Wireless Embedded InterNet working Foundations of Ubiquitous Sensor Networks Triggers and Sensing

June 2008 WEI - L05 sense 1

Wireless Embedded InterNetworking

Foundations of Ubiquitous Sensor Networks

Triggers and Sensing

David E. CullerUniversity of California, Berkeley

June 2008WEI - L05 sense 2

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


An Analog World

• Transducers– Allow us to convert physical phenomena to a voltage

potential in a well-defined way.

R ohm ?

V

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Simplest Analog Device

• Often think of it as an actuator, rather than a sensor– But that’s because of the circuit we put it in

• It is binary (two states) but why is it not digital?

switch

Rain Sensor

Magnetic Reed Contact Switch

Tilt Sensor

Water Level Float Sensor

PhotoInterrupter

Flow Sensor

TemperatureSwitch

Pressure Switch

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To Sample a switch, make it digital

• Many sensor are switches

• Two “states” but not digital– Open => no current

– Closed => no voltage drop

• Cap charges to Vacc when open

• Cap discharges to GND when closed

VD

VtL

VtH

Vacc

GND

switch

D


EPIC I/F Board - Digital Inputs


Getting Input into the MCU


MCU – a system on a chip


Programmed IO


Interrupts

MCU Hardware


Getting a hold of the event

Hardware

Phy

Link

Network

Transport

HplSignal

RadioFlash

MCU

Ports ADCTimers

Sof

twar

e

Kernel

Driver Code

Memory MappedIO registers

Hardware Interrupt

Handler dispatch

event void Boot.booted() {

atomic {

P2IE &= ~PIN7; /* Disable interrupt */

P2IFG &= ~PIN7; /* Clear interrupt flag */

P2DIR &= ~PIN7; /* Configure as input */

P2IES |= PIN7; /* Select Hi->Lo */

P2IE |= PIN7; /* Enable interrupts */

}

}async event void HplSignalPort2.fired() { if ( P2IFG & PIN7 ) { P2IFG &= ~PIN7; post fired(); }

}


Making Sense of Physical Information

• Digital representation of physical phenomenon – Transducer => Signal Conditioning => ADC =>

– 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


Analog to Digital

• What we want

• How we have to get there

SoftwareSensor ADC

PhysicalPhenomena

Voltage ADC Counts Engineering Units

PhysicalPhenomena

Engineering Units


Ratiometric sensor

• Va = Vacc* Rsens / (Rcomp+ Rsens)

• use Vref = Vacc

• D = M * Rsens / (Rcomp+ Rsens)

Vacc

GND

Resistive Sensor

VA

Rcomp

Rsensor


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

)(xf sampled

)(xf

t

ST

V Counts


Sampling Basics

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

What range to use?

rV

tRange Too Small

rV

tRange Too Big

rV

rV

tIdeal Range

rV

rV


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


Sampling Basics

• Converting: ADC counts Voltage

• Converting: Voltage Engineering Units

ADCN

4095

4095

RRADCin

RR

RinADC

VVNV

VV

VVN

t

rV

rV

inV

00355.0

986.0TEMP

986.0)TEMP(00355.0

TEMPC

CTEMP

V

V


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

00355.0

986.0TEMP TEMP

C

V

4095TEMP

RRADC

VVNV

command uint16_t TempInt.get() { uint16_t tval = (uint32_t)760*(uint32_t)val/4096 – 468; return tval;}


Sampling Basics

• What sample rate do we need?– Too little: we can’t 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…

)(xf sampled

)(xf

t


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.

)(xfmaxf

maxsamples 2 ff


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)


Sampling Basics

• Dithering– Quantization errors can result

in large-scale patterns that don’t accurately describe the analog signal

– Introduce random (white) noise to randomize the quantization error.

Direct Samples Dithered Samples


Analog-to-Digital Basics

• So, how do you convert analog signals to a discrete values?

• A software view:1. 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)

2. Enable interrupt and set a bit to start a conversion

3. When interrupt occurs, read sample from data register

4. Wait for a sample period

5. Repeat step 1


Block Diagram (MSP430)


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


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.


ADC Core

• Input– Analog signal

• Output– 12-bit digital value of input

relative to voltage references

• Linear conversion

4095

4095

RRADCin

RR

RinADC

VVNV

VV

VVN

RV

RVinV


SAR ADC

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


SAR ADC• SAR = Successive-Approximation-Register

– Binary search to find closest digital value

1 Sample Multiple cycles


SAR ADC

1 Sample Multiple cycles


Sample and Conversion Timing

• Timing driven by:– TimerA

– TimerB

– Manually using ADC12SC bit

• Signal selection using SHSx

• Polarity selection using ISSH


Voltage Reference

• 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?

Internal External

Vref+ 1.5V, 2.5V, Vcc VeRef+

Vref- AVss VeRef-


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

ns800pF40011.9)kΩ2( Ssample Rt


Software Configuration

• How it looks in code:

ADC12CTL0 = SHT0_2 | REF1_5V |

REFON | ADC12ON;

ADC12CTL1 = SHP;


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?


Conversion Memory

• 16 sample buffer

• Each buffer configures sample parameters

– Voltage reference

– Input channel

– End-of-sequence

• CSTARTADDx indicates where to write next sample


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


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;


post signalReadDone();

}





ADC12CTL1 = SHP;



}


ADC12CTL0 |= ENC;

ADC12IE = 1;


}



}


ADC12CTL0 &= ~ENC;

ADC12CTL0 = 0;

ADC12IE = 0;

ADC12IFG = 0;



}


MCU

Kernel Driver

Interrupts and Tasks

ADC

Application



ADC12CTL1 = SHP;



}


ADC12CTL0 |= ENC;

ADC12IE = 1;


}



}


ADC12CTL0 &= ~ENC;

ADC12CTL0 = 0;

ADC12IE = 0;

ADC12IFG = 0;



}


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

Hardware

Interrupt

Task Task Task

Handler


TinyOS Generic Components

• Multiple instances of a component

• Type polymorphism

• Compile-time configuration

• All of the above


TinyOS Parameterized Interface

• Logically related array of interfaces

• Improved code by handling all interfaces collectively

• Compile time sizing across module boundaries

• Basis fo discovery

Wireless Embedded InterNet working Foundations of Ubiquitous Sensor Networks Triggers and Sensing

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