Micro-Architecture Techniques for Sensor Network Processors

Amir JavidiEECS 598

Feb 25, 2010

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Motivation

• Low performance tasks• Long duration• Small energy supplies

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Papers• [1] L. Nazhandali, B. Zhai, J. Olson, A. Reeves, M. Minuth, R.

Helfand, S. Pant, T. Austin, and D. Blaauw, “Energy optimization of subthreshold voltage sensor network processors,” in Proc. Int. Symp. Computer Architecture, 2005, pp. 197–207.

• [2] S. Hanson, M. Seok, Y-S. Lin, Z. Foo, D. Kim, Y. Lee, N. Liu, D. Sylvester, and D. Blaauw, “A low voltage processor for sensing applications with picowatt standby mode,” IEEE Journal of Solid-State Circuits, pp. 1145-1155, April 2009.

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Energy Budget

• 2g Vanadium oxide battery: 720 mAh• Powers ARM 720T processor at 100MHz for

45hrs

• Thin film zinc/silver oxide battery: 100 μAh/cm2, 1.55 V• For area of 1mm2 average current must be 114pA

(power consumption of 177 pW) for 1 year lifetime

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Performance Requirement

• Blood pressure monitoring (low rate):• Sensing @ 800 bps 10,000 inst/sec

• EEG brain signal monitoring (high rate):• 3200 bps 56,000 inst/sec for filtering,

analysis, compression, and storage

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Architecture/Circuit Techniques• Sub-threshold implementation (Vdd < Vth)

• ISA optimization• Voltage scaling• Power gating• Stack forcing• Data/instruction compression

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Subthreshold design

• Why subthreshold?• Processor operating in lowest super-threshold

voltages deliver too much performance

Performance of sensor network processor applications on embedded targets. Number of times faster than real-time the processor can handle the worst case data stream rate [1].

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Subthreshold Circuit Design

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Subthreshold Energy Optimization

21 ( )2

dd

inst cycle cycle s dd dd leak clk

kVclk sub

E E CPI E C V V I t

t e

SubthresholdEnergy as a function of Voltage[1]

Vmin

energy optimal supply voltage

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ISA Optimization

• Why ISA optimization?• Memory dissipates static/dynamic energy• Memory size leakage• Tradeoff between

memory size and control logic size

Logic Vs memory energy tradeoff [1]

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ISA Optimization

• Impact of ISA optimization on code size and control logic complexity

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Micro-Architecture

• Sensor network processor micro-architecture

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Performance vs. Energy

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Results

• Sensor network processor• ROM/RAM memory• 8 bit data path• 235 mV supply• 182 KHz• 1.38 pJ/inst• 4.1x faster than necessary for mid-bandwidth• 25 years lifetime with 2g vanadium oxide battery

(720 mAh)

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Phoenix Processor

• Focus on lowering standby power• Older 0.18μm technology• Custom leakage-optimized instruction set• Simple data memory compression• Ultra-low-leakage memory cell• Huge tradeoff between standby power and

area and active energy

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Phoenix Processor

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CMOS Technology

• Newer technology (65 nm)• High subthreshold leakage• Small capacitance

• Older technology (180 nm)• 7.7x larger• 647x less total energy consumption

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Voltage Scaling

• Supply voltage of 0.5 V• Mix of subthreshold and near-subthreshold

devices

• Retentive gates high-Vth ~ 0.7 V

• Non-retentive gates medium-Vth ~ 0.5 V

• High-Vth consumes ~ 1000x less leakage power

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Power Gating

• Medium-Vth power switch• Smaller switch ~ 1000x

• Less area overhead • Less charging/discharging power overhead

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CPU Architecture

• 2 stage, 8bit data width, 10bit inst. Width• ALU (add, subtract, shift)• No multiplier• Simple decoder (min set of operations)

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ISA Optimization

• Minimized instruction width (10 bit)• Reduces IMEM standby power dissipation• Efficient operand encoding

• Explicit operand: more flexibility, more frequently used• Implicit operand: less flexibility, less frequently used

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Memory Design

• 64x10b SRAM (IMEM)• Application specific instructions• No power gating

• 64x10b ROM (IROM)• Commonly used instructions• Power gated

• 52x40b SRAM (DMEM) • Data compression• Fine grain power gating

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Memory Design

• Leakage reduction• High-Vth bitcell transistors• Cross coupled inverters:

• Stacked transistors• Increased length

• (0.35μm to 0.50μm)~2x leakage reduction

• Robustness• Full swing read-buffer• Power gated

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Results

• Phoenix processor• 0.5 V power supply• 106 KHz• 2.8 pJ/cycle 297 nW• 226 nW active mode• 35.4 pW standby mode• 915 x 915 μm2

Questions?