[Show abstract][Hide abstract] ABSTRACT: Subthreshold circuits have drawn a strong interest in recent ultralow power research. In this paper, we present a highly efficient subthreshold microprocessor targeting sensor application. It is optimized across different design stages including ISA definition, microarchitecture evaluation and circuit and implementation optimization. Our investigation concludes that microarchitectural decisions in the subthreshold regime differ significantly from that in conventional superthreshold mode. We propose a new general-purpose sensor processor architecture, which we call the Subliminal Processor. On the circuit side, subthreshold operation is known to exhibit an optimal energy point (Knin)- However, propagation delay also becomes more sensitive to process variation and can reduce the energy scaling gain. We conduct thorough analysis on how supply voltage and operating frequency impact energy efficiency in a statistical context. With careful library cell selection and robust static RAM design, the Subliminal Processor operates correctly down to 200 mV in a 0.13-mum technology, which is sufficiently low to operate at V<sub>min</sub> . Silicon measurements of the Subliminal Processor show a maximum energy efficiency of 2.6 pJ/instruction at 360 mV supply voltage and 833 kHz operating frequency. Finally, we examine the variation in frequency and V<sub>min</sub> across die to verify our analysis of adaptive tuning of the clock frequency and V<sub>min</sub> for optimal energy efficiency.
Full-text · Article · Sep 2009 · IEEE Transactions on Very Large Scale Integration (VLSI) Systems
[Show abstract][Hide abstract] ABSTRACT: In this study, we explore the design of a subthreshold processor for use in ultra-low-energy sensor systems. We describe an 8-bit subthreshold processor that has been designed with energy efficiency as the primary constraint. The processor, which is functional below V<sub>dd</sub>=200 mV, consumes only 3.5 pJ/inst at V<sub>dd</sub>=350 mV and, under a reverse body bias, draws only 11 nW at V<sub>dd</sub>=160 mV. Process and temperature variations in subthreshold circuits can cause dramatic fluctuations in performance and energy consumption and can lead to robustness problems. We investigate the use of body biasing to adapt to process and temperature variations. Test-chip measurements show that body biasing is particularly effective in subthreshold circuits and can eliminate performance variations with minimal energy penalties. Reduced performance is also problematic at low voltages, so we investigate global and local techniques for improving performance while maintaining energy efficiency.
Full-text · Article · May 2008 · IEEE Journal of Solid-State Circuits
[Show abstract][Hide abstract] ABSTRACT: A robust, energy efficient subthreshold (sub-V<sub>th</sub>) processor has been designed and tested in a 0.13 mum technology. The processor consumes 11 nW at V<sub>dd</sub> = 160 mV and 3.5 pJ/inst at V<sub>dd</sub> = 350 mV. Variability and performance optimization techniques are investigated for sub-V<sub>th</sub> circuits.
[Show abstract][Hide abstract] ABSTRACT: In this paper we present a second-generation sensor network processor which consumes less than one picoJoule per instruction (typical processors use 100's to 1000's of picoJoules per instruction). As in our first-generation design effort, we strive to build microarchitectures that minimize area to reduce leakage, maximize transistor utility to reduce the energy-optimal voltage, and optimize CPI for efficient processing. The new design builds on our previous work to develop a low-power subthreshold-voltage sensor processor, this time improving the design by focusing on ISA, memory system design, and microarchitectural optimizations that reduce overall design size and improve energy-per-instruction. The new design employs 8-bit datapaths and an ultra-compact 12-bit wide RISC instruction set architecture, which enables high code density via micro-operations and flexible operand modes. The design also features a unique memory architecture with prefetch buffer and predecoded address bits, which permits both faster access to the memory and smaller instructions due to few address bits. To achieve efficient processing, the design incorporates branch speculation and out-of-order execution, but in a simplified form for reduced area and leakage-energy overheads. Using SPICE-level timing and power simulation, we find that these optimizations produce a number of Pareto-optimal designs with varied performance-energy tradeoffs. Our most efficient design is capable of running at 142 kHz (0.1 MIPS) while consuming only 600 fJ/instruction, allowing the processor to run continuously for 41 years on the energy stored in a miniature 1g lithium-ion battery. Work is ongoing to incorporate this design into an intra-ocular pressure sensor.