=
b
=
1
c
X2
X1
f
Sta2c
1-‐hazard
=
glitch
9/23/13
6
Dr. Le Dung Hanoi University of Science and Technology
Removing
sta2c
1-‐hazard
A
B
A
B
A
B
A
B
C
Y
X1
Y
X2
Y
X3
a
c
b
Y
f
A
AND
2
AND
2
A
OR
3
AND
2
Redundant
term
=
consensus
term
Sta2c
hazard
free
Dr. Le Dung Hanoi University of Science and Technology
Dynamic
hazard
Dynamic
Hazard
on
0
1
Dynamic
Hazard
on
1
0
•
A
dynamic
hazard
is
the
possibility
of
an
output
changing
more
than
once
as
a
result
of
a
single
input
change
•
Cause:
different
delay
in
mul2ple
paths
•
Any
circuit
that
is
sta2c
hazard
free
is
also
dynamic
hazard
free
•
Any
circuit
dynamic
hazard
free
9/23/13
7
Dr. Le Dung Hanoi University of Science and Technology
An
example
of
dynamic
hazard
(1)
Dr. Le Dung Hanoi University of Science and Technology
An
example
of
dynamic
hazard
(2)
9/23/13
8
Dr. Le Dung Hanoi University of Science and Technology
An
example
of
dynamic
hazard
(3)
Dr. Le Dung Hanoi University of Science and Technology
Func2on
hazard
•
Func2on
hazards
are
non-‐solvable
hazards
which
occurs
when
more
than
one
input
variable
changes
at
the
same
2me.
•
Func2on
hazards
can
not
be
logically
eliminated
with
actual
specifica2on
of
the
circuit.
The
only
real
way
to
avoid
such
problems
is
to
restrict
the
changing
of
input
variables
so
that
only
one
input
should
change
at
any
given
2me.
9/23/13
9
Dr. Le Dung Hanoi University of Science and Technology
Hazard-‐free
design
• Hazards
are
hard
to
detect
by
hand:
importance
of
simula2on
• The
danger
for
hazards
increases
when
rise
2mes
and
fall
2mes
are
not
equal
• Are
hazards
a
problem?
For
synchronous
circuits,
they
are
not
Unless
they
control
the
clock
of
a
memory
element
For
asynchronous
circuits,
they
always
are
a
problem
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- bai_giang_digital_electronics_analysis_combinational_circuit.pdf