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Low Power Solutions:
A System Design Perspective
Nik Sumikawa
Nik Sumikawa
Contents
1
Low Power: Why?
2
Standard Embedded Solutions
3
Innovative Solutions
4
Solutions for Mobile Platforms
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Low Power: Why?
Power vs. Performance
Technology Scaling
 VLSI
 Embedded
Technology Trend
 Green Stimulus
 Scaling Size
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What You Should Think About
Low power design strategies
 Components: Microcontrollers, peripherals,
ect.
 Low power design with hardware
 Low power design with software
 Low power design in mobile device
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Low Power Embedded Systems
TELOS:
 Low power wireless
embedded system
 Low duty cycle
principle
 Minimizes dynamic
power consumption
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Nik Sumikawa
Low Duty Cycle Principle
Timer or
Interrupt event
Process
Wake
Up
Sleep Mode
Deep
Sleep
Sleep
Prep
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Low Duty Cycle
Low processing to sleep ratio
 Extended sleep period
Responsively:
 fast wake-up and sleep times
Minimize Interrupts:
 Context switching overhead
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Low Duty Cycle: DMA
Direct Memory Access (DMA):
 Controls bus and transfers data with minimal
processor overhead
Significance
 Transfer data while sleeping
 Minimize processor overhead
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Low Duty Cycle
Fails with significant
processing
Alternatives:
 Dynamic Voltage and
Frequency Scaling
(DVFS)
 Dynamic Power
Management (DPM)
Image:
http://www.domainmagnate.com/wpcontent/uploads/2009/03/failure-success.jpg
Nik Sumikawa
Dynamic Power
Design Variables
Energy Source
Capacitance
Frequency
Dynamic
Power
Voltage
P = CVdd2f
Battery
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Reducing Dynamic Power
 Dynamic Voltage and Frequency Scaling
 Scale voltage when sleeping/Idle
 Voltage term quad. proportional to power
 Reduce frequency
 Minimize line capacitance
 Long traces have large capacitance
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Dynamic Power Management
Generalize power
management
Multiple policies
 Single-policy
 Multiple-policy
 Task-scaling
Rajami and Brock [2]
Nik Sumikawa
Single-policy Strategy
Idle Scaling (IS)
 Operate at full speed when processing
workload
 Reduce the frequency and voltage when idle
Goal:
 Reduce the CPU and bus frequencies
 Meet continuous DMA requirements
 Provide acceptable latency when resuming
from idle
Rajami and Brock [2]
Nik Sumikawa
Multi-policy Strategies
Load scaling (LS):
 Balance system operating point with current
or predicted processing demands
 Run system with minimal idle time
Other:
 Manage systems state based on status of the
systems energy source
Rajami and Brock [2]
Nik Sumikawa
Task-scaling Strategies
Application scaling (AS):
 Used for workloads that are difficult to power
manage
• Audio and video processing
• Begin processing next sample immediately
 Operate a lower operating point
 Increases to higher operating point when it
begins to fall behind.
Rajami and Brock [2]
Nik Sumikawa
Results of DPM
 IS: Idle Scaling
LS: Load Scaling
AS: Application Scaling
 Frame-Scaling (FS): perfect knowledge of processing requirements of video frame
Rajami and Brock [2]
Nik Sumikawa
Too Many Low Power States
Disadvantages:
 Confusion
 Wrong low power
state
Solution:
 Minimize the number
of state
 Decrease complexity
Image:
http://kunaljanu.files.wordpress.com/2009/02/
ist2_1457667confusion-1.jpg
Nik Sumikawa
Sources of Power Consumption
Microcontroller
Bus architecture
 On chip communication
 External communication
Memory hierarchy
Peripherals
Rajami and Brock [2]
Nik Sumikawa
Communication Architectures
Advanced Microcontroller Bus Architecture
 ARM bus protocol for system-on-a-chip (SOC)
 Advanced High Performance Bus (AHB)
• Pipelined
• Memory mapped
• Up to 16 masters, 16 slaves
 Advanced Peripheral Bus (APB)
• Non pipelined
• Single master, up to 16 peripherals
Rajami and Brock [2]
Nik Sumikawa
AMBA On-chip Bus
Rajami and Brock [2]
Nik Sumikawa
Power Profiling
86% power consumed by logic
14% power consumed by bus lines
Rajami and Brock [2]
Nik Sumikawa
Power Reduction Techniques
Power Management
 Shut down bus interfaces to idle slaves
Bus Encoding
 Reduces # of line transitions, but not bus
transactions
Traffic Sequencing
 Reduce multiple masters interleaving bus
access
Rajami and Brock [2]
Nik Sumikawa
Power Reduction Techniques
No technique achieves large saving alone
Rajami and Brock [2]
Nik Sumikawa
Power vs Energy
Power is amount of energy over an
amount of time (Watts = Joules / second)
Battery provides finite amount of energy
 Goal: minimize energy use, not just power
In mobile systems we care about energy
 Budget energy to prolong battery life
Rajami and Brock [2]
Nik Sumikawa
Static System Optimization
Compiler techniques
 Instruction energy consumption profiling
• Done empirically
 Instruction reordering
•
•
•
•
Without affecting correctness
Improve register utilization
Reduce memory accesses
Reduce pipeline stalls
Nik Sumikawa
Static System Optimization
Code Compression





Post compilation static optimization
Reduces storage size of instructions
Can have a large impact
Requires complex design space exploration
Goal for mobile system: reduce power
consumption while preserving performance
Nik Sumikawa
Code Compression Challenges
Random access decompression
 Defining decodable block beginnings
 Jump to new locations in program without
decoding all blocks between
Solutions
 Begin compressed blocks on byte boundaries
 Store translation table
• More efficient the compression, larger the table
 Recalculate branch offsets to compressed
addresses
Nik Sumikawa
Code Compression Requirements
Additional hardware
 Additional memory to store table
 Decompression unit
Design decisions
 Table generation/lookup
 Compression technique
Nik Sumikawa
Code Compression Implementation
SPARC ISA
Optimize consumption of complete SOC
Multiple iterations on binary
Instructions split into 4 categories




Group 1: immediate instructions (code = 0)
Group 2: branch instructions (code = 11)
Group 3: dictionary instructions (code = 100)
Group 4: uncompressed instr (code = 101)
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Diagram
Update branch offsets
Optimized Binary
Phase 4
Branch compression
Phase 3
Immediate compression
Phase 2
Markov model
Compiled
Binary
Phase 1
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Nik Sumikawa
As a Result…
Bus Compaction
 Instructions transmitted no longer require
entire bus
 Use the extra lines to transmit the next
compressed instruction
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Decompression Architecture
Pre-Cache
 Decompression engine between
memory/cache
Post-Cache
 Decompression engine between cache/cpu
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Simulation
Full SOC simulation
7 sample apps run
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Results
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INCLUDE?
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Results
Net energy saving observed
 22-82% power savings from code
compression
 What about additional hardware?
Bonus
 Increased performance
 Reduced area
Nik Sumikawa
Verdict
Static power optimization
 Potentially large payoff for little preprocessing
Still more sources of consumption
 We’ve observed SOC savings
 What about peripherals?
Nik Sumikawa
Energy Budget
Voice Call
SMS
Energy
Budget
Emails
Pictures
localization
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Energy Budget: Localization
How much of the energy budget
should be given to localization?
 Depends on the user
Grant increase allotment when
localization is a higher priority
Nik Sumikawa
Localizations Methods
1
2
3
GPS
GSM
WiFi
• Very accurate
• Power Hungry
• Lower accuracy
• Lower power
requirement
• Mod. Accurate
• Mod. Power
requirement
Nik Sumikawa
Power vs. Precision
Localization
Power:
Precision:
amount of energy
required by peripheral
in order to determine
location
Accuracy of the device
used for localization
Constandache, Gaonkar, Sayler,
Choudhury, Cox [3]
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Power Consumption
 30 Second sampling intervals
 Power Consumption:
 GPS: High baseline
 WiFi: Low baseline with high spikes
 GSM: Low baseline with varying spikes
Constandache, Gaonkar, Sayler,
Choudhury, Cox [3]
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Power Consumption
 30 Second sampling
intervals
 Results:
 GPS: increased
baseline
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Localization Accuracy
 Accuracy varied
based on location
ALE: Average
Location Error
 Wifi and GSM
oversampled
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