CAN总线外文文献及译稿文档格式.docx

CAN总线外文文献及译稿文档格式.docx

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Introduction

ControllerAreaNetwork（CAN）wasinitiallycreatedbyGermanautomotivesystemsupplierRobertBoschinthemid-1980sforautomotiveapplicationsasamethodforenablingrobustserialcommunication.Thegoalwastomakeautomobilesmorereliable,safeandfuel-efficientwhiledecreasingwiringharnessweightandcomplexity.Sinceitsinception,theCANprotocolhasgainedwidespreadpopularityinindustrialautomationandautomotive/truckapplications.Othermarketswherenetworkedsolutionscanbringattractivebenefitslikemedicalequipment,testequipmentandmobilemachinesarealsostartingtoutilizethebenefitsofCAN.ThegoalofthisapplicationnoteistoexplainsomeofthebasicsofCANandshowthebenefitsofchoosingCANforembeddedsystemsnetworkedapplications.

CANOverview

Mostnetworkapplicationsfollowalayeredapproachtosystemimplementation.Thissystemeticapproachenablessinteroperabilitybetweenproductsfromdifferentmanufacturers.AstandardwascreatedbytheInternationalStandardsOrganization（ISO）asatemplatetofollowforthislayeredapproach.ItiscalledtheISOOpenSystemsInterconnection（OSI）NetworkLayeringReferenceModel.TheCANprotocolitselfimplementsmostofthelowertwolayersofthisreferencemodel.ThecommunicationmediumportionofthemodelwaspurposelyleftoutoftheBoschCANspecificationtoenablesystemdesignerstoadaptandoptimizethecommunicationprotocolonmultiplemediaformaximumflexibility（twistedpair,singlewire,opticallyisolated,RF,IR,etc.）.Withthisflexibility,however,comesthepossibilityofinteroperabilityconcerns.Toeasesomeoftheseconcerns,theInternationalStandardsOrganizationandSocietyofAutomotiveEngineers（SAE）havedefinedsomeprotocolsbasedonCANthatincludetheMediaDependentsInterfacedefinitionsuchthatallofthelowertwolayersarespecified.

ISO11898isastandardforhigh-speedapplications,ISO11519isastandardforlow-speedapplications,andJ1939（fromSAE）istargetedfortruckandbusapplications.Allthreeoftheseprotocolsspecifya5Vdifferentialelectricalbusasthephysicalinterface.TherestofthelayersoftheISO/OSIprotocolstackarelefttobeimplementedbythesystemsoftwaredeveloper.HigherLayerProtocols（HLPs）aregenerallyusedtoimplementtheupperfivelayersoftheOSIReferenceModel.

HLPsareusedto:

1）Standardizestartupproceduresincludingbitratesused,

2）Distributeaddressesamongparticipatingnodesortypesofmessages,

3）Determinethestructureofthemessages,and

4）Providesystem-levelerrorhandlingroutines.ThisisbynomeansafulllistofthefunctionsHLPsperform;

howeveritdoesdescribesomeoftheirbasicfunctionality.

CANProtocolBasics

CarrierSenseMultipleAccesswithCollisionDetection（CSMA/CD）

TheCANcommunicationprotocolisaCSMA/CDprotocol.TheCSMAstandsforCarrierSenseMultipleAccess.Whatthismeansisthateverynodeonthenetworkmustmonitorthebusforaperiodofnoactivitybeforetryingtosendamessageonthebus（CarrierSense）.Also,oncethisperiodofnoactivityoccurs,everynodeonthebushasanequalopportunitytotransmitamessage（MultipleAccess）.TheCDstandsforCollisionDetection.Iftwonodesonthenetworkstarttransmittingatthesametime,thenodeswilldetectthe‘collision’andtaketheappropriateaction.InCANprotocol,anondestructivebitwisearbitrationmethodisutilized.Thismeansthatmessagesremainintactafterarbitrationiscompletedevenifcollisionsaredetected.Allofthisarbitrationtakesplacewithoutcorruptionordelayofthehigherprioritymessage.

Thereareacoupleofthingsthatarerequiredtosupportnon-destructivebitwisearbitration.First,logicstatesneedtobedefinedasdominantorrecessive.Second,thetransmittingnodemustmonitorthestateofthebustoseeifthelogicstateitistryingtosendactuallyappearsonthebus.CANdefinealogicbit0asadominantbitandalogicbit1asarecessivebit.

Adominantbitstatewillalwayswinarbitrationoverarecessivebitstate,thereforethelowerthevalueintheMessageIdentifier（thefieldusedinthemessagearbitrationprocess）,thehigherthepriorityofthemessage.Asanexample,supposetwonodesaretryingtotransmitamessageatthesametime.Eachnodewillmonitorthebustomakesurethebitthatitistryingtosendactuallyappearsonthebus.Thelowerprioritymessagewillatsomepointtrytosendarecessivebitandthemonitoredstateonthebuswillbeadominant.Atthatpointthisnodelosesarbitrationandimmediatelystopstransmitting.Thehigherprioritymessagewillcontinueuntilcompletionandthenodethatlostarbitrationwillwaitforthenextperiodofnoactivityonthebusandtrytotransmititsmessageagain.

Message-BasedCommunication

CANprotocolisamessage-basedprotocol,notanaddressbasedprotocol.Thismeansthatmessagesarenottransmittedfromonenodetoanothernodebasedonaddresses.EmbeddedintheCANmessageitselfisthepriorityandthecontentsofthedatabeingtransmitted.Allnodesinthesystemreceiveeverymessagetransmittedonthebus（andwillacknowledgeifthemessagewasproperlyreceived）.Itisuptoeachnodeinthesystemtodecidewhetherthemessagereceivedshouldbeimmediatelydiscardedorkepttobeprocessed.Asinglemessagecanbedestinedforoneparticularnodetoreceive,ormanynodesbasedonthewaythenetworkandsystemaredesigned.Forexample,anautomotiveairbagsensorcanbeconnectedviaCANtoasafetysystemrouternodeonly.Thisrouternodetakesinothersafetysysteminformationandroutesittoallothernodesonthesafetysystemnetwork.Thenalltheothernodesonthesafetysystemnetworkcanreceivethelatestairbagsensorinformationfromtherouteratthesametime,acknowledgeifthemessagewasreceivedproperly,anddecidewhethertoutilizethisinformationordiscardit.

AnotherusefulfeaturebuiltintotheCANprotocolistheabilityforanodetorequestinformationfromothernodes.ThisiscalledaRemoteTransmitRequest（RTR）.Thisisdifferentfromtheexampleinthepreviousparagraphbecauseinsteadofwaitingforinformationtobesentbyaparticularnode,thisnodespecificallyrequestsdatatobesenttoit.

Oneadditionalbenefitofthismessage-basedprotocolisthatadditionalnodescanbeaddedtothesystemwithoutthenecessitytoreprogramallothernodestorecognizethisaddition.Thisnewnodewillstartreceivingmessagesfromthenetworkand,basedonthemessageID,decidewhethertoprocessordiscardthereceivedinformation.

CANMessageFrameDescription

CANprotocoldefinefourdifferenttypesofmessages（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.ToaccomplishanRTR,aRemoteFrameissentwiththeidentifieroftherequiredDataFrame.TheRTRbitintheArbitrationFieldisutilizedtodifferentiatebetweenaRemoteFrameandaDataFrame.IftheRTRbitisrecessive,thenthemessageisaRemoteFrame.IftheRTRbitisdominant,themessageisaDataFrame.

TheControlFieldconsistsofsixbits.TheMSBistheIDEbit（signifiesExtendedFrame）whichshouldbedominantforStandardDataFrames.ThisbitdeterminesifthemessageisaStandardorExtendedFrame.InExtendedFrames,thisbitisRB1anditisreserved.ThenextbitisRB0anditisalsoreserved.ThefourLSBsaretheDataLengthCode（DLC）bits.TheDataLengthCodebitsdeterminehowmanydatabytesareincludedinthemessage.ItshouldbenotedthataRemoteFramehasnodatafield,regardlessofthevalueoftheDLCbits.TheDataFieldconsistsofthenumberofdatabytesdescribedintheDataLengthCodeoftheControlField.TheCRCFieldconsistsofa15-bitCRCfieldandaCRCdelimiter,andisusedbyreceivingnodestodetermineiftransmissionerrorshaveoccurred.

TheAcknowledgeFieldisutilizedtoindicateifthemessagewasreceivedcorrectly.Anynodethathascorrectlyreceivedthemessage,regardlessofwhetherthenodeprocessesordiscardsthedata,putsadomi