Wireless Embedded InterNetworking Foundations of Ubiquitous Sensor Networks Triggers and Sensing

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Wireless Embedded InterNetworkingFoundations of
Ubiquitous Sensor Networks Triggers and Sensing

• David E. Culler
• University of California, Berkeley

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An Analog World

• Everything in the physical world is an analog
  signal
• Sound, light, temperature, gravitational force
• Need to convert into electrical signals
• Transducers converts one type of energy to
  another
• Electro-mechanical, Photonic, Electrical,
• Examples
• Microphone/speaker
• Thermocouples
• Accelerometers
• And digitize
• Then manipulate

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An Analog World

• Transducers
• Allow us to convert physical phenomena to a
  voltage potential in a well-defined way.

I
V
R ohm ?
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Simplest Analog Device

• Often think of it as an actuator, rather than a
  sensor
• But thats because of the circuit we put it in
• It is binary (two states) but why is it not
  digital?

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To Sample a switch, make it digital
VD
VtH
VtL

• Many sensor are switches
• Two states but not digital
• Open gt no current
• Closed gt no voltage drop
• Cap charges to Vacc when open
• Cap discharges to GND when closed

D
switch
GND
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EPIC I/F Board - Digital Inputs
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Getting Input into the MCU
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MCU a system on a chip
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Programmed IO
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Interrupts
MCU Hardware
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Getting a hold of the event
event void Boot.booted() atomic P2IE
PIN7 / Disable interrupt / P2IFG PIN7
/ Clear interrupt flag / P2DIR PIN7 /
Configure as input / P2IES PIN7 / Select
Hi-gtLo / P2IE PIN7 / Enable interrupts
/
async event void HplSignalPort2.fired() if (
P2IFG PIN7 ) P2IFG PIN7 post
fired()
Software
Transport
Kernel
Network
Driver Code
Link
Handler dispatch
HplSignal
Phy
Memory Mapped IO registers
Radio
Ports
ADC
Timers
Hardware
Flash
MCU
Hardware Interrupt
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Making Sense of Physical Information

• Digital representation of physical phenomenon
• Transducer gt Signal Conditioning gt ADC gt
• Conversion to physical units
• Calibration and correction
• Here 0 / 1, True / False
• Associating meaning to the reading
• Open / Closed
• Empty / Full
• In Position / Not
• Depends on the specific device taking the reading
• The Context of the device

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Analog to Digital

• What we want
• How we have to get there

Physical Phenomena
Engineering Units
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Ratiometric sensor

• Va Vacc Rsens / (Rcomp Rsens)
• use Vref Vacc
• D M Rsens / (Rcomp Rsens)

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Sampling Basics

• How do we represent an analog signal?
• As a time series of discrete values
• ? On the MCU read the ADC data register
  periodically

V
Counts
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Sampling Basics

• What do the sample values represent?
• Some fraction within the range of values
• ? What range to use?

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Sampling Basics

• Resolution
• Number of discrete values that represent a range
  of analog values
• MSP430 12-bit ADC
• 4096 values
• Range / 4096 Step
• Larger range ? less information
• Quantization Error
• How far off discrete value is from actual
• ½ LSB ? Range / 8192
• Larger range ? larger error

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Sampling Basics

• Converting ADC counts ? Voltage
• Converting Voltage ? Engineering Units

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Sampling Basics

• Converting values in 16-bit MCUs
• vtemp adccount/4095 1.5
• tempc (vtemp-0.986)/0.00355
• ? tempc 0
• Fixed point operations
• Need to worry about underflow and overflow
• Avoid divide and (to a lesser degree) multiply
• Floating point operations
• They can be costly on the node, but not
  ridiculous
• Pay attention to overall all contribution to error

command uint16_t TempInt.get() uint16_t tval
(uint32_t)760(uint32_t)val/4096 468 return
tval
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Sampling Basics

• What sample rate do we need?
• Too little we cant reconstruct the signal we
  care about
• Too much waste computation, energy, resources
• Example
• 2-bytes per sample, 4 kHz ? 8 kB / second
• But the mote only has 10 kB of RAM

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Shannon-Nyquist Sampling Theorem

• If a continuous-time signal contains no
  frequencies higher than , it can be
  completely determined by discrete samples taken
  at a rate
• Example
• Humans can process audio signals 20 Hz 20 KHz
• Audio CDs sampled at 44.1 KHz
• Need to ensure there is no appreciable energy
  above 2x sample.

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Sampling Basics

• Aliasing
• Different frequencies are indistinguishable when
  they are sampled.
• Condition the input signal using a low-pass
  filter
• Removes high-frequency components
• (a.k.a. anti-aliasing filter)

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Sampling Basics

• Dithering
• Quantization errors can result in large-scale
  patterns that dont accurately describe the
  analog signal
• Introduce random (white) noise to randomize the
  quantization error.

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Analog-to-Digital Basics

• So, how do you convert analog signals to a
  discrete values?
• A software view
• Set some control registers
• Specify where the input is coming from (which
  pin)
• Specify the range (min and max)
• Specify characteristics of the input signal
  (settling time)
• Enable interrupt and set a bit to start a
  conversion
• When interrupt occurs, read sample from data
  register
• Wait for a sample period
• Repeat step 1

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Block Diagram (MSP430)
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ADC Features
Texas Instruments MSP430 Atmel ATmega 1281
Resolution 12 bits 10 bits
Sample Rate 200 ksps 76.9 ksps
Internally Generated Reference Voltage 1.5V, 2.5V, Vcc 1.1V, 2.56V
Single-Ended Inputs 12 16
Differential Inputs 0 14 (4 with gain amp)
Left Justified Option No Yes
Conversion Modes Single, Sequence, Repeated Single, Repeated Sequence Single, Free Running
Data Buffer 16 samples 1 sample
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ADCs Resources or Computation

• OS provides a convenient and safe abstraction of
  physical resources
• Operating systems deal with devices, not ADCs.
• TinyOS has strived to provide uniform,
  easy-to-use common abstraction of the ADC.
• Should it?
• ADC and how sampling is performed in on the
  datapath of the application.

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ADC Core

• Input
• Analog signal
• Output
• 12-bit digital value of input relative to voltage
  references
• Linear conversion

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SAR ADC

• SAR Successive-Approximation-Register
• Binary search to find closest digital value

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SAR ADC

• SAR Successive-Approximation-Register
• Binary search to find closest digital value

1 Sample ? Multiple cycles
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SAR ADC
1 Sample ? Multiple cycles
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Sample and Conversion Timing

• Timing driven by
• TimerA
• TimerB
• Manually using ADC12SC bit
• Signal selection using SHSx
• Polarity selection using ISSH

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Voltage Reference
Internal External
Vref 1.5V, 2.5V, Vcc VeRef
Vref- AVss VeRef-

• Voltage Reference Generator
• 1.5V or 2.5V
• REFON bit in ADCCTL0
• Consumes energy when on
• 17ms settling time
• External references allow arbitrary reference
  voltage
• Want to sample Vcc, what Vref to use?

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Sample Timing Considerations

• Port 6 inputs default to high impedance
• When sample starts, input is enabled
• But capacitance causes a low-pass filter effect
• ? Must wait for the input signal to converge

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Software Configuration

• How it looks in code
• ADC12CTL0 SHT0_2 REF1_5V
• REFON ADC12ON
• ADC12CTL1 SHP

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Inputs and Multiplexer

• 12 possible inputs
• 8 external pins (Port 6)
• 1 Vref (external)
• 1 Vref- (external)
• 1 Thermistor
• 1 Voltage supply
• External pins may function as Digital I/O or ADC.
• P6SEL register
• What sort of a MUX is this?

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Conversion Memory

• 16 sample buffer
• Each buffer configures sample parameters
• Voltage reference
• Input channel
• End-of-sequence
• CSTARTADDx indicates where to write next sample

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Conversion Modes

• Single-Channel Single-Conversion
• Single channel sampled and converted once
• Must set ENC (Enable Conversion) bit each time
• Sequence-of-Channels
• Sequence of channels sampled and converted once
• Stops when reaching ADC12MCTLx with EOS bit
• Repeat-Single-Channel
• Single channel sampled and converted continuously
• New sample occurs with each trigger (ADC12SC,
  TimerA, TimerB)
• Repeat-Sequence-of-Channels
• Sequence of channels sampled and converted
  repeatedly
• Sequence re-starts when reaching ADC12MCTLx with
  EOS bit

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Software Configuration

• How it looks in code
• Configuration
• ADC12CTL0 SHT0_2 REF1_5V
• REFON ADC12ON
• ADC12CTL1 SHP
• ADC12MCTL0 EOS SREF_1
• INCH_11
• Reading ADC data
• m_reading ADC12MEM0

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A Software Perspective
command void Read.read() ADC12CTL0 SHT0_2
REF1_5V REFON ADC12ON ADC12CTL1
SHP ADC12MCTL0 EOS SREF_1 INCH_11 call
Timer.startOneShot( 17 ) event void
Timer.fired() ADC12CTL0 ENC ADC12IE
1 ADC12CTL0 ADC12SC task void
signalReadDone() signal Read.readDone(
SUCCESS, m_reading ) async event void
HplSignalAdc12.fired() ADC12CTL0 ENC
ADC12CTL0 0 ADC12IE 0 ADC12IFG
0 m_reading ADC12MEM0 post
signalReadDone()
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A Software Perspective
command void Read.read() ADC12CTL0 SHT0_2
REF1_5V REFON ADC12ON ADC12CTL1
SHP ADC12MCTL0 EOS SREF_1 INCH_11 call
Timer.startOneShot( 17 ) event void
Timer.fired() ADC12CTL0 ENC ADC12IE
1 ADC12CTL0 ADC12SC task void
signalReadDone() signal Read.readDone(
SUCCESS, m_reading ) async event void
HplSignalAdc12.fired() ADC12CTL0 ENC
ADC12CTL0 0 ADC12IE 0 ADC12IFG
0 m_reading ADC12MEM0 post
signalReadDone()
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A Software Perspective
command void Read.read() ADC12CTL0 SHT0_2
REF1_5V REFON ADC12ON ADC12CTL1
SHP ADC12MCTL0 EOS SREF_1 INCH_11 call
Timer.startOneShot( 17 ) event void
Timer.fired() ADC12CTL0 ENC ADC12IE
1 ADC12CTL0 ADC12SC task void
signalReadDone() signal Read.readDone(
SUCCESS, m_reading ) async event void
HplSignalAdc12.fired() ADC12CTL0 ENC
ADC12CTL0 0 ADC12IE 0 ADC12IFG
0 m_reading ADC12MEM0 post
signalReadDone()
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A Software Perspective
command void Read.read() ADC12CTL0 SHT0_2
REF1_5V REFON ADC12ON ADC12CTL1
SHP ADC12MCTL0 EOS SREF_1 INCH_11 call
Timer.startOneShot( 17 ) event void
Timer.fired() ADC12CTL0 ENC ADC12IE
1 ADC12CTL0 ADC12SC task void
signalReadDone() signal Read.readDone(
SUCCESS, m_reading ) async event void
HplSignalAdc12.fired() ADC12CTL0 ENC
ADC12CTL0 0 ADC12IE 0 ADC12IFG
0 m_reading ADC12MEM0 post
signalReadDone()
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A Software Perspective
command void Read.read() ADC12CTL0 SHT0_2
REF1_5V REFON ADC12ON ADC12CTL1
SHP ADC12MCTL0 EOS SREF_1 INCH_11 call
Timer.startOneShot( 17 ) event void
Timer.fired() ADC12CTL0 ENC ADC12IE
1 ADC12CTL0 ADC12SC task void
signalReadDone() signal Read.readDone(
SUCCESS, m_reading ) async event void
HplSignalAdc12.fired() ADC12CTL0 ENC
ADC12CTL0 0 ADC12IE 0 ADC12IFG
0 m_reading ADC12MEM0 post
signalReadDone()
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Interrupts and Tasks
command void Read.read() ADC12CTL0 SHT0_2
REF1_5V REFON ADC12ON ADC12CTL1
SHP ADC12MCTL0 EOS SREF_1 INCH_11 call
Timer.startOneShot( 17 ) event void
Timer.fired() ADC12CTL0 ENC ADC12IE
1 ADC12CTL0 ADC12SC task void
signalReadDone() signal Read.readDone(
SUCCESS, m_reading ) async event void
HplSignalAdc12.fired() ADC12CTL0 ENC
ADC12CTL0 0 ADC12IE 0 ADC12IFG
0 m_reading ADC12MEM0 post
signalReadDone()
Application
Kernel
Driver
MCU
ADC
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Interrupts and Tasks

• Tasks are run-to-completion
• Used to signal application events
• Break up computation in the application
• Interrupts
• Generated by the hardware
• Preempt execution of tasks
• Interrupts and tasks can schedule new tasks

Task
Task
Task
Handler
Interrupt
Hardware
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TinyOS Generic Components

• Multiple instances of a component
• Type polymorphism
• Compile-time configuration
• All of the above

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TinyOS Parameterized Interface

• Logically related array of interfaces
• Improved code by handling all interfaces
  collectively
• Compile time sizing across module boundaries
• Basis fo discovery