computing for embedded systems - ptolemy...

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State of the Art LectureIEEE Instrumentation and Measurement

Technology ConferenceBudapest, Hungary, May 21-23, 2001

Computing for Embedded Systems

Edward A. LeeUC Berkeley

computers without actuators andsensors are destined to look like this.

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What is an Embedded System?One or more computers

but not first-and-foremost a computerInteraction with physical processes

sensors, actuators, processesReactive

operating at the speed of the environmentHeterogeneous

hardware/software, mixed architecturesNetworked

adaptive software, shared data, resource discovery

primeexample

today

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Why is Embedded SW an Issue?Embedded systems becoming networked

more complex, more vulnerablecan no longer use static point designs

Focus on non-functional properties is new for SWreal-time, fault recovery, power, security, robustness

Neglected areacomputer science has largely ignored itbest-of-class methods don’t help much

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E.g. Object-Oriented DesignCall/return imperative semanticsConcurrency is via ad-hoc calling conventions

band-aids: futures, proxies, monitorsPoorly models the environment

which does not have call/return semanticsNothing at all to say about time

ComponentEntityCompositeEntity

AtomicActor

CompositeActor

0..10..n

«Interface»Actor

+getDirector() : Director+getExecutiveDirector() : Director+getManager() : Manager+inputPortList() : List+newReceiver() : Receiver+outputPortList() : List

«Interface»Executable

+fire()+initialize()+postfire() : boolean+prefire() : boolean+preinitialize()+stopFire()+terminate()+wrapup()

Director

Object modelingemphasizes inheritanceand procedural interfaces.

We need to emphasizesconcurrency andcommunicationabstractions.

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E.g. Real-Time Corba

Component specification includesworst case execution timetypical execution timecached execution timepriorityfrequencyimportance

This is an elaborate prayer…

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Alternative View of SW Architecture:Actors with Ports and Attributes

PortPort

Actor ActorLink

Relation

ActorPort

connection

connection conn

ectio

n

Link

Link

Attributes Attributes

Attributes

Model of Computation:

• Messaging schema• Flow of control• Concurrency

Examples:

• Time triggered• Process networks• Discrete-event systems• Dataflow systems• Publish & subscribe

Key idea: The model of computation is part of the frameworkwithin which components are embedded rather than part of thecomponents themselves.

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Examples of Actors+PortsSoftware Architectures

Simulink (The MathWorks)Labview (National Instruments)OCP, open control platform (Boeing)SPW, signal processing worksystem (Cadence)System studio (Synopsys)ROOM, real-time object-oriented modeling (Rational)Port-based objects (U of Maryland)I/O automata (MIT)VHDL, Verilog, SystemC (Various)Polis & Metropolis (UC Berkeley)Ptolemy & Ptolemy II (UC Berkeley)…

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What a Program Might Look Like

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Ptolemy II – An Open SourceSoftware Laboratory

Ptolemy II –

Emphasis is on building a frameworksupporting experimentation withmodels of computation and theirinteractions.

http://ptolemy.eecs.berkeley.edu

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Example: Controlling anInverted Pendulum

The Furuta pendulum hasa motor controlling theangle of an arm, fromwhich a free-swingingpendulum hangs. Theobjective is to swing thependulum up and thenbalance it.

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Execution

An execution of themodel displays varioussignals and at the bottomproduces a 3-D animationof the physical system.

Model by Johan Eker

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Top-Level Model

The top-level is a continuous-time model that specifies thedynamics of the physical system as a set of nonlinear ordinarydifferential equations, and encapsulates a closed loop controller.

Framework by Jie Liu

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Higher-Order Components

Complexsubmodels aregeneratedautomaticallyfrom equationsdescribing thephysicaldynamics. Framework by Jie Liu

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A Modal Controller

The controlleritself is modal,with three modesof operation,where a differentcontrol law isspecified foreach mode.

Framework by Xiaojun Liu

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The Discrete Controllers

Three discretesubmodels(dataflowmodels) specifycontrol laws foreach of threemodes ofoperation.

Framework by Steve Neuendorffer

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The Graphical AnimatorA 3-D graphicalanimation isconstructedmodularly as aparametricscene graphthat is driven bythe physicalmodel.

Framework by Chamberlain Fong

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The Key Idea

Components are actors with portsInteraction is governed by a model of computation

flow of controlmessaging protocols

So what is a model of computation?

It is the “laws of physics” governing theinteraction between componentsIt is the modeling paradigm

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Model of Computation

What is a component? (ontology)States? Processes? Threads? Differential equations?Constraints? Objects (data + methods)?

What knowledge do components share? (epistemology)Time? Name spaces? Signals? State?

How do components communicate? (protocols)Rendezvous? Message passing? Continuous-time signals?Streams? Method calls? Events in time?

What do components communicate? (lexicon)Objects? Transfer of control? Data structures? ASCII text?

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Domains – Ptolemy II Realizations ofModels of Computation

CSP – concurrent threads with rendezvousCT – continuous-time modelingDE – discrete-event systemsDDE – distributed discrete-event systemsDT – discrete time (cycle driven)FSM – finite state machinesGiotto – time driven cyclic modelsGR – graphicsPN – process networksSDF – synchronous dataflowxDF – other dataflow

Each of these defines a component ontology and an interactionsemantics between components. There are many morepossibilities!

Each isrealized as adirector and areceiver classin Ptolemy II

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Heterogeneity – Hierarchical Mixturesof Models of Computation

Modal ModelsFSM + anything

Hybrid systemsFSM + CT

Mixed-signal systemsDE + CTDT + CT

Complex systemsResource managementSignal processingReal time

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Domain

Domain

Domain

Hierarchical, Compositional Models

Schedulers (Directors) arenested hierarchically,each interacting withcomponents through theExecutable interface.Directors themselvesimplement this sameinterface, so the model iscompositional.

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Key Advantages

Domains are specializedleantargetedoptimizableunderstandable

Domains are mixable (hierarchically)structureddisciplined interactionunderstandable interaction

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Ptolemy IIOpen source frameworkBlock diagram user interfaceXML persistent file formatJava basedNetwork integratedFully extensibleExperiment with models of computation

http://ptolemy.eecs.berkeley.edu

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Example Ptolemy II Experiment:Connecting to Smart Sensors

Ethernet Hub

AgilentNCAP

TelemonitorTilt sensor

1451.2

Simple demonstration project forDARPA software-enabled control(SEC) project.

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Networked Smart Sensors

Ethernet

AgilentNCAP

TelemonitorTilt sensor

1451.2

HTTPinterface

HTTP queries

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Abstraction of the Sensoras a Software Component

Ethernet

AgilentNCAP

TelemonitorTilt sensor

1451.2

HTTP queries

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Discovery – Realizing More SpecializedCommunication with the Sensors

Ethernet

AgilentNCAP

TelemonitorTilt sensor

Hub

JINI

1451.2

Register a Service

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Ethernet

AgilentNCAP

TelemonitorTilt sensor

Hub

JINI

1451.2

Discover a Service

Discovery – Realizing More SpecializedCommunication with the Sensors

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Ethernet

AgilentNCAP

TelemonitorTilt sensor

Hub

JINI

1451.2

Download Interface Software

Discovery – Realizing More SpecializedCommunication with the Sensors

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Ethernet

AgilentNCAP

TelemonitorTilt sensor

Hub

JINI

1451.2

Communicate

This wouldprovide a vendor-neutral softwarecomponent.

Discovery – Realizing More SpecializedCommunication with the Sensors

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Smart Sensor + Ptolemy II

Tilt sensor connected to a plotter.

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Actuator Setup

Proxy

serial port

IR tower

Lego Mindstorm

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Distributed Lego Controller

Tilt sensor data published, controller subscribes.

left = 4 * xTilt - 2 * yTiltright = 4 * xTilt + 2 * yTilt

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Issues Raised

Concurrency management with I/O

Separate threads handling I/O communication?Rendezvous with computational thread?Publish & subscribe buffering strategy?How to maintain time consistency?How to ensure no deadlock?Is HTTP a good way to communicate with sensors?

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Conclusions

Actors with ports are better than objects withmethods for embedded system design.

Models of computation matter a great deal, andspecialization can help.

Ptolemy II provides an excellent openarchitecture for experimentation.

http://ptolemy.eecs.berkeley.edu