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Micro-Architecture Techniques for Sensor Network Processors
Amir Javidi
EECS 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 ( )
2dd
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?