Designing Low Power Wireless Systems

Telos / Tmote Sky

Joe PolastreUC Berkeley

Moteiv Corporation

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Faster, Smaller, Numerous

year

log

(p

eo

ple

pe

r c

om

pu

ter)

Streaming Data to/from the

Physical World

Moore’s Law “Stuff” (transistors, etc)

doubling every 1-2 years

Bell’s Law New computing class

every 10 years

3

ApplicationsDensity & Scale

Sample Rate & Precision

MobilityLow Latency

Disconnection & Lifetime

Monitoring Habitat Monitoring Integrated Biology Structural Monitoring

Interactive and Control Pursuer-Evader Intrusion Detection Automation

4

Berkeley Motes Timeline

WeC“Smart Rock”

1999 2000

Rene’“Experimentation”

2001 2002 2003 2004

Dot“Scale”

Mica“Open Experimental Platform”

Spec “Mote on a chip”

Telos“Integrated Platform”

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Low Power Operation

Efficient Hardware Integration and Isolation

Complementary functionality (DMA, USART, etc) Selectable Power States (Off, Sleep, Standby) Operate at low voltages and low current

Run to cut-off voltage of power source

Efficient Software Fine grained control of hardware Utilize wireless broadcast medium Aggregate

6

Typical WSN Application

sleepw

akeu

p

Short active time• processing• data acquisition• communication

Po

wer

Time

Communications Periodic

Data Collection Network Maintenance

Triggered Events Detection/Notification

Duty Cycled Sleep 99+% of time

Active time is very short Milliseconds or less

Long Lifetime Months to Years without

changing batteries Power management is the key

to WSN success

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Design Principles

Key to Low Duty Cycle Operation:

Sleep – majority of the time Wakeup – quickly start processing Active – minimize work & return to sleep

For long lived wireless networks, optimize sleep, then wakeup, then active current consumption and processing time For low duty cycle networks, active mode optimizations (like

dynamic voltage scaling) provide insignificant benefits

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Sleep

Majority of time, node is asleep >99%

Minimize sleep current through Isolating and shutting down individual circuits Using low power hardware

Need RAM retention Run auxiliary hardware components from low

speed oscillators (typically 32kHz) Perform ADC conversions, DMA transfers, and bus

operations while microcontroller core is stopped

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Overhead of switching from Sleep to Active Mode Reduce wasted energy due to switching modes

Wakeup

Microcontroller Radio (IEEE 802.15.4)

1– 10 ms typical

1.6 ms

-0.5 0 0.5 1 1.5 2 2.50

10

20

30

Time (ms)

Cu

rren

t (m

A)

osc onload regs

cap chargingenter

rx rx

10ns – 4ms typical292 ns

Texas Instruments MSP430 Fx1xx Chipcon CC2420

Time (ns)

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Active

Microcontroller Fast processing, low active

power Avoid external oscillators

Radio High data rate, low power

tradeoffs Increased complexity vs

robusness to noise

External Flash (stable storage) Data logging, network code

reprogramming, aggregation High power consumption Long writes

Radio vs. Flash 250kbps radio sending 1 byte

Energy : 1.5J Duration : 32s

Atmel flash writing 1 byte Energy : 3J Duration : 78s

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Selecting a Radio

Narrowband Low bit rate (< 250kbps) Lower frequencies

higher range Simple channel modulation Susceptible to noise

(narrow frequency use) Low power consumption

(<15mA) Fast wakeup times

(some may be clocked by MCU)

Examples:RFM TR1000, Chipcon CC1020

Wideband High bit rate (100kbps+) High frequencies Global ISM

band at 2.4GHz Complex channel modulation Robust to noise

(using spreading codes) High power consumption

(>20mA) Slow wakeup times

(must start external oscillators)

Examples:IEEE 802.15.4, Bluetooth

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Microcontroller Memory Trends

Available RAM has stayed fairly constant

Instead of increasing RAM, extra die space used for hardware modules DMA: increases

performance AND lowers power consumption

1975 1980 1985 1990 1995 2000 20050

20

40

60

80

100

120

140

Year

Kilo

byt

es

FlashRAM

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Accelerators vs Modules

Hardware Modules Software routines pushed

into hardware Lose flexibility

Example: encryption Isolated to specific

component Radio or Microcontroller

Examples: Packet handling support Encryption Data busses and Timers

Accelerators Break modules up into

accelerators Let software tie them

together Considerable flexibility Spec (Jason Hill thesis)

Examples: RF Interrupt Handling Encryption Simple DMA for Tx/Rx

Unfortunately, most manufacturers are moving to Modules, not AcceleratorsExamples: Newly released Chipcon CC2430, Ember EM250

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Putting it all togetherLow Power Microcontroller

WirelessTransceiver

Real Time Clock32.768kHz

for low power modes

Low ESR fast starting oscillator

Disconnect unused peripherals

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Telos

Applications Monitoring – H/VAC,

Structural, Environmental, Medical

Principles Low Power

Long Lifetime Easy to use Robust hardware and

software High Performance

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Wireless sensor module for building applications

Standards Based USB IEEE 802.15.4/Zigbee TinyOS Expansion to other sensors

Low Power Hardware designed from

software principles for low power operation

Isolation, buffering, fast wakeup from sleep

Low Cost Integrated design 50m range indoors 125m range outdoors

IEEE 802.15.4 New wireless standard

for low power communication CC2420 radio 250kbps 2.4GHz ISM band Zigbee-compatible

Telos

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Low Power Operation

TI MSP430 -- Advantages over other microcontrollers 16-bit core 12-bit ADC < 50nA port leakage

(vs. 1A for Atmels) Double buffered data

buses Interrupt priorities Calibrated DCO

Integrated wireless module Buffers and Transistors Switch on/off each

sensor and componentsubsystem

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Hardware Isolation

Experiences from Great Duck Island One component failure kills entire system Must isolate and detect failures Remove/Turn off voltage regulators

Each “sub-circuit” on Telos is isolated Microcontroller turns on/off Fine-grained control of power consumption Reduce node failures from a single faulty component

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Minimize Power Consumption

Compare to using the AVR MCU and 802.15.4 radio

Sleep Majority of the time, including peripherals Telos: 5.1A AVR: 30A

Wakeup As quickly as possible to process and return to sleep Telos: 290ns typical, 6s max AVR: 60s max internal oscillator, 4ms external

Active Get your work done and get back to sleep Telos: 4-8MHz 16-bit AVR: 8MHz 8-bit

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CC2420 Transceiver Fast data rate, robust signal

250kbps : 2Mchip/s : DSSS 2.4GHz : Offset QPSK : 5MHz 16 channels in 802.15.4 -94dBm sensitivity

Low voltage operation 1.8V minimum supply

Software assistance for low power microcontrollers 128byte TX/RX buffers for full packet support Automatic address decoding and automatic

acknowledgements Hardware encryption/authentication Link quality indicator (assist software link estimation)

samples error rate of first 8 chips of packet (8 chips/bit)

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AVR + CC1000 0.2 ms wakeup 30 W sleep 33 mW active 21 mW radio 19 kbps 2.5V min

2/3 of AA capacity

Power Calculation Comparison Design for low power

AVR + CC2420 0.2 ms wakeup 30 W sleep 33 mW active 45 mW radio 250 kbps 2.5V min

2/3 of AA capacity

Telos (TI MSP) 0.006 ms wakeup 2 W sleep 3 mW active 45 mW radio 250 kbps 1.8V min

8/8 of AA capacity

Supporting mesh networking with a pair of AA batteries reporting data once every 3 minutes using synchronization (<1% duty cycle)

328 days 945 days453 days

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Duty Cycle vs LifetimeLifetime (days) vs Period (seconds)

0.10

1.00

10.00

100.00

1000.00

10000.00

0.01 0.1 1 10 100 1000 10000

AVR+CC1000

AVR+CC2420

MSP430+CC2420

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Supporting Software Pushing information up the stack

Pac

ket

Yie

ldL

ink

Qu

alit

y In

dic

ato

r

Distance250ft0ft

0%

100%

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Increasing Robustness Golden Image

Problem: Faulty software causes the system to halt Solution: Store known good image in write protected flash

STM25P80

SPI

WriteProtect

MicrocontrollerFlash

USB

USB Power

Write OK

X

Write FAIL

USB Disconnected

Next year marks the release of MCUs with 1MB Flash and Protected Segments

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Entering the Golden Image

Watchdog Count number of resets

Voltage Maintain a low power state

User Input Button presses

Other options Grenade timer (XSM/Trio)

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Key Contributions

New design approach derived from our experience with resource constrained wireless sensor networks Active mode needs to run quickly to completion Wakeup time is crucial for low power operation

Wakeup time and sleep current set the minimum energy consumed Sleep most of the time

Principles for increased robustness Isolation: Fine grained software control Protected Golden Image Careful microcontroller/radio selection to meet app requirements

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Want to experiment with Telos? Constraints:

Up to 4 powered hubs in a chain USB cables up to 5m in length Up to 127 devices on a USB bus

Practical testbed limits: 30m radius About a hundred motes Usable for a large room

Low cost approach Off the shelf hardware

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