Circuit Design Techniques for Low Power DSPs

1. Circuit Design Techniques for Low Power DSPs Simone Gambini Marghoob Mohiyuddin Melinda Ler

2. Motivation • Energy Per Operation (EOP) important • For energy-constrained systems, e.g., battery-powered devices • Supply voltage scaling can be used to reduce energy consumption • Leakage limits scaling to above a certain supply voltage • Conventional techniques for low power/energy design may not be beneficial • Low power designs also have performance constraints (apart from power) • Design should meet throughput constraints while minimizing energy consumption • Want to explore tradeoffs in design given these constraints Free Template from www.brainybetty.com

3. Problem Statement • Study the impact of low power design techniques for different circuits with performance constraints • Effects of process variations and temperature • Supply voltage scaling • Using parallelism to reduce power • Architectural approaches • Multiplier designs used as case studies • Fundamental block in many DSP systems Free Template from www.brainybetty.com

4. Prior Work • If energy per operation (EOP) used as an optimization metric then an optimal choice of Vdd exists [1] • Technology, micro-architecture and architecture affect the EOP • Minimum EOP point at EOP optimal Vdd shown to shift with different micro-architectures [2] • Performance constraint not taken into account Free Template from www.brainybetty.com

5. Project Outline • Technology characterization • Simulated leakage and delay for 90nm technology node across process corners and temperature • Modeling EOP • Using simulated delay and leakage data for FO1 and FO4 ring oscillators • Extrapolated to get predictions for EOP behavior for different micro-architectures • Characterizing test circuits to validate our predictions • Delay validation • Power validation Free Template from www.brainybetty.com

6. Technology Characterization: Process and Temperature Variation Effects • EOP Model • Variations in EOP behavior for SVT and LVT across corners • Variations of minimal EOP with temperature Free Template from www.brainybetty.com

7. Tradeoff exists between LVT and HVT process options for different operating frequencies with intermediate switching activities Technology Characterization: Process Options Free Template from www.brainybetty.com

8. Pipelining For constant throughput, pipelining allows for lower energy per operation at lower supply voltages Parallelism Due to overhead in hardware and increase in logic delay, parallel structures increases minimal energy per operation Parallelism and Pipelining Free Template from www.brainybetty.com

9. Case Study: Multipliers • Investigated multiple architectures • Wallace tree multiplier • Array multiplier • Serial multiplier • Impact of parallelism on EOP behavior • Technology characterization predicts that parallelism increases EOP for low supply voltages • Leakage becomes the dominating factor at low supply • Higher leakage factor means optimal Vdd should increase with parallelism Free Template from www.brainybetty.com

10. Flow for EOP estimation • Circuit synthesis (Module Compiler) • Extraction of activity factor (ModelSim) • Correlated input vectors generated with Matlab • Gates annotated with activity factors and capacitances • Delay simulation for critical paths (Spectre) • Over multiple Vdds • Power estimation (PowerPrime) • Dynamic and leakage power for single supply voltage • Power scaling with Vdd extrapolated using scaling factors from FO4 inverter chain Free Template from www.brainybetty.com

11. Simulated EOPs for Multipliers • Parallelism yields energy benefits only above a certain Vdd • Tradeoff between different architectures at different bitwidths • Wallace tree is better than carry save for higher bitwidths • Architectural decisions affect EOP strongly Free Template from www.brainybetty.com

12. Delay Validation • Simulated critical path delays vs. extrapolated delays from inverter chain • Delay scales proportionally to inverter delay Free Template from www.brainybetty.com

13. Leakage power for the critical paths for a specific input vector Leakage power for a NAND4 gate with different input vectors A large variation in the leakage power over input vectors Total leakage power depends on the statistical distribution of input vectors over time Leakage does not scale the same way as inverter leakage Leakage Validation Free Template from www.brainybetty.com

14. Leakage Validation • Top figure shows leakage current for different input vectors • Bottom figure shows leakage current for different input vectors weighted by the probability of the input vectors Free Template from www.brainybetty.com

15. Leakage Validation • Sensitivity of the optimal Vdd point with respect to the leakage energy • For sub-threshold operation, the optimal Vdd is not very sensitive to leakage energy [2] Free Template from www.brainybetty.com

16. Conclusions & Future Work • For low power systems operating below a certain operating frequency, parallelism would not be the ideal option due to leakage • This trend is expected to be reinforced in the future technology nodes • Fast and accurate leakage estimation tools needed • Should take into account the state-dependent behavior of leakage Free Template from www.brainybetty.com

17. References [1] D. Markovic, V. Stojanovic, B. Nikolic, M. A. Horowitz, and R. W. Brodersen, “Methods for True Energy-Performance Optimization,” IEEE Journal of Solid-State Circuits, vol. 39, pp. 1282–1293, Aug. 2004 [2] B. H. Calhoun and A. Chandrakasan, “Characterization and Modeling of Minimum Energy Per Operation Point,” in Proc. IEEE International Symposium on Low Power Electronics and Design, Newport Beach, California, Aug. 2004, pp. 90–95. [3] A. Wang and A. Chandrakasan, “A 180mV FFT Processor Using Subthreshold Circuit Techniques,” in Proc. IEEE International Solid-State Circuits Conference, Feb. 2004. [4] Y. Taur and T. H. Ning, Fundamentals of Modern VLSI Devices. Cambridge University Press, 1999. Free Template from www.brainybetty.com