1. 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
Copyright Agrawal, 2009
ELEC6270/5270 Spring 09, Lecture 3

2. Course Objectives Low-power is a current need in VLSI design.
Learn basic ideas, concepts, theory and methods.
Gain experience with techniques and tools.
Copyright Agrawal, 2009
ELEC6270/5270 Spring 09, Lecture 3

3. Student Evaluation Homeworks (25%) ~ Four Class Project (25%)
Class Test (25%)
Final Exam (25%): Thursday, May 7, 2009, 4:00 – 6:30PM, Broun 125.
Copyright Agrawal, 2009
ELEC6270/5270 Spring 09, Lecture 3

4. Power Consumption of VLSI Chips
Why is it a concern?
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5. 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.
“Unfortunately, if nothing changes these chips will produce as much heat, for their proportional size, as a nuclear reactor ”
Patrick P. Gelsinger
Senior Vice President General Manager
Digital Enterprise Group INTEL CORP.
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6. VLSI Chip Power Density
Sun’s
4004
8008
8080
8085
8086
286
386
486
Pentium® P6 1 10
100
1000
10000
1970
1980
1990
2000
2010
Year
Power Density (W/cm2)
Surface
Rocket
Nozzle
Nuclear
Reactor
Hot Plate
Source: Intel
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ELEC6270/5270 Spring 09, Lecture 3

7. 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
160
170
175
183
Source:
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8. Recent Data Source: http://www.eetimes.com/story/OEG20040123S0041
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9. 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
Copyright Agrawal, 2009
ELEC6270/5270 Spring 09, Lecture 3

10. VLSI Building Blocks Finite-state machine (FSM) Bus
Flip-flops and shift registers
Memories
Datapath
Processors
Analog circuits
RF components
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ELEC6270/5270 Spring 09, Lecture 3

11. Specific Topics in Low-Power
Power dissipation in CMOS circuits
Device technology
Low-power CMOS technologies
Energy recovery methods
Circuit and gate level methods
Logic synthesis
Dynamic power reduction techniques
Leakage power reduction
System level methods
Microprocessors
Arithmetic circuits
Low power memory technology
Test Power
Power estimation
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12. Some Examples Copyright Agrawal, 2009
ELEC6270/5270 Spring 09, Lecture 3

13. 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.
Copyright Agrawal, 2009
ELEC6270/5270 Spring 09, Lecture 3

14. Binary Counter: Original Encoding
Present state
Next state a b A B 1 a b A B CK
CLR
A = a’b + ab’
B = a’b’ + ab’
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15. Binary Counter: Gray Encoding
Present state
Next state a b A B 1 a b A B CK
CLR
A = a’b + ab
B = a’b’ + a’b
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ELEC6270/5270 Spring 09, Lecture 3

16. Three-Bit Counters Binary Gray-code State No. of toggles
000 -
001 1
010 2
011
100 3
110
101
111
Av. Transitions/clock = 1.75
Av. Transitions/clock = 1
Copyright Agrawal, 2009
ELEC6270/5270 Spring 09, Lecture 3

17. 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
1.0 6 4
0.6667 3 14 8
0.5714 30 16
0.5333 5 62 32
0.5161
126 64
0.5079 ∞ -
0.5000
Copyright Agrawal, 2009
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18. FSM State Encoding 2(0.3+0.4) + 1(0.1+0.1) = 1.6
Transition
probability
based on
PI statistics
0.6
0.6 11 01
0.3
0.3
0.1
0.1
0.4
0.4 00 01 00 11
0.1
0.1
0.9
0.9
0.6
0.6
Expected number of state-bit transitions:
2( ) + 1( ) = 1.6
1( ) + 2(0.1) = 1.0
State encoding can be selected using a power-based cost function.
Copyright Agrawal, 2009
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19. 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
Xk/Zk
Clock can be stopped
when (Xk, Sk) combination
occurs. Sj
Xj/Zk
Copyright Agrawal, 2009
ELEC6270/5270 Spring 09, Lecture 3

20. Clock-Gating in Moore FSM
Combinational
logic PI PO
Flip-flops
Clock
activation
logic
Latch
L. Benini and G. De Micheli,
Dynamic Power Management,
Boston: Springer, 1998. CK
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ELEC6270/5270 Spring 09, Lecture 3

21. 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: N
N/2
Number of bit transitions
after inversion encoding
N/2 N
Number of bit transitions
Copyright Agrawal, 2009
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22. Bus-Inversion Encoding Logic
Sent data
Received data
Polarity
decision
logic
Bus register
M. Stan and W. Burleson, “Bus-Invert Coding for Low Power I/O,” IEEE Trans. VLSI Systems, vol. 3, no. 1, pp , March 1995.
Polarity bit
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23. Clock-Gating in Low-Power Flip-FlopD
D Q CK
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24. Reduced-Power Shift Register
D Q
D Q
D Q
D Q
Output
multiplexer
D Q
D Q
D Q
D Q
CK(f/2)
Flip-flops are operated at full voltage and half the clock frequency.
Copyright Agrawal, 2009
ELEC6270/5270 Spring 09, Lecture 3

25. Power Consumption of Shift Register
16-bit shift register, 2μ CMOS
P = C’VDD2f/n
1.0
0.5
0.25
0.0
Deg. Of parallelism
Freq (MHz)
Power (μW) 1
33.0
1535 2
16.5
887 4
8.25
738
Normalized power
C. Piguet, “Circuit and Logic Level
Design,” pages in W. Nebel
and J. Mermet (ed.), Low Power
Design in Deep Submicron
Electronics, Springer, 1997.
Degree of parallelism, n
Copyright Agrawal, 2009
ELEC6270/5270 Spring 09, Lecture 3

26. Books on Low-Power Design (1)
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: Wiley-Interscience, 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.
Copyright Agrawal, 2009
ELEC6270/5270 Spring 09, Lecture 3

27. Books on Low-Power Design (2)
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: Wiley-Interscience, 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.
Copyright Agrawal, 2009
ELEC6270/5270 Spring 09, Lecture 3

28. Books Useful in Low-Power Design
A. Chandrakasan, W. J. Bowhill and F. Fox, Design of High-Performance 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.
Copyright Agrawal, 2009
ELEC6270/5270 Spring 09, Lecture 3

29. 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 [Hint: You should be able to solve this problem without the help of the reference.]
Copyright Agrawal, 2009
ELEC6270/5270 Spring 09, Lecture 3

30. Solution: Bus Encoding
Un-coded bus: Two consecutive bits on a wire can be 00, 01, 10 and 11, each with a probability 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).
Copyright Agrawal, 2009
ELEC6270/5270 Spring 09, Lecture 3

31. 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 1
2.0000 8
0.4981 2
1.5000 16
= 1/4 3
1.1670 32
1/8 4
0.9375 64
1/16
Copyright Agrawal, 2009
ELEC6270/5270 Spring 09, Lecture 3