Ultra-Low-Power Wireless Sensor Node Programmable in Java

ICC 2012 – E2NETS workshop
OTTAWA – CANADA – JUNE 10-15 2012

E2NETS Workshop
Energy Efficiency in Wireless Networks & Wireless Networks for Energy Efficiency

Ultra-Low-Power Sensor Nodes
Featuring a Virtual Runtime Environment

EMANUELE LATTANZI ALESSANDRO BOGLIOLO
DiSBeF University of Urbino & NeuNet

1/25
alessandro.bogliolo@uniurb.it


Agenda
• Introduction
• HW-SW platform
• Power states
• DPM issues and solutions
• Experimental results

2/25


Introduction
• Need for energy efficient wireless sensor nodes
– Lifetime maximization
– Compatibility with energy harvesters
• Attractiveness of runtime virtual environment
– Development
– Deployment
– Re-use/Re-programming
• Availability of ultra-low-power micro controller units
(MCUs) featuring
– 16-bit RISC architectures clocked at tens of MHz
– 16kbytes of main memory
– 64kbytes of flash memory
– Voltage-frequency scaling
– Low-power modes with sub-ms wake-up time

3/25


Objective
• Ultra-low power sensor node
• Java-compatible virtual run-time environment
• Full exploitation of low-power modes
• Off-the-shelf low-cost components
• Open-hardware PCB
• Open-source software stack

4/25


Agenda
• Introduction
• HW-SW platform
• Power states

5/25


HW platform
• TI MSP430F54xxa MCU
– 16-bit RISC
– 8.9mA @ 25MHz
– 6 LP modes
– 0.1-73 uA
– 5us wake-up

6/25


SW stack Contiki OS
• Open-source real-time OS for sensor networks
and networked embedded systems
– Portability
– Multi-tasking
– Memory efficiency
– Event-driven organization
– Elementary DPM which exploits the stand-by state of
the MCU and wakes it up every 10ms to keep timing
coherence

7/25


SW stack Darjeeling VM
• Open-source VM for extremely limited devices
– Java compatibility
– Limited requirements (10kbytes of RAM)
– Bytecode efficiency (infuser)
– Runs as a single process
– Supports multi-threading
– Preemptive round-robin scheduling

8/25


Agenda
• Introduction
• HW-SW platform
• Power states

9/25


Power modes
CPU Clock Memory

Active mode    1-10mW
Self wake up [1-10us]

Standby mode    1-100uW
External interrupt [<1ms]

Sleep mode    1uW
Reboot at external interrupt [1-100ms]

Hibernation mode    100nW

10/25


SW-stack dimension
OS VM Int OS VM
Active boot boot hand sch sch
Vtask

Standby

Sleep

Hibernation

11/25


Wake-up transitions
OS VM Int OS VM
Active boot boot hand sch sch
Vtask

Standby

Sleep

Hibernation

12/25


Agenda
• Introduction
• HW-SW platform
• Power states

13/25


Standby mode
Int OS VM
ContikiOS schedules hand sch sch
Vtask
a self-wakeup
every 10ms

There are 3 cases:
a) The VM has no tasks to resume a

14/25


Standby mode
Int OS VM
Vtask
a self-wakeup
every 10ms

There are 3 cases:

a) The OS has no proc. to resume b

15/25


Standby mode
Int OS VM
Vtask
a self-wakeup
every 10ms

There are 3 cases:

a) The OS has no processes to resume b

a) The VM has a task to resume Actual wake up

16/25


Standby mode abstract state diagram
Active 1-10mW Vtask
c Exploitation issues:
a The self-loop longer than 10ms
1-10mW
No power saving
Standby b Not supported by the SW
1-100uW stack (VM always active)

1-10uW Not supported by Contiki OS
Lack of timing info

Stanby mode is not compatible with the SW stack!

17/25


Standby mode improvements
• Modified Contiki OS to make it able to dynamically adjust
the INTERVAL of timer interrupts

• Modified Darjeeling VM scheduler to make it able to set
the OS timer and suspends the VM
PROCESS_WAIT_EVENT_UNTIL()

• Implemented a timed standby mode with just-in-time
predictive wake-up

• Mounted an external low-power (0.3uW) real-time clock
(RTC) to preserve timing accuracy

18/25


Standby mode modified state diagram
6.6mW Vtask

4.5uW + 153.72uJ/T Standby.a(T) Families of
low-power states
Standby.b(T) depending on T
4.5uW + 0.33uJ/T

4.5uW + 0.3uW Standby.t

4.5uW Standby

19/25


Sleep mode issues
• Unable to issue self-events
– Wakup can only be triggered by external interrupts
– Cannot be exploited if there are processes/tasks that
need to resume at a given time (e.g., periodic
monitoring tasks typical of many WSN applications)
• Lack of timing information
– No information about the time elapsed since last shut
down
– No time stamps associated with external interrupts
Sleep mode can ony be exploited in case of time-
independent reactive applications
20/25


Sleep mode improvements
• Implemented a timed sleep mode with just in
time predictive wakeup based on the external
RTC
• Used the external RTC to provide relative and
absolute timing information at wake up

1.5uW + 0.3uW Sleep.t

1.5uW Sleep

21/25


Hibernation mode issues
• Unable to issue self-events
– As for Sleep mode
• Lack of timing information
– As for Sleep mode
• Lack of data retention
– Requires a complete reboot
– Impossible to resume a process/task

Hibernation mode can ony be exploited in case of
memory-less reactive applications
22/25


Hibernation mode improvements
• Used the external RTC as for the Sleep mode
• Implemented a native method in the main of the
VM to save and restore the heap of the VM in
flash memory

1.5uW + 0.3uW Hibernation.t

1.5uW Hibernation

23/25


Agenda
• Introduction
• HW-SW platform
• Power states

24/25


Characterization results

Data refer to a MSP430F2618 MCU powered at 3V and clocked at 16MHz

25/25


Experimental results
6.6mW

Average power consumption of the MCU
used to execute a periodic monitoring
110uW
task which keeps the CPU busy for 1s

6.9uW

Once Once
per hour per day

26/25


Conclusions
• Ultra-low-power sensor node
• Java-compatible virtual runtime environment
• Power consumption ranging from 6.6mW to
0.1uW (in Hibernation)
• Capable of reacting to external events and
resume execution from any low-power state
• All low-power modes directly exploitable from
the Java runtime environment
• Open-HW / Open-SW approach
27/25

Ultra-Low-Power Wireless Sensor Node Programmable in Java

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