Transcript Lecture 3: Introduction to Low Power Design

ELEC 5270/6270 Spring 2009
Low-Power Design of Electronic Circuits
Introduction to Low Power Design
Vishwani D. Agrawal
James J. Danaher Professor
Dept. of Electrical and Computer Engineering
Auburn University, Auburn, AL 36849
[email protected]
http://www.eng.auburn.edu/~vagrawal/COURSE/E6270_Spr09/course.html
Copyright Agrawal, 2009
ELEC6270/5270 Spring 09, Lecture 3
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Course Objectives
Low-power is a current need in VLSI
design.
 Learn basic ideas, concepts, theory and
methods.
 Gain experience with techniques and tools.

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Student Evaluation
Homeworks (25%) ~ Four
 Class Project (25%)
 Class Test (25%)
 Final Exam (25%): Thursday, May 7,
2009, 4:00 – 6:30PM, Broun 125.

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Power Consumption of VLSI Chips
Why is it a concern?
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ISSCC, Feb. 2001, Keynote
“Ten years from now,
microprocessors will run at
10GHz to 30GHz and be capable
of processing 1 trillion operations
per second – about the same
number of calculations that the
world's fastest supercomputer
can perform now.
Patrick P. Gelsinger
Senior Vice President
General Manager
Digital Enterprise Group
INTEL CORP.
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“Unfortunately, if nothing
changes these chips will produce
as much heat, for their
proportional size, as a nuclear
reactor. . . .”
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VLSI Chip Power Density
Sun’s
Surface
Power Density (W/cm2)
10000
Rocket
Nozzle
1000
Nuclear
Reactor
100
8086
Hot Plate
10 4004
8008 8085
386
286
8080
1
1970
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1980
P6
Pentium®
486
1990
Year
Source: Intel
2000
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SIA Roadmap for Processors (1999)
Year
1999
2002
2005
2008
2011
2014
Feature size (nm)
180
130
100
70
50
35
Logic transistors/cm2
6.2M
18M
39M
84M
180M
390M
Clock (GHz)
1.25
2.1
3.5
6.0
10.0
16.9
Chip size (mm2)
340
430
520
620
750
900
Power supply (V)
1.8
1.5
1.2
0.9
0.6
0.5
High-perf. Power (W)
90
130
160
170
175
183
Source: http://www.semichips.org
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Recent Data
Source: http://www.eetimes.com/story/OEG20040123S0041
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Low-Power Design
 Design
practices that reduce power
consumption at least by one order of
magnitude; in practice 50% reduction
is often acceptable.
 Low-power design methods:
Algorithms and architectures
 High-level and software techniques
 Gate and circuit-level methods
 Test power

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VLSI Building Blocks
Finite-state machine (FSM)
 Bus
 Flip-flops and shift registers
 Memories
 Datapath
 Processors
 Analog circuits
 RF components

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Specific Topics in Low-Power


Power dissipation in CMOS circuits
Device technology



Circuit and gate level methods




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Logic synthesis
Dynamic power reduction techniques
Leakage power reduction
System level methods

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Low-power CMOS technologies
Energy recovery methods
Microprocessors
Arithmetic circuits
Low power memory technology
Test Power
Power estimation
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Some Examples
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State Encoding for a Counter

Two-bit binary counter:
 State
sequence, 00 → 01 → 10 → 11 → 00
 Six bit transitions in four clock cycles
 6/4 = 1.5 transitions per clock

Two-bit Gray-code counter
 State
sequence, 00 → 01 → 11 → 10 → 00
 Four bit transitions in four clock cycles
 4/4 = 1.0 transition per clock

Gray-code counter is more power efficient.
G. K. Yeap, Practical Low Power Digital VLSI Design, Boston:
Kluwer Academic Publishers (now Springer), 1998.
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Binary Counter: Original
Encoding
Present
state
a
0
0
1
1
a
Next state
b
0
1
0
1
A = a’b + ab’
B = a’b’ + ab’
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A
b
A
0
1
1
0
B
1
0
1
0
B
CK
CLR
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Binary Counter: Gray Encoding
Present
state
a
0
0
1
1
Next state
b
0
1
0
1
A = a’b + ab
B = a’b’ + a’b
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a
A
A
0
1
0
1
B
1
1
0
0
B
b
CK
CLR
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Three-Bit Counters
State
Binary
No. of toggles
Gray-code
State
No. of toggles
000
-
000
-
001
1
001
1
010
2
011
1
011
1
010
1
100
3
110
1
101
1
111
1
110
2
101
1
111
1
100
1
000
3
000
1
Av. Transitions/clock = 1.75
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Av. Transitions/clock = 1
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N-Bit Counter: Toggles in Counting Cycle



Binary counter: T(binary) = 2(2N – 1)
Gray-code counter: T(gray) = 2N
T(gray)/T(binary) = 2N-1/(2N – 1) → 0.5
Bits
T(binary)
T(gray)
T(gray)/T(binary)
1
2
2
1.0
2
6
4
0.6667
3
14
8
0.5714
4
30
16
0.5333
5
62
32
0.5161
6
126
64
0.5079
∞
-
-
0.5000
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FSM State Encoding
Transition
probability
based on
PI statistics
0.6
11
0.3
0.4
00
0.6
0.6
0.1
01
0.3
0.1
0.4
01
00
0.9
0.6
0.1
0.1
11
0.9
Expected number of state-bit transitions:
2(0.3+0.4) + 1(0.1+0.1) = 1.6
1(0.3+0.4+0.1) + 2(0.1) = 1.0
State encoding can be selected using a power-based cost function.
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FSM: Clock-Gating

Moore machine: Outputs depend only on
the state variables.

If a state has a self-loop in the state transition
graph (STG), then clock can be stopped
whenever a self-loop is to be executed.
Xi/Zk
Si
Sk
Sj
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Xj/Zk
Xk/Zk
Clock can be stopped
when (Xk, Sk) combination
occurs.
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Clock-Gating in Moore FSM
PI
Flip-flops
Combinational
logic
Clock
activation
logic
CK
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Latch
PO
L. Benini and G. De Micheli,
Dynamic Power Management,
Boston: Springer, 1998.
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Bus Encoding for Reduced Power

Example: Four bit bus
0000 → 1110 has three transitions.
 If bits of second pattern are inverted, then 0000 →
0001 will have only one transition.

Bit-inversion encoding for N-bit bus:
Number of bit transitions
after inversion encoding

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N
N/2
0
0
N/2
Number of bit transitions
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Sent data
Received data
Bus-Inversion Encoding Logic
Polarity
decision
logic
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Bus register
Polarity bit
M. Stan and W. Burleson, “Bus-Invert
Coding for Low Power I/O,” IEEE
Trans. VLSI Systems, vol. 3, no. 1,
pp. 49-58, March 1995.
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Clock-Gating in Low-Power Flip-Flop
D
D
Q
CK
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Reduced-Power Shift Register
D
Q
D
Q
D
Q
D
Q
multiplexer
D
D
Q
D
Q
D
Q
D
Output
Q
CK(f/2)
Flip-flops are operated at full voltage and half the clock frequency.
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Power Consumption of Shift Register
P = C’VDD2f/n
16-bit shift register, 2μ CMOS
Deg. Of
parallelism
Freq
(MHz)
Power
(μW)
1
33.0
1535
2
16.5
887
4
8.25
738
C. Piguet, “Circuit and Logic Level
Design,” pages 103-133 in W. Nebel
and J. Mermet (ed.), Low Power
Design in Deep Submicron
Electronics, Springer, 1997.
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Normalized power
1.0
0.5
0.25
0.0
1
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4
Degree of parallelism, n
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Books on Low-Power Design (1)
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L. Benini and G. De Micheli, Dynamic Power Management Design Techniques and
CAD Tools, Boston: Springer, 1998.
T. D. Burd and R. A. Brodersen, Energy Efficient Microprocessor Design, Boston:
Springer, 2002.
A. Chandrakasan and R. Brodersen, Low-Power Digital CMOS Design, Boston:
Springer, 1995.
A. Chandrakasan and R. Brodersen, Low-Power CMOS Design, New York: IEEE
Press, 1998.
J.-M. Chang and M. Pedram, Power Optimization and Synthesis at Behavioral
and System Levels using Formal Methods, Boston: Springer, 1999.
M. S. Elrabaa, I. S. Abu-Khater and M. I. Elmasry, Advanced Low-Power Digital
Circuit Techniques, Boston: Springer, 1997.
R. Graybill and R. Melhem, Power Aware Computing, New York: Plenum
Publishers, 2002.
S. Iman and M. Pedram, Logic Synthesis for Low Power VLSI Designs, Boston:
Springer, 1998.
J. B. Kuo and J.-H. Lou, Low-Voltage CMOS VLSI Circuits, New York: WileyInterscience, 1999.
J. Monteiro and S. Devadas, Computer-Aided Design Techniques for Low Power
Sequential Logic Circuits, Boston: Springer, 1997.
S. G. Narendra and A. Chandrakasan, Leakage in Nanometer CMOS
Technologies, Boston: Springer, 2005.
W. Nebel and J. Mermet, Low Power Design in Deep Submicron Electronics,
Boston: Springer, 1997.
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Books on Low-Power Design (2)
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N. Nicolici and B. M. Al-Hashimi, Power-Constrained Testing of VLSI Circuits,
Boston: Springer, 2003.
V. G. Oklobdzija, V. M. Stojanovic, D. M. Markovic and N. Nedovic, Digital System
Clocking: High Performance and Low-Power Aspects, Wiley-IEEE, 2005.
M. Pedram and J. M. Rabaey, Power Aware Design Methodologies, Boston:
Springer, 2002.
C. Piguet, Low-Power Electronics Design, Boca Raton: Florida: CRC Press, 2005.
J. M. Rabaey and M. Pedram, Low Power Design Methodologies, Boston:
Springer, 1996.
S. Roudy, P. K. Wright and J. M. Rabaey, Energy Scavenging for Wireless Sensor
Networks, Boston: Springer, 2003.
K. Roy and S. C. Prasad, Low-Power CMOS VLSI Circuit Design, New York: WileyInterscience, 2000.
E. Sánchez-Sinencio and A. G. Andreaou, Low-Voltage/Low-Power Integrated
Circuits and Systems – Low-Voltage Mixed-Signal Circuits, New York: IEEE
Press, 1999.
W. A. Serdijn, Low-Voltage Low-Power Analog Integrated Circuits,
Boston:Springer, 1995.
S. Sheng and R. W. Brodersen, Low-Power Wireless Communications: A
Wideband CDMA System Design, Boston: Springer, 1998.
G. Verghese and J. M. Rabaey, Low-Energy FPGAs, Boston: springer, 2001.
G. K. Yeap, Practical Low Power Digital VLSI Design, Boston:Springer, 1998.
K.-S. Yeo and K. Roy, Low-Voltage Low-Power Subsystems, McGraw Hill, 2004.
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Books Useful in Low-Power Design
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A. Chandrakasan, W. J. Bowhill and F. Fox, Design of HighPerformance Microprocessor Circuits, New York: IEEE Press,
2001.
R. C. Jaeger and T. N. Blalock, Microelectronic Circuit Design,
Third Edition, McGraw-Hill, 2006.
S. M. Kang and Y. Leblebici, CMOS Digital Integrated Circuits,
New York: McGraw-Hill, 1996.
E. Larsson, Introduction to Advanced System-on-Chip Test
Design and Optimization, Springer, 2005.
J. M. Rabaey, A. Chandrakasan and B. Nikolić, Digital Integrated
Circuits, Second Edition, Upper Saddle River, New Jersey:
Prentice-Hall, 2003.
J. Segura and C. F. Hawkins, CMOS Electronics, How It Works,
How It Fails, New York: IEEE Press, 2004.
N. H. E. Weste and D. Harris, CMOS VLSI Design, Third Edition,
Reading, Massachusetts: Addison-Wesley, 2005.
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Problem: Bus Encoding
A 1-hot encoding is to be used for reducing the capacitive power
consumption of an n-bit data bus. All n bits are assumed to be
independent and random. Derive a formula for the ratio of power
consumptions on the encoded and the un-coded buses. Show that
n ≥ 4 is essential for the 1-hot encoding to be beneficial.
Reference: A. P. Chandrakasan and R. W. Brodersen, Low Power
Digital CMOS Design, Boston: Kluwer Academic Publishers, 1995,
pp. 224-225. [Hint: You should be able to solve this problem
without the help of the reference.]
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Solution: Bus Encoding
Un-coded bus: Two consecutive bits on a wire can be 00, 01, 10 and
11, each with a probability 0.25. Considering only the 01 transition,
which draws energy from the supply, the probability of a data pattern
consuming CV 2 energy on a wire is ¼. Therefore, the average per
pattern energy for all n wires of the bus is CV 2n/4.
Encoded bus: Encoded bus contains 2n wires. The 1-hot encoding
ensures that whenever there is a change in the data pattern, exactly
one wire will have a 01 transition, charging its capacitance and
consuming CV 2 energy. There can be 2n possible data patterns and
exactly one of these will match the previous pattern and consume no
energy. Thus, the per pattern energy consumption of the bus is 0 with
probability 2–n, and CV 2 with probability 1 – 2–n. The average per
pattern energy for the 1-hot encoded bus is CV 2(1 – 2–n).
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Solution: Bus Encoding (Cont.)
Power ratio
=
Encoded bus power / un-coded bus power
=
4(1 – 2–n)/n → 4/n for large n
For the encoding to be beneficial, the above power ratio should be
less than 1. That is, 4(1 – 2–n)/n ≤ 1, or 1 – 2–n ≤ n/4, or n/4 ≥ 1
(approximately) → n ≥ 4.
The following table shows 1-hot encoded bus power ratio as a
function of bus width:
n
4(1 – 2–n)/n
n
4(1 – 2–n)/n
1
2.0000
8
0.4981
2
1.5000
16
0.2500 = 1/4
3
1.1670
32
1/8
4
0.9375
64
1/16
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