1. Designing Low Power Wireless Systems Telos / Tmote Sky
Joe Polastre
UC Berkeley
Moteiv Corporation

2. 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

3. Applications Monitoring Interactive and Control
Sample Rate & Precision
Disconnection & Lifetime
Density & Scale
Low Latency
Mobility
Monitoring
Habitat Monitoring
Integrated Biology
Structural Monitoring
Interactive and Control
Pursuer-Evader
Intrusion Detection
Automation

4. Berkeley Motes Timeline
Rene’
“Experimentation”
Mica
“Open Experimental Platform”
Telos
“Integrated Platform”
WeC
“Smart Rock”
Dot
“Scale”
Spec “Mote on a chip”
1999
2000
2001
2002
2003
2004

5. Low Power Operation Efficient Hardware Efficient Software
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
Short active time
processing
data acquisition
communication
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
wakeup
Power
sleep
Time

7. 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

8. Sleep Majority of time, node is asleep Minimize sleep current through
>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

9. Wakeup Overhead of switching from Sleep to Active Mode
Reduce wasted energy due to switching modes
Microcontroller
Radio (IEEE )
cap charging
enter rx
load regs
osc on rx
Time (ns)
Texas Instruments MSP430 Fx1xx
Chipcon CC2420
292 ns
1.6 ms
10ns – 4ms typical
1– 10 ms typical

10. Active Microcontroller Radio External Flash (stable storage)
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

11. 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
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 , Bluetooth

12. 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

13. 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 Accelerators
Examples: Newly released Chipcon CC2430, Ember EM250

14. Putting it all together
Low ESR fast starting oscillator
Low Power Microcontroller
Wireless Transceiver
Real Time Clock kHz for low power modes
Disconnect unused peripherals

15. Telos Applications Principles
Monitoring – H/VAC, Structural, Environmental, Medical
Principles
Low Power  Long Lifetime
Easy to use
Robust hardware and software
High Performance

16. Telos Wireless sensor module for building applications Standards Based
USB
IEEE /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
New wireless standard for low power communication
CC2420 radio
250kbps
2.4GHz ISM band
Zigbee-compatible

17. Low Power Operation
TI MSP 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

18. 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

19. Minimize Power Consumption
Compare to using the AVR MCU and radio
Sleep
Majority of the time, including peripherals
Telos: 5.1mA
AVR: 30mA
Wakeup
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

20. CC2420 Transceiver Fast data rate, robust signal Low voltage operation
250kbps : 2Mchip/s : DSSS
2.4GHz : Offset QPSK : 5MHz
16 channels in
-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)

21. 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

22. Duty Cycle vs Lifetime

23. Supporting Software Pushing information up the stack
100%
Link Quality Indicator
Packet Yield 0%
0ft
250ft
Distance

24. 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 Protect
Write OK
USB
Write FAIL
USB Power X
USB Disconnected
Next year marks the release of MCUs with 1MB Flash and Protected Segments

25. 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)

26. 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

27. 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