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VLSI
Prof. Vojin G. Oklobdzija
References (used for creation of the presentation material):
[1] Mead, Conway, “Introduction to VLSI Systems”, Addison Wesley
Publishing.
[2] Glasser, Dobberpuhl, “The Design and Analysis of VLSI Circuits”,
Addison Wesley Publishing.
[3] Weste, Eshraghian, “Principles of CMOS VLSI Design”, Addison
Wesley Publishing.
[4] Shoji, “CMOS Digital Circuits Technology”, Prentice Hall.
Historical Overview
•
•
•
•
nMOS era: 1970-85
Pass-transistor design
CMOS existed early but took off 1985 on
Domino CMOS, 1982
– NORA
– DCVSL
• CPL, DPL
– DCVS-PG
– SRPL
– LEAP
• SOI-CMOS
Fall 2004
Prof. V.G. Oklobdzija: High-Performance System Design
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n-MOS Design Era 1970-85
LSI started with nMOS:
• pass-transistor design experience:
- Flourished at the beginning of the nMOS era
(popularized by Mead-Conway book)
- Allows high density layout and compact design
style
- Fast: outperforming gate based design
- Low in power
• Drawbacks:
– Not compatible with existing design tools
– Exhibiting testability and reliability problems
Fall 2004
Prof. V.G. Oklobdzija: High-Performance System Design
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Review of CMOS
Prof. Vojin G. Oklobdzija
CMOS Basics
Vdd
Karnaugh Map
of Function F
( A + B)
A
F
0
1
0
1
1
1
1
0
F
B
(A
A
B)
B
C
Function F and its Dual
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Prof. V.G. Oklobdzija: High-Performance System Design
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CMOS Basics
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Prof. V.G. Oklobdzija: High-Performance System Design
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CMOS Basics
covering ones :
D( A + B C )
B
A
D
C
D
0
1
1
0
0
1
covering zeroes :
D + A (B +C)
C
0
0
0
0
1
0
0
0
A
A
Fall 2004
0
B
F
B
1
D
D
Prof. V.G. Oklobdzija: High-Performance System Design
7
CMOS Basics
A complex path example:
VDD
E
B
D
A
C
Output
A
B
C
D
E
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Prof. V.G. Oklobdzija: High-Performance System Design
8
CMOS Basics
Primitive gates:
Fall 2004
More complex blocks
are realizable in CMOS
Prof. V.G. Oklobdzija: High-Performance System Design
9
CMOS Deficiencies:
Various remedies:
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Muli-Input NOR
function
in CMOS is slow
Prof. V.G. Oklobdzija: High-Performance System Design
10
CMOS Deficiencies and Remedies
Faster, one-level
realizations of XOR
function
XOR
A
A
B
AB
B
+
AB
AB
+
AB
B
(a)
Fall 2004
(b)
Prof. V.G. Oklobdzija: High-Performance System Design
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CMOS Deficiencies and Remedies
XNOR
Faster, one-level
realizations of XOR
function
A
AB
+
AB
AB
+
AB
B
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Prof. V.G. Oklobdzija: High-Performance System Design
12
CMOS Basic
Inverter Transfer
function:
VDD
VOH
Logic voltage levels are VOH VIH
and VOL
and VIL and VIH
Vout
VIL
The inverter transfer
function lie
within the shaded region
VOL
0
VOL
VIL
VIH
VOH
VDD
Vin
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Prof. V.G. Oklobdzija: High-Performance System Design
13
CMOS Basic: Inverter Characteristic
p “ON”
n “OFF’’
BOTH p & n “ON”
A
B
VDD
0.5VDD
C
D
0
0
Fall 2004
p ”OFF”
n ”ON”
Vtn
0.5VDD
Leakage
Currents
E
VDD + Vtp
VDD
Prof. V.G. Oklobdzija: High-Performance System Design
14
CMOS Basic: Inverter Characteristic
VDD
VDD
T2
V0(t)
Vin(t)
T1
t
CL
+ VDD
0
t
+ VDD
0.9 VDD
0.1 VDD
t
td
tf
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tr
Prof. V.G. Oklobdzija: High-Performance System Design
15
CMOS Basic: Inverter Characteristic
Transistors during the transition
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Prof. V.G. Oklobdzija: High-Performance System Design
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CMOS Basic: Inverter Switching
VDD
p-DEVICE
VDD
p-DEVICE
t=0
Ic
Ic
CL
R
c
VO
n-DEVICE
CL
VO
n-DEVICE
SATURATION :
VO  VDD  Vin
VDD
p-DEVICE
LINEAR :
0  VO  VDD  Vin
VDD
p-DEVICE
R
c
Ic
Ic
t=0
CL
n-DEVICE
SATURATION
Fall 2004
VO
CL
n-DEVICE
VO
LINEAR
Prof. V.G. Oklobdzija: High-Performance System Design
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CMOS Basic: Power
tp
• During the static
state there is no
current
VDD
V
0
tf
• Current is only
present during
transistion:
- Short circuit current
(crow-bar current)
- Charging and
discharging of the
output capacitor
- Leakage Current
Fall 2004
tr
t
VDD
V
0
t
VDD
I
0
Prof. V.G. Oklobdzija: High-Performance System Design
t
18
CMOS Basic: Power
PCMOS=kCLV2DDfo
This is an E=mc2 of low-power design
There are three ways to control power:
- Reducing Power-Supply Voltage (most effective
!!)
- Reducing the switching activity
- Reducing
k (various ways)
CL (technology scaling etc.)
- Reducing the required frequency of operation (?)
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Prof. V.G. Oklobdzija: High-Performance System Design
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CMOS Basic: Delay
• Which one of the three designs is the fastest ?
• How can we find this out without simulation ?
a7
Case 1
CL
a0
(a)
a7
Case 3
a7
a6
a5
a4
a3
a2
a1
CL
a0
(c)
Case 2
a4
CL
a3
a0
(b)
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Learn about Logical Effort !
Prof. V.G. Oklobdzija: High-Performance System Design
20
CMOS Basic: Delay
Charge:
Ic
Cin1
Discharge
Id
Fall 2004
Cout
Cin2
Discharge
Id
Prof. V.G. Oklobdzija: High-Performance System Design
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CMOS Basic: Delay
RNOR
Ic
Id
RND7
Id
Cin1
Cin2
RND2
Cout
Delay can be approximated with:
RND7Cin1+RNORCin2+RND2Cout
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Prof. V.G. Oklobdzija: High-Performance System Design
22
CMOS Basic: Delay
Delay of a signal path in CMOS logic is
dependent on:
• Fan-in of a gate
– Represented as a resistance of the pullup/down transistor path of the gate
• Fan-out of a gate
– Represented as a capacitive load at the
output
• Number of CMOS blocks in the path.
• Wire delay connecting various blocks.
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Prof. V.G. Oklobdzija: High-Performance System Design
23
CMOS Basic: Delay
Delay of a signal path in CMOS logic can be
reduced by:
• Making the transistors larger in order to
minimize resistance of a pull-up/down path in
the gate
• Making the transistors smaller in order to
minimize the capacitive load of each gate
• Reducing the number of CMOS blocks in the
path.
• Bringing the blocks closer and/or choosing the
less wire intensive topology.
– Note that these requirements are often
contradictory
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Prof. V.G. Oklobdzija: High-Performance System Design
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CMOS Basic: Delay
• How to estimate delay and critical
timing in CMOS circuits ?
• How to determine the proper
transistor sizing in order to make a
compromise with contradicting
requirements ?
• How to choose the right circuit
topology ?
The Answer:
“Logical Effort”
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Prof. V.G. Oklobdzija: High-Performance System Design
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Pass-Transistor Design
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Prof. V.G. Oklobdzija: High-Performance System Design
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Pass-Transistor Design
Another way of looking at Karnaugh Map: AND function
A
A
B
F
0
1
0
0
1
1
1
0
F
B
B
A
Fall 2004
B
Prof. V.G. Oklobdzija: High-Performance System Design
B
27
Pass-Transistor Design
A
X
F
Y
A
Two-variable function
Fall 2004
X
0
0
1
1
0
0
1
1
B
B
B
B
B
B
B
B
Y
0
1
0
1
B
B
B
B
0
1
0
1
B
B
F
0
A
A
1
AB
AB
AB
AB
AB
AB
A+B
A B
B
B
A B
A B
B
B
Prof. V.G. Oklobdzija: High-Performance System Design
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Pass-Transistor Design
“Threshold Voltage Drop” problem:
A=Vdd
+ V
th
-
B=Vdd
Fmax = Vdd-Vth
B
Vdd
Vdd
Vdd
+ V V
+
th
th
--
Fmax = Vdd-Vth
Cout
Cout
A
(a)
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(b)
Prof. V.G. Oklobdzija: High-Performance System Design
29
Pass-Transistor Design
Solving the “Threshold Voltage Drop” problem in CMOS:
+Vdd
A=Vdd
+ V
th
Vdd
+ V
th
Fmax= Vdd
In=Vdd
ON
Vdd
-
+
Cout
Cin
Vdd A=0V
(a)
Fall 2004
(b)
Prof. V.G. Oklobdzija: High-Performance System Design
30
Pass-Transistor Design
A
A
B
B
P0
P1
F(A,B)
P2
P3
Fall 2004
Function Generator
Prof. V.G. Oklobdzija: High-Performance System Design
31
Pass-Transistor Design
Full 1-bit
Adder
A
S
A
A
S
CO
A
A
CO
B
Fall 2004
B
C
C
Prof. V.G. Oklobdzija: High-Performance System Design
32
Pass-Transistor Design
Compact
ALU
Example
(IBM
PC/RT)
Circ. 1984
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Prof. V.G. Oklobdzija: High-Performance System Design
33
Control Lines
A - inputs
Odd
Operation
Output
Control
B - inputs
Even
Odd
A
Even
K1
K2
Qn
A
B
B
Odd
Even
0
0
0
0
1
1
0
0
1
1
0
0
1
0
0
1
0
1
1
0
0
1
1
0
0
1
Arithmetic
A+B
Add
A+B+1
A-B
Subtract
0
0
1
0
1
1
0
1
0
0
1
0
1
B-A
Subtract
0
0
1
1
0
0
1
0
1
1
0
0
1
B+1
Increment
0
0
1
1
1
0
0
0
1
1
0
0
1
2s compl
0
0
1
1
1
0
0
1
0
0
1
0
1
Increment
0
0
1
0
1
1
0
1
1
0
0
0
1
2s compl
0
0
1
1
0
0
1
1
1
0
0
0
1
+1
A+1
+1
Logical
1
1
1
0
0
0
0
0
0
0
0
0
0
0
B
1
1
0
0
0
0
0
0
1
0
1
0
0
1
1
0
0
0
0
0
1
0
1
0
0
0
1
1
0
0
1
0
1
0
1
0
1
0
0
1
1
0
0
1
0
1
1
0
1
0
0
0
1
1
0
1
0
1
0
0
0
0
0
0
0
1
1
0
1
0
1
0
0
1
0
1
0
0
1
1
0
0
0
0
0
0
0
0
0
1
1
1
1
0
1
0
1
0
1
0
1
0
1
1
1
1
0
1
0
1
0
0
0
0
0
1
1
1
1
0
0
1
0
1
0
1
0
1
1
1
1
1
0
0
1
0
1
1
0
1
0
1
1
1
1
0
1
0
1
0
0
1
0
1
1
1
0
1
0
0
1
0
1
0
1
0
1
1
1
0
1
0
0
1
0
1
1
0
1
0
1
1
0
1
0
1
0
1
0
0
1
0
1
1
1
0
A
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Prof. V.G. Oklobdzija: High-Performance System Design
34
Pass-Transistor
Design
Function Generator
VH
f
A
Compact ALU
Example
(IBM PC/RT)
B
A
B
K2
CO
K2
VH
CO
CI
K2
A
K1
A
B
B
Carry Generator
Fall 2004
Prof. V.G. Oklobdzija: High-Performance System Design
35
Using Pass-Transistor Design to Speedup Addition
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Prof. V.G. Oklobdzija: High-Performance System Design
36