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5
Introduction to Embedded System DesignProfessor Dhananjay V. GadreElectronics and Communication EngineeringNetaji Subhas University of TechnologyBadri SubudhiElectrical Engineering DepartmentIndian Institute of Technology, JammuLecture 02Introduction Continued with Project DemosWelcomeback.Sowearegoingtoresumeourdiscussionsandinthissessionwearegoingtocompare embedded systems and general purpose computing systems.(Refer Slide Time: 00:33)Sothefirstdifferenceisthatembeddedsystemsarededicatedforspecifictasks.Theymaybeonetaskortheremaybefewtasks,theremaybemorethanonetask,butthatnumberisfixed.Asagainstgeneralpurposecomputingsystemwhereyoucanputanyapplications,youcanadd applications embedded systems are different from such general purpose system.AsanexampleifIhaveamicrowaveoventheembeddedcomputerinthemicrowaveovenisdesignedonlytocookfood.IcannotreprogramitandexpectittowashclothesthatisakindofglaringdifferencethatIhaveandbetweenanembeddedcomputerandageneralpurposecomputer.Theseconddifferenceisthatembeddedcomputerscanbeimplementedusingawidevarietyofprocessors.Asyousawaprocessormicrocontrollersmallerthanagrainofriceandonthe
otherextremebigmicrocontrollermorethananinchsquare,butwhenyoulookatgeneralpurposecomputingdevicesusuallyyouusethehighestperformanceprocessorthatisavailable. The third difference is that embedded systems are cost sensitive.Whatisthemeaning?Becauseyouarebuyingaproductyouarenotbuyingacomputer.Theembeddedcomputerthatformpartofthatembeddedsystemgadgetcarrythesmallfractionofcostandthereforeembeddedcomputers,embeddedsystemsareverycostsensitiveandthefourthdifferenceisthatembeddedsystemsaresupposedtoofferandworkunderrealtimeconstraints.Themeaningisthatwheneverthereisainputchange,environmentalchange,theembeddedsystemsaresupposedtoreacttothatchangeandprovideanoutputinatimelyfashion.Ifthereisdelayinproducingtheoutputitmayleadtofatality,itmayleadtolossoflife.Asanexampleiftheembeddedcomputerwhichisflyinganaircraftandthepilotwantstheaircrafttoclimbandtheembeddedcomputerdoesnotperformappropriatelyandkeepsonflyingatthe same level it may lead to a crash.Nowyoudonotexpectsuchabehavioroutofageneralpurposecomputingsystemsoftentimeyoumayhavenoticedthatyoupressakeyandyourlaptopordesktopdoesnotresponditisallrightbecauseitisnotgoingtokillanybodyandsothereisvastdifferencebetweentheexpectation of an embedded computer in terms of requiring real time response.(Refer Slide Time: 03:10)
Thefourthdifferenceorfifthdifferenceisthatembeddedsystemsaredesignedtooperateinanextremeenvironmentalconditions.Takeaexampleofmissilesystemthatmissilecouldbelaunchedfromglacierenvironmentoritcouldbelaunchfromdesertsandtheymustperform.Usually embedded systems run out of a ROM.MeaningthattheprogramthatisstoredintheembeddedcomputerisstoredintheROMthatisreadonlymemoryapermanentmemory.ButoftentimesnotoftentimesmostofthetimestheprogramthatrunsonyourdesktoporlaptopcomputerisrunningintheRAM.YesadesktoporalaptopsystemalsohasaROMwhichisusedatthebeginningwhenyoupoweronyourdeviceandthatprogramiscalledbootupprogramcallBIOSBasicInputOutputSystem.Oncethatcompletesexecutionthetransferthecontrolistransferredtotheoperatingsystemontheharddiskandthentotheprogramthatyouwouldliketorun.TheseprogramsareloadedintheRAMofthegeneralpurposecomputingsystemsothatisavastdifferencebetweenhowanembeddedsystemoperatesandhowageneralpurposecomputingsystemoperates.Anotherdifferenceisthatembeddedsystemhasresourceconstraints.Whatareresources of a computer?Thecomputationalcapabilityisaresourcethewaytheembeddedsystemorthecomputercommunicateswiththeoutsideworldisresourceandyouseewhenyouhaveageneralpurposecomputingsystemsuchasyourlaptopordesktopithasmanywaysofcommunicatingwiththeoutsideworld.IthasEthernet,ithasWiFi,ithasFireWire,ithasUSB and so on.ButdoesyourmicrowavehaveEthernetconnectioncanyoucontrolitwithyourphonethroughBluetoothmostprobablynot.Although,inrecenttimestherearecertainexamplesofsuchembeddedapplicationswithconnectivity,butthatconnectivityisbasedontherequirementnotbecauseyouwanttoflauntthatyourcomputerhassomanywaysofcommunicating with the outside world.Soembeddedsystemshaveresourceconstraints,thereareexamplesofforexampleawashingmachinewhichiscalledIOTwashingmachinethatwashingmachinecancommunicatewiththeoutsideworldthroughinternetandthepurposeisnotthatyouwouldwanttocontrolthiswashing machine sitting in your office.
Butwhenthewashingmachineisoperatingathomeandencounterslossinperformanceitcancommunicatethattotheserviceproviderthemaintenanceguysothattheycancomeandprovidepreventivemaintenancebeforethewashingmachineactuallystopsworking.Sothisisanexamplewhereyouhaveacommunicationprotocol,communicationlinkonyourembeddeddevicebutitisoutofnecessityasafeaturenotbecauseyouhaveyouwanttosaythat your washing machine has so many ways of communicating with the outside world.Embeddedsystemsareinfrequentlyreprogrammed.Whatisthemeaning?Thatonceyouloadaprograminanembeddedapplicationsyoudonoteverchangeit,youdonotchangeitmostofthetimesandinmostcasesyoudonoteverchangeit,butinageneralpurposecomputingdeviceeverytimeyourunanapplicationsanewapplicationyouarereprogrammingthatdevice.HowmanytimeshaveyouencounteredasituationthatsomebodyhascometoyourhomesayingthattheywouldliketoupgradethefirmwareofyourwashingmachineIhaveneverencounteredit,itdoesnothappenandsothatisthemeaningofthispointthatembeddedsystemsareinfrequentlyreprogrammed.Embeddedsystemshavehardreliabilityandcorrectness constraints.Allcomputersmustoperatecorrectly,butembeddedsystemshaveanadditionalresponsibilitythattheymustcontinuetoprovideservicereliablynotallmostofthem,manyofthem,someofthem.Whybecausefailuretodosomayleadtolossoflife.Asanexamplemedicaldeviceslikeanx-raymachineyoudonotwantthecomputerinthex-raymachinetofail and it may give needlessly high dose of x-ray to the patient under the x-ray machine.Andinanaircraftyoudonotwantthecomputerflyingtheaircrafttofailleadingtolossoflifeandsoembeddedcomputersareexpectedsomeoftheembeddedcomputersareexpectedto have hard reliability requirements.
(Refer Slide Time: 07:50)Nowthatwehaveseenmajordifferencesbetweenembeddedcomputersandgeneralpurposecomputingdevicesitistimetogothroughcertainterminologiesthatweuse.Andformanyofyouorsomeofyouitmaybearepeatofthingsthatyoualreadyknow,butitdoesnotheard.Sothefirstterminologyiswhatisacomputer?Acomputerisnothing,butasystemwhichhasaCPUthatisacentralprocessingunit,memoryandinputoutputports.Letmedrawadiagram to illustrate it.(Refer Slide Time: 08:27)SothisisyourCPU,thisisyourmemoryandusuallyyouwouldrequiretwotypesofmemories.OneistheROMwhichstoresyourprogramtheotheristheRAMwhichweuse
forvariablesandthenyouwouldhavecertainIOdevicesmaybeaninputdeviceandanoutputdeviceletmeerasethisandwriteithereandhowdoestheCPUcommunicatewiththese devices through buses.Thesebusesarefordataandforaddressandanothersetofbusiswhatwecallasthecontrolbuswhichtellswhetheryouarereadinginformationorwritinginformationandallthese.SothisisthediagramforacomputerhereistheCPU,ROM,RAMthatismemoryandinputoutput ports.(Refer Slide Time: 10:18)Thesecondterminologyisamicroprocessor.AmicroprocessorisnothingbutaCPUonasinglechip.Howdidithappen?Inthe50sand60stheCPUwasmadewithdiscretecomponentsordiscreteintegratedcircuitsinthe60s.In1971,astartupatthattimethatwascalledIntel.AndtodayIntelisagiantmadeaninterestingdevicewhatitdidwasitintegratedthe functionality of a CPU on the single substrate of silicon. What does a CPU entail?