Smart Dust: Communicating with a CubicMillimeter Computer

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Smart Dust Communicating with a Cubic-Millimeter
Computer

• Presentation by
• Hörður Mar Tómasson
• 13. October 2006

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The Smart Dust project

• Went on at UC Berkeley 1998-2001
• Primary investigator Kristofer S.J. Pister

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The goal

• 1 mm³ motes
• with onboard sensors,
• CPUs
• and wireless communications facilities
• forming the basis of a sensor network

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Fundamental goal

• To explore the limitations of microfabrication
  technology

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Ideas for uses for smart dust

• Surveillance networks for defense
• Monitoring environmental conditions
• Human-computer interfaces
• Inventory and product quality control
• Tracking movements of animals

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Power considerations

• Batteries 1 J/mm³ storage
• Capacitors 10 mJ/mm³ usable storage
• Solar cells 1 J/(mm² day) in sunlight or
• 1-10 mJ/(mm² day) indoors
• Optical receiver 0.1 nJ/bit
• Optical transmitter 1 nJ/bit
• A/D converter 1 nJ/sample
• Computation 1 pJ/instruction

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Power considerations

• 1000 8-bit operations per sample will not make a
  big difference in power used.
• 1 mJ per day from a solar cell indoors will be
  sufficient for making a measurement every second,
  processing the result and transmitting it.

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Low-energy computation

• Smaller transistors with less parasitic
  capacitance consume less dynamic power.
• Reduced supply voltage also means less dynamic
  power.
• Leakage currents can be decreased by reverse
  biasing the channel-to-source junction.
• Clock rates of 1-100 kHz are sufficient for
  working with some important types of physical
  signals.

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Wireless communication

• Radio communication currently requires several mW
  of power and preferably antennas longer than a
  millimeter.
• Semiconductor lasers and diode receivers can use
  less power and are more directional.
• The Smart Dust project explored optical
  communication.

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Passive reflective systems

• A MEMS corner cube reflector (CCR) with a side
  that can be tilted
• Less than 1 nJ used per transition
• The mote can use the CCR to communicate with a
  base station equipped with a light source.

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Active steered laser systems

• Semiconductor laser
• Collimating lens
• MEMS steerable micromirror

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Optical receiver

• An imaging receiver has several benefits.
• Only one pixel receives the signal but the
  ambient light is divided between the pixels.
• Several signals can be received in parallel.
• The authors did an experiment with a laser and a
  video camera.
• A smart pixel has an integrated receiver.

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Ad hoc mote networks

• If the motes can communicate directly with each
  other, they can form ad hoc multihop networks to
  carry the data around.
• This is an interesting problem for network
  algorithm design.

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An Ultra-Low Energy Microcontroller for Smart
Dust Wireless Sensor Networks

• Presentation by
• Hörður Mar Tómasson
• 13. October 2006

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Creators of the microcontroller

• Brett A. Warneke
• Kristofer S.J. Pister

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Application

• The microcontroller was developed for this
  prototype smart dust mote.

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Architectural features

• Highly independent subsystems
• Component-level clock gating in decoder
• Processor halt mode
• Guarded ALU inputs
• Multiple busses
• Harvard architecture
• Load-store RISC

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Main oscillator

• Runs continuously at a few kHz
• Operates real time clock and five timers
• One timer for each sensor sampling period
• One timer for invoking the transmitter
• One timer for invoking the receiver
• One timer for waking up the datapath

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Other oscillators

• 100 kHz for driving the sensor ADC
• 8 MHz for sampling a 1 Mb/s optical signal

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ADC automation

• The ADC is configurable to different levels of
  automation.
• At the minimum level, the sensor and sample and
  hold are activated.
• At the maximum level, the voltage is compared to
  a threshold and, if the threshold is exceeded,
  converted and stored in the SRAM along with a
  time stamp.

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Transmitter

• The processor core uses two registers to specify
  what memory blocks contain data to be
  transmitted.
• The transmitter formats the data into packets and
  transmits them asynchronously to the CCR.

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Four types of received packets

• Short sync packets trigger the transmitter.
• Immidiate packets contain an instruction that is
  immediately executed.
• Program packets are streamed to the program
  memory.
• Data packets are streamed to the data memory.

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Trimmable oscillator
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Specifications
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RF Telemetry System for an Implantable Bio-MEMS
Sensor

• Presentation by
• Hörður Mar Tómasson
• 13. October 2006

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The long range goal

• NASA wants to develop implantable sensors to
  monitor physiological parameters of humans during
  space flights.
• It would be of great benefit to have contactless
  powering and data readout for the implants.

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Advantages of contactless powering and telemetry

• The inductor/antenna is small in size.
• There is no need to implant batteries.
• The circuit only operates when interrogated,
  avoiding heating of the surrounding tissue and
  extending the life span of the sensor.
• Feed-through wires not needed, enhancing mobility
  and reducing risk of infection.

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This paper

• A system for contactless powering and RF
  telemetry from an implantable bio-MEMS sensor.
• A square spiral inductor/antenna
• A MEMS capacitive pressure sensor
• A pick-up antenna

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Spiral inductor/antenna
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MEMS pressure sensor
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Pick-up system

• A printed circuit with mounted components
• Spiral inductor/antenna, printed
• MMIC low noise amplifier, mounted on
• Antenna matching network, mounted discrete
  components, ?-network
• Output connector

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Operating principle

• The idea is to send pulses down into the implant
  and detect the decaying sine response.

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Parameters

• Desired frequency range 200 - 700 MHz
• Expected capacitance of pressure sensor 0.3 - 4
  pF
• Expected required parameters for square inductor
  150 nH and Q10
• Several inductors with different geometries were
  tried.

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Fabrication of the inductor

• High resistance silicon wafer
• Spin-on glass coating
• Chrome/gold metallization
• The goal is to have high Q.

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The next step

• Exploring coupling between the inductor and the
  pick-up antenna through stratified media