Power Issues in

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Power Issues in Embedded Systems
Wong Weng Fai
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Outline

• The Big Picture
• Whos got the Power?
• Whats in the bag of tricks?

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The Big Picture

• Phenomenal increase in processor speed
• 3GHz Pentium 4 by the end of the year
• Shrinkage in size
• Mobility highly desired
• BUT battery technology not improving at the same
  rate

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Batteries Not Included

• Nickel-based batteries
• Nickel-Iron
• The first rechargeable, old technology
• Nickel-cadmium and Nickel-Metal-Hydride
• High energy density good for motors
• Lithium-based batteries
• Promising because lithium releases electrons
  easily
• Problem with battery life, dangerous to handle
• Others
• Zinc-air batteries can work a laptop for 10
  hours

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Some Terminologies

• Power is the rate of energy consumption
• Power ? energy
• Energy depends on how long you run the thing!
• Optimizing for speed optimizing for energy?
• Some researchers look at average power

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Back to Basics
Gate oxide insulator
Gate
?
?
N source
N drain
?
?
?
?
?
?
?
?
P- substrate
?
N-Channel Metallic Oxide Semiconductor Field
Effect Transistor
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Back to Basics ACTION!
? ? ? ? ? ?
Gate oxide insulator
-

Gate
? ? ? ? ? ? ? ? ? ?

? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
N source
N drain
?
?
-
? ? ?
?
?
?
?
?
?
?
P- substrate
?
N-Channel Metallic Oxide Semiconductor Field
Effect Transistor
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CMOS
VDD
P-channel MOSFET
Input 0 0V 1 5V
Output
CMOS Inverter
N-channel MOSFET
GND
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CMOS
VDD
P-channel MOSFET
Input 0 0V
Output 0
CMOS Inverter
N-channel MOSFET
GND
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CMOS
VDD
P-channel MOSFET
Input 1 5V
Output 1
CMOS Inverter
N-channel MOSFET
GND
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Power in CMOS
P total power VDD supply voltage f clock
frequency N switching (gate transition per
clock cycle) Ileak leakage power
Istatic static power QSC quantity of charge
carried by short-circuit current per
transistion
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Power in CMOS
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Switching Power

• Accounts for most (90) of power
• Two major factor is supply voltage and frequency
• Voltage scaling
• Frequency scaling

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Short Circuit Power

• During switching, there is a short period of time
  when both gates are ON
• ? a path from VDD to ground
• ? power dissipation

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Leakage Power

• Diode leakage
• Source (and drain) together with substrate forms
  a diode
• At times, this diode can be reverse-biased during
  which current can leak
• Sub-threshold leakage
• Even when gate is not completely on, enough of a
  channel can form for some movement of charges
  from source to drain

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Static Power

• Reduced voltage feeding
• Both gates can be weakly on
• Weak current flow from VDD to ground
• Other parasitic current flows
• Due to imperfect manufacturing or operating
  conditions

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A Digression The Problems Of Scaling down

• Latch-up effect
• Short-channel effect
• Punch-through effect
• Hot electron effect
• Gate erosion

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Latch-up Effect
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Tricks in the bag

• Voltage Scaling
• Frequency Scaling
• Power Gating

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Voltage Scaling

• Lower VDD
• For the same circuit and technology, this leads
  to higher gate delay
• Total delay, ?, is made up of two components, ?
  ?1 ?2
• ?1 is a constant
• ?2 ? VDD

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Frequency Scaling

• Widely used in many processors
• Intel SpeedStep on mobile processors
• Leads to lower performance
• Obvious!

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Power Gating

• Turn off power to parts of the circuit
• Can be problematic for circuits with memory

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What About Memory?

• SRAM
• Implemented using CMOS
• DRAM
• Entirely different technology
• Implemented using capacitors

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SRAM
CMOS SRAM Cell
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DRAM
Single Transistor DRAM cell
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Model or Measure?

• Hardware measurement
• Measures the amount of current consumed
• Depends on how the circuit is designed
• Cannot get core CPU power breakdowns

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Software Estimation

• SPICE simulation
• Very slow
• PowerMill from Synopsys
• CAD Tools
• Part of a lot of CAD tool chains, eg. Synopsys
• Architectural based simulation
• Eg SimplePower, WATTCH etc.

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Putting it Together System Power

• Reference
• Marc A. Viredaz and Deborah A. Wallach, Power
  Evaluation of a Handheld Computer A Case Study.
  Compaq Western Research Lab Technical Report
  2001/1. May 2001.
• http//research.compaq.com/wrl/techreports/abstrac
  ts/2001.1.html

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Dealing with it

• System / OS
• Algorithms
• Architecture
• Circuit/Logic
• Technology

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Technology

• Low threshold, low voltage
• Various technological issues as discussed

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Circuit/Logic

• Even within CMOS, there are different types of
  logic families that consumes different amount of
  energy
• Transistor size
• Layout
• Asynchronous circuits
• Clocking consumes a lot of power
• Pipeline retiming

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Architecture / Compiler

• Trade off area for power

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Architecture / Compiler

• Trade off area for power
• Shorter wires less power
• Parallelism and concurrency
• Directives to allow compiler to do
• Voltage scaling
• Frequency scaling
• Power gating
• One more degree of freedom activity

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Algorithms

• Low power algorithms
• Parallelism and concurrency
• A under-research area

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System / OS

• System level power management
• Heuristics for transiting between various power
  modes
• Operating environment sensitive power management
• Battery or plugged-in?
• Power-domain specific management schemes

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Reducing Processor Power

• Energy conscious code generation
• Reduce switching
• Instruction scheduling
• Use of Gray code instead of binary
• Low power modes
• Instruction compression
• Parallelism and concurrency

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Reducing Memory Power

• Reduce memory accesses
• All compiler techniques for reducing cache misses
• Use registers
• Memory reference compaction
• Power aware page allocation
• Group active pages together

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Reducing Peripheral Power

• Communication
• Different power modes for communicating devices
• Data compression
• Adaptation in view of traffic and power
• Disk
• Spin-down and different power modes (when?)
• Display

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Summary

• Some research opportunities still exist
• Especially in algorithms and operating systems
• An integrated approach is needed
• All levels of the system cooperating with one
  another