Designing Low Power Wireless Systems Telos / Tmote Sky

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

2. Moore’s Law “Stuff” (transistors, etc) doubling every 1-2 years Bell’s Law New computing class every 10 years Faster, Smaller, Numerous Streaming Data to/from the Physical World log (people per computer) year

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

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

5. 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. 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 Typical WSN Application • Short active time • processing • data acquisition • communication 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 • >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. Microcontroller Radio (IEEE 802.15.4) Wakeup • Overhead of switching from Sleep to Active Mode • Reduce wasted energy due to switching modes cap charging enterrx osc on load regs rx Time (ns) Texas Instruments MSP430 Fx1xx Chipcon CC2420 1.6 ms 292 ns 10ns – 4ms typical 1– 10 ms typical

10. 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 Active

11. 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 Selecting a Radio

12. Available RAM has stayed fairly constant Instead of increasing RAM, extra die space used for hardware modules DMA: increases performance AND lowers power consumption Microcontroller Memory Trends

13. 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 Accelerators vs Modules 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 WirelessTransceiver Real Time Clock32.768kHzfor low power modes Disconnect unused peripherals

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

16. 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

17. 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 eachsensor and componentsubsystem Low Power Operation

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 802.15.4 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 • 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)

21. 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 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 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% Packet Yield Link Quality Indicator 0% 0ft 250ft Distance

24. Microcontroller STM25P80 SPI WriteProtect Increasing Robustness • Golden Image • Problem: Faulty software causes the system to halt • Solution: Store known good image in write protected flash Flash Write OK Write FAIL USB 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. 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 Want to experiment with Telos?