往屆10通信2文獻(xiàn)理工科技學(xué)院

CANBusControllerAreaNetwork(CAN)wasinitiallycreatedbyGermanautomotivesystemrRobertBoschinthemid-1980sforautomotiveapplicationsasamethodforenablingrobustserialcommunication.Thegoalwastomakeautosmorereliable,safeandfuel-efficientwhiledecreasingwiringharnessweightandcomplexity.Sinceitsinception,theCANprotocolhasgainedwidespreadpopularityinindustrialautomationandautomotive/truckapplications.Othermarketswherenetworkedsolutionscanbringattractivebenefitslikemedicalequipment,testequipmentandmachinesarealsostartingtoutilizethebenefitsofCAN.ThegoalofthisapplicationnoteistoexinsomeofthebasicsofCANandshowthebenefitsofchoosingCANforembeddedsystemsnetworkedapplications.CANMostnetworkapplicationsfollowalayeredapproachtosystemimplementation.Thissystemeticapproachenablessinteroperabilitybetweenproductsfromdifferentmanufacturers.AstandardwascreatedbytheInternationalStandardsOrganization(ISO)asatemtetofollowforthislayeredapproach.ItiscalledtheISOOpenSystemsInterconnection(OSI)NetworkLayeringReferenceModel.TheCANprotocolitselfimplementsmostofthelowertwolayersofthisreferencemodel.ThecommunicationmediumportionofthemodelwaspurposelyleftoutoftheBoschCANspecificationtoenablesystemdesignerstoadaptandoptimizethecommunicationprotocolonmultiplemediaforumflexibility(twistedpair,singlewire,opticallyisolated,RF,IR,etc.).Withthisflexibility,however,comesthepossibilityofinteroperabilityconcerns.Toeasesomeoftheseconcerns,theInternationalStandardsOrganizationandSocietyofAutomotiveEngineers(SAE)havedefinedsomebasedonCANthatincludetheMediaDependentsInterfacedefinitionsuchthatallofthelowertwolayersarespecified.ISO11898isastandardforhigh-speedapplications,ISO11519isastandardforlow-speedapplications,andJ1939(fromSAE)istargetedfortruckandbusapplications.Allthreeoftheseprotocolsspecifya5Vdifferentialelectricalbusasthephysicalinterface.TherestofthelayersoftheISO/OSIprotocolstackarelefttobeimplementedbythesystemsoftwaredeveloper.HigherLayerProtocols(HLPs)aregenerallyusedtoimplementtheupperfivelayersoftheOSIReferenceModel.HLPsareusedStandardizestartupproceduresincludingbitratesDistributeaddressesamongparticipatingnodesortypesofDeterminethestructureofthemessages,Providesystem-levelerrorhandlingroutines.ThisisbynomeansafulllistofthefunctionsHLPsperform;howeveritdoesdescribesomeoftheirbasicfunctionality.CANProtocolCarrierSenseMultipleAccesswithCollisionDetectionTheCANcommunicationprotocolisaCSMA/CDprotocol.TheCSMAstandsforCarrierSenseMultipleAccess.Whatthismeansisthateverynodeonthenetworkmustmonitorthebusforaperiodofnoactivitybeforetryingtosendamessageonthebus(CarrierSense).Also,oncethisperiodofnoactivityoccurs,everynodeonthebushasanequalopportunitytotransmitamessage(MultipleAccess).TheCDstandsforCollisionDetection.Iftwonodesonthenetworkstarttransmittingatthesametime,thenodeswilldetectthe‘collision’andtaketheappropriateaction.InCANprotocol,anondestructivebitwisearbitrationmethodisutilized.Thismeansthatmessagesremainintactafterarbitrationiscompletedevenifcollisionsaredetected.Allofthisarbitrationtakescewithoutcorruptionordelayofthehigherprioritymessage.Thereareacoupleofthingsthatarerequiredtosupportnon-destructivebitwisearbitration.First,logicstatesneedtobedefinedasdominantorrecessive.Second,thetransmittingnodemustmonitorthestateofthebustoseeifthelogicstateitistosendactuallyappearsonthebus.CANdefinealogicbit0asadominantbitandalogicbit1asarecessivebit.Adominantbitstatewillalwayswinarbitrationoverarecessivebitstate,thereforethelowerthevalueintheMessageIdentifier(thefieldusedinthemessagearbitrationprocess),thehigherthepriorityofthemessage.Asanexample,supposetwonodesaretryingtotransmitamessageatthesametime.Eachnodewillmonitorthebustomakesurethebitthatitistryingtosendactuallyappearsonthebus.Thelowerprioritymessagewillatsomepointtrytosendarecessivebitandthemonitoredstateonthebuswillbeadominant.Atthatpointthisnodelosesarbitrationandimmediaystopstransmitting.Thehigherprioritymessagewillcontinueuntilcompletionandthenodethatlostarbitrationwillwaitforthenextperiodofnoactivityonthebusandtrytotransmititsmessageagain.Message-BasedCANprotocolisamessage-basedprotocol,notanaddressbasedprotocol.Thismeansthatmessagesarenottransmittedfromonenodetoanothernodebasedonaddresses.EmbeddedintheCANmessageitselfisthepriorityandthecontentsofthedatabeingtransmitted.Allnodesinthesystemreceiveeverymessagetransmittedonthebus(andwillacknowledgeifthemessagewasproperlyreceived).Itisuptoeachnodeinthesystemtodecidewhetherthemessagereceivedshouldbeimmediaydiscardedorkepttobeprocessed.Asinglemessagecanbedestinedforoneparticularnodetoreceive,ormanynodesbasedonthewaythenetworkandsystemaredesigned.Forexample,anautomotiveairbagsensorcanbeconnectedviaCANtoasafetysystemrouternodeonly.Thisrouternodetakesinothersafetysysteminformationandroutesittoallothernodesonthesafetysystemnetwork.Thenalltheothernodesonthesafetysystemnetworkcanreceivethelatestairbagsensorinformationfromtherouteratthesametime,acknowledgeifthemessagewasreceivedproperly,anddecidewhethertoutilizethisinformationordiscardit.AnotherusefulfeaturebuiltintotheCANprotocolistheabilityforanodetorequestinformationfromothernodes.ThisiscalledaRemoteTransmitThisisdifferentfromtheexampleinthepreviousparagraphbecauseinsteadofwaitingforinformationtobesentbyaparticularnode,thisnodespecificallyrequestsdatatobesenttoit.Oneadditionalbenefitofthismessage-basedprotocolisthatadditionalnodescanbeaddedtothesystemwithoutthenecessitytoreprogramallothernodestorecognizethisaddition.Thisnewnodewillstartreceivingmessagesfromthenetworkand,basedonthemessageID,decidewhethertoprocessordiscardthereceivedinformation.CANMessageFrameCANprotocoldefinefourdifferenttypesofmessages(orFrames).ThefirstandmostcommontypeofframeisaDataFrame.Thisisusedwhenanodetransmitsinformationtoanyorallothernodesinthesystem.SecondisaRemoteFrame,whichisbasicallyaDataFramewiththeRTRbitsettosignifyitisaRemoteTransmitRequest.Theothertwoframetypesareforhandlingerrors.OneiscalledanErrorFrameandoneiscalledanOverloadFrame.ErrorFramesaregeneratedbynodesthatdetectanyoneofthemanyprotocolerrorsdefinedbyCAN.Overloaderrorsaregeneratedbynodesthatrequiremoretimetoprocessmessagesalreadyreceived.DataFramesconsistoffieldsthatprovideadditionalinformationaboutthemessageasdefinedbytheCANspecification.EmbeddedintheDataFramesareArbitrationFields,ControlFields,DataFields,CRCFields,a2-bitAcknowledgeFieldandanEndofFrame.TheArbitrationFieldisusedtoprioritizemessagesonthebus.SincetheCANprotocoldefinesalogical0asthedominantstate,thelowerthenumberinthearbitrationfield,thehigherprioritythemessagehasonthebus.Thearbitrationfieldconsistsof12-bits(11identifierbitsandoneRTRbit)or32-bits(29identifierbits,1-bittodefinethemessageasanextendeddataframe,anSRRbitwhichisunused,andanRTRbit),dependingonwhetherStandardFramesorExtendedFramesarebeingutilized.ThecurrentversionoftheCANspecification,version2.0B,defines29-bitidentifiersandcallsthemExtendedFrames.PreviousversionsoftheCANspecificationdefined11-bitidentifierswhicharecalledStandardFrames.Asdescribedintheprecedingsection,theRemoteTransmitRequest(RTR)isusedbyanodewhenitrequiresinformationtobesenttoitfromanothernode.To anRTR,aRemoteFrameissentwiththeidentifieroftherequiredDataFrame.TheRTRbitintheArbitrationFieldisutilizedtodifferentiatebetweenaRemoteFrameandaDataFrame.IftheRTRbitisrecessive,thenthemessageisaRemoteFrame.IftheRTRbitisdominant,themessageisaDataFrame.TheControlFieldconsistsofsixbits.TheMSBistheIDEbit(signifiesExtendedFrame)whichshouldbedominantforStandardDataFrames.ThisbitdeterminesifthemessageisaStandardorExtendedFrame.InExtendedFrames,thisbitisRB1anditis.ThenextbitisRB0anditisalso.ThefourLSBsaretheDataLengthCode(DLC)bits.TheDataLengthCodebitsdeterminehowmanydatabytesareincludedinthemessage.ItshouldbenotedthataRemoteFramehasnodatafield,regardlessofthevalueoftheDLCbits.TheDataFieldconsistsofthenumberofdatabytesdescribedintheDataLengthCodeoftheControlField.TheCRCFieldconsistsofa15-bitCRCfieldandaCRCdelimiter,andisusedbyreceivingnodestodetermineiftransmissionerrorshaveoccurred.TheAcknowledgeFieldisutilizedtoindicateifthemessagewasreceivedcorrectly.Anynodethathascorrectlyreceivedthemessage,regardlessofwhetherthenodeprocessesordiscardsthedata,putsadominantbitonthebusintheACKSlotbitThelasttwomessagetypesareErrorFramesandOverloadFrames.WhenanodedetectsoneofthemanytypesoferrorsdefinedbytheCANprotocol,anErrorFrameoccurs.OverloadFrameslthenetworkthatthenodesendingtheOverloadFrameisnotreadytoreceiveadditionalmessagesatthistime,orthatintermissionhasbeenviolated.Theseerrorswillbediscussedinmoredetailinthenextsection.Fast,RobustBecauseCANwasinitiallydesignedforuseinautos,aprotocolthatefficientlyhandlederrorswascriticalifitwastogainmarketacceptance.Withreleaseofversion2.0BoftheCANspecification,theumcommunicationratewasincreased8xovertheversion1.0specificationto1Mbit/sec.Atthisrate,eventhemosttime-criticalparameterscanbetransmittedseriallywithoutlatencyconcerns.Inadditiontothis,theCANprotocolhasacomprehensivelistoferrorsitcandetectthatensurestheintegrityofmessages.CANnodeshavetheabilitytodeterminefaultconditionsandtransitiontodifferentmodesbasedontheseverityofproblemsbeingencountered.Theyalsohavetheabilitytodetectshortdisturbancesfrompermanentfailuresandmodifytheirfunctionalityaccordingly.CANnodescantransitionfromfunctioninglikeanormalnode(beingabletotransmitandreceivemessagesnormally),toshuttingdowncompley(bus-off)basedontheseverityoftheerrorsdetected.ThisfeatureiscalledFaultConfinement.NofaultyCANnodeornodeswillbeabletomonopolizeallofthebandwidthonthenetworkbecausefaultswillbeconfinedtothefaultynodesandthesefaultynodeswillshutoffbeforebringingthenetworkdown.ThisisverypowerfulbecauseFaultConfinementguaranteesbandwidthforcriticalsysteminformation.TheCANprotocolwasoptimizedforsystemsthatneedtotransmitandreceiverelativelysmallamountsofinformation(ascomparedtoEthernetorUSB,whicharedesignedtomovemuchlargerblocksofdata)reliablytoanyorallothernodesonthenetwork.CSMA/CDallowseverynodetohaveanequalchancetogainaccesstothebus,andallowsforsmoothhandlingofcollisions.Sincetheprotocolismessage-based,notaddressbased,allmessagesonthebusreceiveeverymessageandacknowledgeeverymessage,regardlessofwhetherinneedsthedataornot.Thisallowsthebustooperateinnode-to-nodeormulticastmessagingformatswithouthavingtosenddifferenttypesofmessages.Fast,robustmessagetransmissionwithfaultconfinementisalsoabigplusforCANbecausefaultynodeswillautomaticallydropoffthebusnotallowinganyonenodefrombringinganetworkdown.Thiseffectivelyguaranteesthatbandwidthwillalwaysbeavailableforcritic