Designing Low Power Wireless Systems Telos / Tmote Sky Joe Polastre UC Berkeley Moteiv Corporation.
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Transcript Designing Low Power Wireless Systems Telos / Tmote Sky Joe Polastre UC Berkeley Moteiv Corporation.
Designing Low Power
Wireless Systems
Telos / Tmote Sky
Joe Polastre
UC Berkeley
Moteiv Corporation
Faster, Smaller, Numerous
Moore’s Law
“Stuff”
(transistors, etc)
doubling every 1-2
years
Bell’s Law
New
computing class
every 10 years
Streaming Data
to/from the
Physical World
log (people per computer)
year
2
Applications
Monitoring
Habitat Monitoring
Integrated Biology
Structural Monitoring
Interactive and Control
Pursuer-Evader
Intrusion Detection
Automation
3
Berkeley Motes Timeline
Rene’
“Experimentation”
Mica
Telos
“Open Experimental Platform”
“Integrated Platform”
WeC
“Smart Rock”
Spec
Dot
“Scale”
1999
2000
2001
2002
“Mote on a chip”
2003
2004
4
Low Power Operation
Efficient Hardware
Integration and Isolation
Selectable Power States (Off, Sleep, Standby)
Operate at low voltages and low current
Complementary functionality (DMA, USART, etc)
Run to cut-off voltage of power source
Efficient Software
Fine grained control of hardware
Utilize wireless broadcast medium
Aggregate
5
Typical WSN Application
Communications
Periodic
Sleep 99+% of time
Active time is very short
Detection/Notification
Duty Cycled
Data Collection
Network Maintenance
Triggered Events
• processing
• data acquisition
• communication
Milliseconds or less
Long Lifetime
Power
Short active time
Months to Years without
changing batteries
Power management is the key
to WSN success
sleep
Time
6
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
Using low power hardware
individual circuits
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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Wakeup
Overhead of switching from Sleep to Active Mode
Reduce wasted energy due to switching modes
Microcontroller
Radio (IEEE 802.15.4)
Current (mA)
30
load regs
Texas Instruments MSP430 Fx1xx
292 ns
10ns – 4ms typical
osc on
enter
rx
rx
20
10
0
-0.5
Time (ns)
cap charging
0
0.5
1
1.5
Time (ms)
2
2.5
Chipcon CC2420
1.6 ms
1– 10 ms typical
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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 : 1.5mJ
Duration : 32ms
Atmel flash writing 1 byte
Energy : 3mJ
Duration : 78ms
10
Selecting a Radio
Narrowband
Wideband
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
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
11
Microcontroller Memory Trends
140
Flash
RAM
120
Kilobytes
100
80
60
40
20
0
1975
Available RAM has
stayed fairly
constant
Instead of
increasing RAM,
extra die space
used for hardware
modules
1980
1985
1990
Year
1995
2000
DMA: increases
performance AND
lowers power
consumption
2005
12
Accelerators vs Modules
Hardware Modules
Software routines pushed
into hardware
Lose flexibility
Example: encryption
Break modules up into
accelerators
Let software tie them
together
Considerable flexibility
Spec (Jason Hill thesis)
Radio or Microcontroller
Examples:
Accelerators
Isolated to specific
component
Packet handling support
Encryption
Data busses and Timers
Examples:
RF Interrupt Handling
Encryption
Simple DMA for Tx/Rx
Unfortunately, most manufacturers are moving to Modules, not Accelerators
Examples: Newly released Chipcon CC2430, Ember EM250
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Putting it all together
Low Power
Microcontroller
Low ESR fast
starting oscillator
Wireless
Transceiver
Real Time Clock
32.768kHz
for low power modes
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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Telos
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
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Low Power Operation
TI MSP430 -- Advantages
over other
microcontrollers
16-bit core
12-bit ADC
< 50nA port leakage
(vs. 1mA for Atmels)
Double buffered data
buses
Interrupt priorities
Calibrated DCO
Integrated wireless
module
Buffers and Transistors
Switch on/off each
sensor and component
subsystem
17
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
Wakeup
Majority of the time, including peripherals
Telos: 5.1mA
AVR: 30mA
As quickly as possible to process and return to sleep
Telos: 290ns typical, 6ms max
AVR: 60ms 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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Power Calculation Comparison
Design for low power
AVR + CC1000
0.2 ms wakeup
30 mW sleep
33 mW active
21 mW radio
19 kbps
2.5V min
2/3 of AA capacity
AVR + CC2420
0.2 ms wakeup
30 mW 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 mW 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)
453 days
328 days
945 days
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Duty Cycle vs Lifetime
Lifetime (days) vs Period (seconds)
10000.00
AVR+CC1000
AVR+CC2420
MSP430+CC2420
1000.00
100.00
10.00
1.00
0.10
0.01
0.1
1
10
100
1000
10000
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Supporting Software
Pushing information up the stack
100%
Link Quality Indicator
Packet Yield
0%
0ft
250ft
Distance
23
Increasing Robustness
Golden Image
Problem: Faulty software causes the system to halt
Solution: Store known good image in write protected flash
Microcontroller
Flash
ST
M25P80
SPI
Write OK
Write FAIL
USB
USB Power
X
Write
Protect
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
User Input
Button
a low power state
presses
Other options
Grenade
timer (XSM/Trio)
25
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
26
Want to experiment with Telos?
Constraints:
Practical testbed limits:
Up to 4 powered hubs in a chain
USB cables up to 5m in length
Up to 127 devices on a USB bus
30m radius
About a hundred motes
Usable for a large room
Low cost approach
Off the shelf hardware
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