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Chapter 2
CMOS Logic
Application-Specific Integrated Circuits
Michael John Sebastian Smith
Addison Wesley, 1997
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
The MOS Transistor
Gate Oxyde
Gate
Source
Polysilicon
n+
Field-Oxyde
Drain
(SiO2)
n+
p+ stopper
p-substrate
Bulk Contact
CROSS-SECTION of NMOS Transistor
EGRE 427 Advanced Digital Design
Figures
from
material provided with
DigitalCircuits,
Integrated
Circuits,
A Design
Figures
from
Application-Specific
Integrated
Michael
John
Sebastian
Perspective,
Jan Rabaey,
Smith,
Addison by
Wesley,
1997 Prentice Hall, 1996
The MOS Transistor
Figure 2.3 An N-channel MOS transistor
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
Threshold Voltage: Concept
+
S
VGS
-
D
G
n+
n+
n-channel
Depletion
Region
p-substrate
B
EGRE 427 Advanced Digital Design
Figures
from
material provided with
DigitalCircuits,
Integrated
Circuits,
A Design
Figures
from
Application-Specific
Integrated
Michael
John
Sebastian
Perspective,
Jan Rabaey,
Smith,
Addison by
Wesley,
1997 Prentice Hall, 1996
The Threshold Voltage
EGRE 427 Advanced Digital Design
Figures
from
material provided with
DigitalCircuits,
Integrated
Circuits,
A Design
Figures
from
Application-Specific
Integrated
Michael
John
Sebastian
Perspective,
Jan Rabaey,
Smith,
Addison by
Wesley,
1997 Prentice Hall, 1996
Current-Voltage Relations
VGS
VDS
S
G
n+
–
V(x)
ID
D
n+
+
L
x
p-substrate
B
MOS transistor and its bias conditions
EGRE 427 Advanced Digital Design
Figures
from
material provided with
DigitalCircuits,
Integrated
Circuits,
A Design
Figures
from
Application-Specific
Integrated
Michael
John
Sebastian
Perspective,
Jan Rabaey,
Smith,
Addison by
Wesley,
1997 Prentice Hall, 1996
Current-Voltage Relations
EGRE 427 Advanced Digital Design
Figures
from
material provided with
DigitalCircuits,
Integrated
Circuits,
A Design
Figures
from
Application-Specific
Integrated
Michael
John
Sebastian
Perspective,
Jan Rabaey,
Smith,
Addison by
Wesley,
1997 Prentice Hall, 1996
Transistor in Saturation
VGS
VDS > VGS - VT
G
D
S
n+
EGRE 427 Advanced Digital Design
-
VGS - VT
+
n+
Figures
from
material provided with
DigitalCircuits,
Integrated
Circuits,
A Design
Figures
from
Application-Specific
Integrated
Michael
John
Sebastian
Perspective,
Jan Rabaey,
Smith,
Addison by
Wesley,
1997 Prentice Hall, 1996
I-V Relation
VDS = VGS-VT
Saturation
ID (mA)
VGS = 4V
1
0.0
VGS = 3V
1.0
2.0
3.0
VDS (V)
VGS = 2V
VGS = 1V
4.0
5.0
(a) ID as a function of VD S
0.020
÷ID
Triode
Square Dependence
2
VGS = 5V
0.010
Subthreshold
Current
0.0
2.0
VT1.0
VGS (V)
3.0
(b) ID as a function of VGS
(for VDS = 5V).
NMOS Enhancement Transistor: W = 100 m, L = 20 m
EGRE 427 Advanced Digital Design
Figures
from
material provided with
DigitalCircuits,
Integrated
Circuits,
A Design
Figures
from
Application-Specific
Integrated
Michael
John
Sebastian
Perspective,
Jan Rabaey,
Smith,
Addison by
Wesley,
1997 Prentice Hall, 1996
Velocity Saturation
(a)
(b)
(c)
Figure 2.4 MOS N-channel transistor characteristics for a generic
0.5 m process. (a) IV curves for several short channel
devices. (b) IV characteristics represented as a surface.
(c) Linear IV characteristic due to velocity saturation
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
MOS Logic Levels
Figure 2.5 CMOS logic levels. (a) A strong
‘0’. (b) A weak ‘1’. (c) A weak ‘0’. (d) A
strong ‘1’.
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
MOS Transistors as Switches
Figure 2.1 CMOS transistors as switches. (a) An N-channel transistor. (b) A P-channel transistor. (c) A CMOS inverter
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
CMOS Logic
Figure 2.2 CMOS logic. (a) A two-input NAND gate. (b) A two-input NOR gate.
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
MOS IC Fabrication
Figure 2.6 IC fabrication Grow crystalline silicon (1); make wafer (2-3); grow and oxide layer (4);
apply liquid photoresist (5); mask exposure (6); cross-section showing exposed
photoresist (7); etch the oxide layer (8); ion implantation (9-10); strip resist (11); strip
oxide (12); repeat steps similar to 4-12 for subsequent layers (12-20 times for a typical
CMOS process).
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
CMOS Device Layers
Figure 2.7The drawn layers, final
layout, and phantom cell
view of the standard cell
shown in figure 1.3
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
Drawn vs. Actual Transistor Layers
Figure 2.9 The transistor layers (a) a drawn P-channel layout. (b) The corresponding silicon cross-section.
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
Drawn vs. Actual Interconnect Layers
Figure 2.10 The interconnect layers. (a) The drawn layers. (b) The corresponding structure.
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
Typical CMOS Layout Rules
Figure 2.11
EGRE 427 Advanced Digital Design
The MOSIS SCMOS design
rules (rev. 7). Dimensions are
in l.
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
Typical CMOS Library Cells
Figure 2.12 Naming and numbering conventions for complex CMOS cells. (a) An
AND_OR_INVERT cell. (b) An OR-AND-INVERT cell
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
Constructing a CMOS Logic Cell
Figure 2.13 Constructing a CMOS AOI221 cell. (a) Use Demorgan’s theorem to “push” inversion bubbles to
the inputs. (b) Build the pull-up and pull-down networks from PMOS and NMOS transistors. (c) Adjust
transistor sizes so that the n and p based networks have the same drive strength - to ensure equal rise and
fall times.
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
CMOS Transmission Gates
Figure 2.14 CMOS transmission gate (TG). (a) a P and N transistor implementation. (b) A
common symbol. (c) The charge sharing problem.
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
CMOS Implementation of a Multiplexer
Figure 2.15 A CMOS multiplexer (MUX). (a) A TG implementation without buffering. (b) The corresponding logic
symbol. (c) The IEEE standard symbol. (d) an alternate (non-standard) IEEE symbol. (e) An inverting,
buffered implementation and its logic symbol. (f) a non-inverting, buffered implementation and its symbol.
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
Implementing an Mutiplexer Using an
OAI22 Cell
Figure 2.16 An inverting 2:1 mux based on an OAI22 cell
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
CMOS Latch
Figure 2.17 CMOS latch. (a) A positive-enable latch using transmission gates (b) Operation
when enable is high. (b) Operation when enable is low.
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
CMOS Flip-Flop
Figure 2.18 CMOS flip-flop. (a) Negative
edge triggered master-slave. (b)
Master loads when clock is high.
(c) Slave loads output value of
master latch when clock goes
low. (d) Waveforms illustrating
setup, hold, and propagation
times.
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
Datapath Logic Cells
Figure 2.20 A datapath adder. (a) A full adder (FA) cell. (b) A 4-bit adder. (c) Wiring layout using 2 level metal. (d) The
datapath layout
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
Datapath Elements
Figure 2.21 Symbols for a datapath adder. (a) A generic symbol. (b) An alternate symbol. (c) A symbol with control lines.
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
Ripple Carry Adder
Figure 2.22 The ripple carry adder (RCA). (a) A conventional RCA. (b) An implementation using alternate cells for even
and odd bits.
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
Carry-Save Adder
Figure 2.23 The carry-save adder (CSA). (a) A CSA cell. (b) A 4-bit CSA. (c) Symbol for a CSA. (d) A 4-input CSA. (e)
The datapath for a 4-bit adder using CSAs. (f) A pipelined adder. (g) The datapath for the pipelined
version.
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
Lookahead Carry Adder
Figure 2.24 The Brent-Kung carry-lookahead adder. (a) Carry generation. (b) Cell to generate look-ahead terms. (c)
Arrangement of cells. (d) and (e) Simplified representations of parts a and c. (f) The lookahead logic for
an 8 bit adder. (g) An 8 bit Brent-Kung CLA.
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
Carry Select Adder
Figure 2.25 The conditional-sum adder (a) A 1-bit conditional adder. (b) The multiplexer to select sums and carries. (c)
A 4-bit conditional-sum adder
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
Comparison of Adder Implementations
Figure 2.26 Delay and area comparison for datapath adders. (a) Delay normalized to a two-input NAND logic cell
delay. (b) Adder area.
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
Array Multiplier
Figure 2.27 A 6-bit array
multiplier using a
final carry-propagate
adder.
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
Other Datapath Elements
Figure 2.32 Symbols for datapath elements. (a) An N-bit wide register. (b) An N-bit wide two-input NAND array. (c)
An N-bit wide two-input NAND array with a control input. (d) An N-bit wide MUX. (e) An N-bit wide
incrementer/decrementer. (f) An N-bit wide all zeros detector. (g) An N-bit wide all ones detector. (h) An
N-bit wide adder/subtracter.
EGRE 427 Advanced Digital Design
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997
I/O Cells

I/O pads are specalized to
connect to the actual pins of
the device



Electrostatic discharge (ESD)
High(er) drive capability to
drive larger capacitances
(bonding pad, bond wire,
device pin, PCB trace > 20pF)
Different types of I/O pads are
provided to perform different
functions




Digital input
Digital Output
Digital Bi-directional
Analog In/Output
EGRE 427 Advanced Digital Design
Figure 2.33 A tri-state bidirectional output buffer with I/O pad.
Figures from Application-Specific Integrated Circuits, Michael John Sebastian
Smith, Addison Wesley, 1997