Transcript Logic Gates 1

Topics
Combinational logic functions.
 Static complementary logic gate structures.

Modern VLSI Design 3e: Chapter 3
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Combinational logic expressions
Combinational logic: function value is a
combination of function arguments.
 A logic gate implements a particular logic
function.
 Both specification (logic equations) and
implementation (logic gate networks) are
written in Boolean logic.

Modern VLSI Design 3e: Chapter 3
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Gate design
Why designing gates for logic functions is
non-trivial:
– may not have logic gates in the library for all
logic expressions;
– a logic expression may map into gates that
consume a lot of area, delay, or power.
Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR
Boolean algebra terminology

Function:
f = a’b + ab’
a is a variable; a and a’ are literals.
 ab’ is a term.
 A function is irredundant if no literal can be
removed without changing its truth value.

Modern VLSI Design 3e: Chapter 3
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Completeness
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A set of functions f1, f2, ... is complete iff every Boolean function can
be generated by a combination of the functions.
NAND is a complete set; NOR is a complete set; {AND, OR} is not
complete.
NOT = NAND with shorted inputs.
AND = NAND followed by a NOT.
OR = Two inverters followed by a NAND.
Transmission gates are not complete.
If your set of logic gates is not complete, you can’t design arbitrary
logic.
Modern VLSI Design 3e: Chapter 3
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Static complementary gates
Complementary: have complementary
pullup (p-type) and pulldown (n-type)
networks.
 Static: do not rely on stored charge.
 Simple, effective, reliable; hence
ubiquitous.

Modern VLSI Design 3e: Chapter 3
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Static complementary gate
structure
Pullup and pulldown networks:
VDD
pullup
network
out
inputs
pulldown
network
VSS
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Inverter
+
a
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out
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Inverter layout
VDD
+
a
tub ties
out transistors
a
out
(tubs not
shown)
GND
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NAND gate
+
out
b
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a
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NAND layout
VDD
+
out
b
a
out tub
ties
b
a
GND
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NOR gate
+
b
a
out
Modern VLSI Design 3e: Chapter 3
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NOR layout
b
VDD
a
tub ties
b
out
out
a
GND
Modern VLSI Design 3e: Chapter 3
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AOI/OAI gates
AOI = and/or/invert; OAI = or/and/invert.
 Implement larger functions.
 Pullup and pulldown networks are compact:
smaller area, higher speed than
NAND/NOR network equivalents.
 AOI312: and 3 inputs, and 1 input
(dummy), and 2 inputs; or together these
terms; then invert.

Modern VLSI Design 3e: Chapter 3
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AOI example
out = [ab+c]’:
invert
symbol
pull-up circuit
or
and
pull-down circuit
Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR
Pullup/pulldown network design
Pullup and pulldown networks are duals.
 To design a CMOS logic circuit for a
combinational expression, first design a
pull-down network for it, then compute the
dual to get the pull-up network.

Modern VLSI Design 3e: Chapter 3
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Dual network construction
a
c
b
b
dummy
a
c
dummy
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Dual network construction
The pull-up part of a CMOS implementation of f can be
obtained by writing a sum-of-product expression for f and
likewise the pull-down part can be obtained by writing a
sum-of-product expression for f’.
Note: As we discussed in class, it is not always possible
to use p-type transistors in the pull-up circuit and/or ntype transistors in the pull-down circuit unless we use
inverter gates for negated inputs in certain cases. For
example, (A’B + CD) cannot be realized all with p-type
transistors unless we use an inverter to generate A’.
Modern VLSI Design 3e: Chapter 3
Copyright  1998, 2002 Prentice Hall PTR
Dual network construction
(A’B + CD)’ = (A +B’)(C’+D’)
vDD
C
D
A
B
(A’B + CD)’
GND
A
C
B
D
(A’B + CD)
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GND
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