1. Copyright Agrawal & Srivaths, 2007 Low-Power Design and Test, Lecture 1 1 Low-Power Design and Test Introduction Vishwani D. Agrawal Auburn University, USA vagrawal@eng.auburn.edu Srivaths Ravi Texas Instruments India Srivaths.ravi@ti.com Hyderabad, July 30-31, 2007 http://www.eng.auburn.edu/~vagrawal/hyd.html

2. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 12 Acknowledgments  The lecturers wish to thank Synopsys (India) and Sequence Design (India) for their help in preparation of demos for this course. Special thanks to Rahul Prasad [rprasad@sequencedesign.com] Visit http://www.sequencedesign.com/ for more informationhttp://www.sequencedesign.com/ Bhavesh Shah [Bhavesh.Shah@synopsys.com] Anantha Bhat [Anantha.Bhat@synopsys.com] Visit http://www.synopsys.com/ for more informationhttp://www.synopsys.com/

3. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 13 Course Objective  Low-power is a current need in VLSI design.  Learn basic ideas, concepts and methods.  Gain experience with CAD tools.

4. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 14 Course Schedule Day 1, Monday, July 30, 2007 9 – 10:30AMLecture 1Introduction (30*)VA 10:30 – 11AMCoffee break 11 – 12:30PMLecture 2Dynamic and static power in CMOS(39)VA 12:30 – 2PMLunch 2 – 3:30PMLecture 3Logic-level power estimation(56)VA 3:30 – 4PMCoffee break 4 – 5:30PMLecture 4High-level power analysis(55)SR Day 2, Tuesday, July 31, 2007 9 – 10:30AMLecture 5Gate-level power optimization (45)VA 10:30 – 11AMCoffee break 11 – 12:30PMLecture 6Memory and multicore design (40)VA 12:30 – 2PMLunch 2 – 3:30PMLecture 7High-level power reduction and management(50)SR 3:30 – 4PMCoffee break 4 – 5:30PMLecture 8Test power (35)SR * Number of slides

5. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 15 Introduction Why is it a concern? Power Consumption of VLSI Chips

6. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 16 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.

7. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 17 VLSI Chip Power Density 4004 8008 8080 8085 8086 286 386 486 Pentium® P6 1 10 100 1000 10000 19701980199020002010 Year Power Density (W/cm 2 ) Hot Plate Nuclear Reactor Rocket Nozzle Sun’s Surface Source: Intel 

8. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 18 SIA Roadmap for Processors (1999) Year199920022005200820112014 Feature size (nm) 180130100705035 Logic transistors/cm 2 6.2M18M39M84M180M390M Clock (GHz) 1.252.13.56.010.016.9 Chip size (mm 2 ) 340430520620750900 Power supply (V) 1.81.51.20.90.60.5 High-perf. Power (W) 90130160170175183 Source: http://www.semichips.orghttp://www.semichips.org

9. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 19 Recent Data Source: http://www.eetimes.com/story/OEG20040123S0041

10. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 110 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

11. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 111 VLSI Building Blocks  Finite-state machine (FMS)  Bus  Flip-flops and shift registers  Memories  Datapath  Processors

12. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 112 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.

13. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 113 Binary Counter: Original Encoding Present state Next state abAB 0001 0110 1011 1100 A = a’b + ab’ B = a’b’ + ab’ ABAB a b CK CLR

14. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 114 Binary Counter: Gray Encoding Present state Next state abAB 0001 0111 1000 1110 A = a’b + ab B = a’b’ + a’b ABAB a b CK CLR

15. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 115 Three-Bit Counters BinaryGray-code State No. of toggles State 000-000- 00110011 01020111 01110101 10031101 10111111 11021011 11111001 00030001 Av. Transitions/clock = 1.75 Av. Transitions/clock = 1

16. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 116 N-Bit Counter: Toggles in Counting Cycle  Binary counter: T(binary) = 2(2 N – 1)  Gray-code counter: T(gray) = 2 N  T(gray)/T(binary) = 2 N-1 /(2 N – 1) → 0.5 BitsT(binary)T(gray)T(gray)/T(binary) 1221.0 2640.6667 31480.5714 430160.5333 562320.5161 6126640.5079 ∞--0.5000

17. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 117 FSM State Encoding 11 0100 0.1 0.4 0.3 0.6 0.9 0.6 01 1100 0.1 0.4 0.3 0.6 0.9 0.6 Expected number of state-bit transitions: 1(0.3+0.4+0.1) + 2(0.1) = 1.0 Transition probability based on PI statistics State encoding can be selected using a power-based cost function. 2(0.3+0.4) + 1(0.1+0.1) = 1.6

18. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 118 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. Sj Si Sk Xi/Zk Xk/Zk Xj/Zk Clock can be stopped when (Xk, Sk) combination occurs.

19. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 119 Clock-Gating in Moore FSM Combinational logic Latch Clock activation logic Flip-flops PI CK PO L. Benini and G. De Micheli, Dynamic Power Management, Boston: Springer, 1998.

20. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 120 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 0 N/2N N N/2 0 Number of bit transitions after inversion encoding

21. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 121 Bus-Inversion Encoding Logic Polarity decision logic Sent data Received data 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.

22. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 122 Clock-Gating in Low-Power Flip-Flop D Q D CK

23. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 123 Reduced-Power Shift Register D Q D CK(f/2) multiplexer Output Flip-flops are operated at full voltage and half the clock frequency.

24. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 124 Power Consumption of Shift Register P = C’V DD 2 f/n Degree of parallelism, n 1 2 4 Normalized power 1.0 0.5 0.25 0.0 Deg. Of parallelism Freq (MHz) Power (μW) 133.01535 216.5887 48.25738 16-bit shift register, 2μ CMOS 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.

25. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 125 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.

26. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 126 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.

27. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 127 Other 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.  N. H. E. Weste and D. Harris, CMOS VLSI Design, Third Edition, Reading, Massachusetts, Addison-Wesley, 2005.  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.

28. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 128 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.]

29. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 129 Solution: Bus Encoding Un-coded bus: Two consecutive bits 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 2 n/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 ).

30. Copyright Agrawal & Srivaths, 2007Low-Power Design and Test, Lecture 130 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: n4(1 – 2 –n )/nn 12.000080.4981 21.5000160.2500 = 1/4 31.1670321/8 40.9375641/16