Power Optimization and Prediction Techniques for FPGAs by Jason Helge Anderson A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Department of Electrical and Computer Engineering University of Toronto c Copyright by Jason Helge Anderson 2005 Power Optimization and Prediction Techniques for FPGAs Jason Helge Anderson Doctor of Philosophy, 2005 Graduate Department of Electrical and Computer Engineering University of Toronto Abstract Field-programmable gate arrays (FPGAs) are a popular choice for digital circuit implementa- tion because of their growing density and speed, short design cycle, and steadily decreasing cost. Power consumption, specifically leakage power, has become a major concern for the semi- conductor industry and its customers. FPGAs are less power-efficient than custom ASICs, due to the overhead required to provide programmability. Despite this, power has been largely ig- nored by the FPGA research community, whose prime focus to date has centered on improving FPGA speed and area-efficiency. This dissertation presents new techniques for optimizing and predicting the power consumption of FPGAs. First, two novel computer-aided design (CAD) techniques for FPGA leakage power reduction are presented. The proposed techniques are unique in that they substantially reduce leakage power, while imposing no cost, meaning that they have no impact on FPGA area-efficiency, speed, or fabrication cost. Following this, the circuit-level design of low-power FPGA interconnect is considered. A family of new low-power FPGA routing switches is proposed. The switches significantly reduce iii dynamic and leakage power in the interconnect, with varying amounts of area and/or perfor- mance cost. The proposed switches require only minor changes to traditional FPGA routing switches, allowing them to be easily incorporated into current FPGAs. Next, a new power-aware technology mapping algorithm for look-up-table-based FPGAs is described. The algorithm takes an activity-conscious approach to logic replication, and allows trade-offs between circuit performance and power. The dynamic power of mapping solutions produced by the proposed algorithm is shown to be considerably less than competing techniques. Finally, the topic of early dynamic power estimation for FPGAs is addressed. Empirical models are developed for the prediction of interconnect capacitance and switching activity in FPGA designs. The proposed models can be applied early in the design process, when detailed routing data is incomplete or unavailable, thereby reducing design effort and cost. iv Acknowledgments Throughout my life, I have had great teachers and mentors. First, thank you to my advisor, Professor Farid Najm, for his guidance throughout my doctoral studies. He helped to set the direction for this research by encouraging me to pursue paths that initially were unfamiliar to me. His steady, solid, and rigorous approach to research is something I aim to practice in my own career. Many thanks to Professors Zhu, Rose, Chow, Sheikholeslami, and Tessier for reviewing and evaluating this work. I also acknowledge Profs. Brown and Rose for the significant and lasting roles they played in early stages of my research career. I am indebted to my mentors at Xilinx, who, during my four years in San Jose, helped me to develop professional and technical skills that enabled me to produce and succeed in graduate school. Thanks Rajeev, Sudip, Jim, and Sandor! Thanks also to the members of the Xilinx place and route team who seamlessly accepted a remote, part-time guy in Toronto. I am grateful to Xilinx for giving me the opportunity to continue to work part-time during my PhD studies, and for allowing us to demonstrate a portion of this work using Xilinx technology. Thanks to Tim Tuan of Xilinx Research Labs for being a great sounding board, and for his technical assistance. To all of my friends within Xilinx: thanks for the welcome diversions throughout my PhD. I have thoroughly enjoyed being in graduate school and it's not an experience I would trade for any other. Many thanks go to my fellow graduate students in the Computer Group and especially, my office mates in LP392, both current and former, in particular: Imad, Navid, Denis, Lesley, Andy, Paul, Tom, Peter, Anish, Ian, Aaron, Mehrdad, Vince, and Warren. Thanks to Kelly Chan for the dependable and conscientious administrative support. I wish to express my appreciation to the Natural Sciences and Engineering Research Council (NSERC) of Canada for the PGS scholarship, and to the Government of Ontario for the OGS and OGS-ST scholarships. Finally, thank you to my wife, Mary, for her love and support throughout our 8 years together, and for always encouraging me to do whatever I enjoy. v Acknowledgements vi Contents Acknowledgments v List of Figures xi List of Tables xv 1 Introduction 1 1.1 Field-Programmable Gate Arrays . 1 1.2 Motivation . 2 1.3 Thesis Contributions . 4 1.4 Thesis Organization . 7 2 Background and Related Work 9 2.1 Introduction . 9 2.2 Power Dissipation in CMOS Circuits . 9 2.2.1 Dynamic Power . 9 2.2.2 Static (Leakage) Power . 12 2.3 FPGA Architecture and Hardware Structures . 20 2.4 Power Dissipation in FPGAs . 26 2.4.1 Dynamic Power . 26 2.4.2 Static (Leakage) Power . 26 2.5 FPGA Power Optimization . 28 2.5.1 Leakage Power Optimization . 28 2.5.2 Dynamic Power Optimization (Architecture/Circuit Techniques) . 29 2.5.3 Dynamic Power Optimization (CAD Techniques) . 33 2.6 Summary . 38 vii Contents 3 CAD Techniques for Leakage Optimization 39 3.1 Introduction . 39 3.2 FPGA Hardware Structures . 40 3.3 Active Leakage Power Optimization via Polarity Selection . 45 3.3.1 Experimental Study and Results . 51 3.4 Active Leakage Power Optimization via Leakage-Aware Routing . 63 3.4.1 Experimental Study and Results . 65 3.5 Summary . 69 4 Circuit Techniques for Low-Power Interconnect 71 4.1 Introduction . 71 4.2 Preliminaries . 72 4.2.1 Related Work . 72 4.2.2 FPGA Interconnect Structures . 73 4.3 Low-Power Routing Switch Design . 74 4.4 Slack Analysis . 81 4.5 Experimental Study . 82 4.5.1 Methodology . 82 4.5.2 Leakage Power Results . 85 4.5.3 Dynamic Power Results . 92 4.5.4 Summary of Results . 94 4.6 Summary . 96 5 Power-Aware Technology Mapping 99 5.1 Introduction . 99 5.2 Preliminaries . 100 5.2.1 Power and Logic Replication . 101 5.3 Algorithm Description . 104 5.3.1 Generating K-Feasible Cuts . 104 5.3.2 Costing Cuts . 106 5.3.3 Mapping . 110 5.4 Experimental Study and Results . 110 5.4.1 Methodology . 110 5.4.2 Results . 113 5.5 Impact of Research . 118 viii Contents 5.6 Summary . 118 6 Power Prediction Techniques 121 6.1 Introduction . 121 6.2 Background . 123 6.2.1 Target FPGA Architecture . 123 6.2.2 Prediction Methodology Overview . 124 6.3 Switching Activity Prediction . 125 6.3.1 Activity Analysis . 126 6.3.2 Noise in Switching Activity . 129 6.3.3 Prediction Model . 130 6.3.4 Results and Discussion . 134 6.4 Interconnect Capacitance Prediction . 139 6.4.1 Related Work . 139 6.4.2 Noise in Interconnect Capacitance . 140 6.4.3 Prediction Model . 143 6.4.4 Results and Discussion . 146 6.5 Summary . 151 7 Conclusions 153 7.1 Summary and Contributions . 153 7.2 Future Work . 155 7.2.1 Extensions of this Research Work . 155 7.2.2 Additional Power-Related Research Directions . 157 7.3 Closing Remarks . 158 A Power Estimation Model Regression Analysis Results 159 A.1 Switching Activity Prediction . 159 A.2 Capacitance Prediction . 159 References 163 ix Contents x List of Figures 2.1 Equivalent circuit for a CMOS gate charging and discharging a capacitor [Yeap 98]. 10 2.2 Leakage mechanisms in an NMOS transistor. 13 2.3 Gate oxide leakage dependence on oxide thickness and gate bias [Thom 98]. 15 2.4 Scaling of gate length and supply voltage [Doyl 02]. 16 2.5 Scaling of VDD, VT H , and tox with process generation [Taur 02]. 17 2.6 Scaling of subthreshold leakage power density and dynamic power density [Nowa 02]. 18 2.7 ASIC leakage optimization techniques. 20 2.8 (a) Abstract FPGA architecture; (b) logic block; (c) LUT. 21 2.9 Logic blocks in Xilinx and Altera commercial FPGAs. 23 2.10 FPGA routing switch. 24 2.11 Multiplexers as deployed in FPGA routing switches and a LUT. 25 2.12 Dynamic power breakdown in Xilinx Virtex-II [Shan 02]. 27 2.13 Leakage power breakdown in Xilinx Spartan-3 [Tuan 03]. 28 2.14 Multiplexer leakage reduction techniques [Rahm 04]. 30 2.15 Dual-VDD FPGA structures. ..