車載網(wǎng)絡(luò)CAN數(shù)據(jù)連路層基本概念MessagesInformationRoutingBitratePrioritiesRemoteDataRequestMultimasterArbitrationSafetyErrorSignallingandRecoveryTimeFaultConfinementConnectionsSingleChannelBusvaluesAcknowledgmentSleepMode/Wake-upOscillatorToleranceMessagesInformationonthebusissentinfixedformatmessagesofdifferentbutlimitedlengthWhenthebusisfreeanyconnectedunitmaystarttotransmitanewmessage.InformationRoutingInCANsystemsaCANnodedoesnotmakeuseofanyinformationaboutthesystemconfiguration(e.g.stationaddresses).Thishasseveralimportantconsequences:(1)SystemFlexibility:NodescanbeaddedtotheCANnetworkwithoutrequiringanychangeinthesoftwareorhardwareofanynodeandapplicationlayer.

InformationRouting(2)MessageRouting:ThecontentofamessageisnamedbyanIDENTIFIER.TheIDENTIFIERdoesnotindicatethedestinationofthemessage,butdescribesthemeaningofthedata,sothatallnodesinthenetworkareabletodecidebyMessageFilteringwhetherthedataistobeacteduponbythemornot.InformationRouting (3)Multicast:AsaconsequenceoftheconceptofMessageFilteringanynumberofnodescanreceiveandsimultaneouslyactuponthesamemessage.InformationRouting (4)DataConsistency:WithinaCANnetworkitisguaranteedthatamessageissimultaneouslyacceptedeitherbyallnodesorbynonode.Thusdataconsistencyofasystemisachievedbytheconceptsofmulticastandbyerrorhandling.BitrateThespeedofCANmaybedifferentindifferentsystems.However,inagivensystemthebit-rateisuniformandfixed.PrioritiesTheIDENTIFIERdefinesastaticmessagepriorityduringbusaccess.RemoteDataRequestBysendingaREMOTEFRAMEanoderequiringdatamayrequestanothernodetosendthecorrespondingDATAFRAME.

TheDATAFRAMEandthecorrespondingREMOTEFRAMEarenamedbythesameIDENTIFIER.MultimasterWhenthebusisfreeanyunitmaystarttotransmitamessage.Theunitwiththemessageofhigherprioritytobetransmittedgainsbusaccess.ArbitrationWheneverthebusisfree,anyunitmaystarttotransmitamessage.If2ormoreunitsstarttransmittingmessagesatthesametime,thebusaccessconflictisresolvedbybitwisearbitrationusingtheIDENTIFIER.ArbitrationThemechanismofarbitrationguaranteesthatneitherinformationnortimeislost.IfaDATAFRAMEandaREMOTEFRAMEwiththesameIDENTIFIERareinitiatedatthesametime,theDATAFRAMEprevailsovertheREMOTEFRAME.ArbitrationDuringarbitrationeverytransmittercomparesthelevelofthebittransmittedwiththelevelthatismonitoredonthebus.Iftheselevelsareequaltheunitmaycontinuetosend.Whena’recessive’levelissentanda’dominant’levelismonitored(seeBusValues),theunithaslostarbitrationandmustwithdrawwithoutsendingonemorebit.SafetyInordertoachievetheutmostsafetyofdatatransfer,powerfulmeasuresforerrordetection,signalingandself-checkingareimplementedineveryCANnode.SafetyErrorDetection:Fordetectingerrorsthefollowingmeasureshavebeentaken:(1)Monitoring(transmitterscomparethebitlevelstobetransmittedwiththebitlevelsdetectedonthebus) (2)CyclicRedundancyCheck (3)BitStuffingSafetyTheerrordetectionmechanismshavethefollowingproperties:

(1)upto5randomlydistributederrorsinamessagearedetected.(2)bursterrorsoflengthlessthan15inamessagearedetected.Safety (3)errorsofanyoddnumberinamessagearedetected. (4)Totalresidualerrorprobabilityforundetectedcorruptedmessages:lessthanmessageerrorrate*4.7*10-11.ErrorSignalingand

RecoveryTimeCorruptedmessagesareflaggedbyanynodedetectinganerror.Suchmessagesareabortedandwillberetransmittedautomatically.ErrorSignallingand

RecoveryTimeTherecoverytimefromdetectinganerroruntilthestartofthenextmessageisatmost31bittimes,ifthereisnofurthererror.FaultConfinementCANnodesareabletodistinguishshortdisturbancesfrompermanentfailures.Defectivenodesareswitchedoff.ConnectionsTheCANserialcommunicationlinkisabustowhichanumberofunitsmaybeconnected.Thisnumberhasnotheoreticallimit.Practicallythetotalnumberofunitswillbelimitedbydelaytimesand/orelectricalloadsonthebusline.SingleChannelThebusconsistsofasinglechannelthatcarriesbits.Fromthisdataresynchronizationinformationcanbederived.SingleChannelThewayinwhichthischannelisimplementedisnotfixedinthisspecification.E.g.singlewire(plusground),twodifferentialwires,opticalfibres,etc.BusvaluesThebuscanhaveoneoftwocomplementarylogicalvalues:’dominant’or’recessive’.Duringsimultaneoustransmissionof’dominant’and’recessive’bits,theresultingbusvaluewillbe’dominant’.

BusvaluesForexample,incaseofawired-ANDimplementationofthebus,the’dominant’levelwouldberepresentedbyalogical’0’andthe’recessive’levelbyalogical’1’.Physicalstates(e.g.electricalvoltage,light)thatrepresentthelogicallevelsarenotgiveninthisspecification.AcknowledgmentAllreceiverschecktheconsistencyofthemessagebeingreceivedandwillacknowledgeaconsistentmessageandflaganinconsistentmessage.SleepMode/Wake-upToreducethesystem’spowerconsumption,aCAN-devicemaybesetintosleepmodewithoutanyinternalactivityandwithdisconnectedbusdrivers.Thesleepmodeisfinishedwithawake-upbyanybusactivityorbyinternalconditionsofthesystem.SleepMode/Wake-upOnwake-up,theinternalactivityisrestarted,althoughtheMACsublayerwillbewaitingforthesystem’soscillatortostabilizeanditwillthenwaituntilithassynchronizeditselftothebusactivity(bycheckingforelevenconsecutiverecessivebits),beforethebusdriversaresetto"on-bus"again.OscillatorToleranceTheBitTimingrequirementsallowceramicresonatorstobeusedinapplicationswithtransmissionratesofupto125kbit/sasaruleofthumb;ForthefullbusspeedrangeoftheCANprotocol,aquartzoscillatorisrequired.MESSAGETRANSFERMessage一詞在CAN通訊中有特殊的內(nèi)涵：首先它是一個(gè)位流序列有限的長(zhǎng)度并且有固定的格式；FrameFormatsTherearetwodifferentformatswhichdifferinthelengthoftheIDENTIFIERfield:StandardFrames--Frameswiththenumberof11bitIDENTIFIERExtendedFrames--framescontaining29bitIDENTIFIERFrameTypesfourdifferentframetypesDATAFRAMEREMOTEFRAMEERRORFRAMEOVERLOADFRAME示波器采集的

CAN總線數(shù)據(jù)幀DATAFRAMESTARTOFFRAMEARBITRATIONFIELDARBITRATIONFIELD

---------StandardFormatARBITRATIONFIELD

---------ExtendedFormatExtendedFormatStandardFormatARBITRATIONFIELD

---------ExtendedFormatCONTROLFIELD(StandardFormat

aswellasExtendedFormat)DATALENGTHCODE(StandardFormataswellasExtendedFormat)DATAFIELDCRCFIELD

將要發(fā)送的數(shù)據(jù)比特序列當(dāng)作一個(gè)多項(xiàng)式f(x)的系數(shù)，在發(fā)送方用收發(fā)雙方預(yù)先約定的生成多項(xiàng)式G(x)去除，求得一個(gè)余數(shù)多項(xiàng)式。

將余數(shù)多項(xiàng)式加到數(shù)據(jù)多項(xiàng)式之后發(fā)送到接收端。CRC校驗(yàn)基本工作原理

接收端用同樣的生成多項(xiàng)式G(x)去除接收數(shù)據(jù)多項(xiàng)式f’(x)，得到計(jì)算余數(shù)多項(xiàng)式。

如果計(jì)算余數(shù)多項(xiàng)式與接收余數(shù)多項(xiàng)式相同，則表示傳輸無(wú)差錯(cuò)；如果計(jì)算余數(shù)多項(xiàng)式不等于接收余數(shù)多項(xiàng)式，則表示傳輸有差錯(cuò)，由發(fā)送方重發(fā)數(shù)據(jù)，直至正確為止。CRC校驗(yàn)基本工作原理CRC生成多項(xiàng)式G(x)由協(xié)議規(guī)定，生成多項(xiàng)式的結(jié)構(gòu)和檢錯(cuò)效果要經(jīng)過(guò)嚴(yán)格的數(shù)學(xué)分析和試驗(yàn)后確定。CRC校驗(yàn)基本工作原理發(fā)送方接收方發(fā)送數(shù)據(jù)f(x)數(shù)據(jù)字段校驗(yàn)字段接收數(shù)據(jù)f’(x)CRC校驗(yàn)基本工作原理實(shí)際發(fā)送生成多項(xiàng)式生成多項(xiàng)式接收正確接收出錯(cuò)CRC校驗(yàn)的工作過(guò)程描述：

在發(fā)送端，將發(fā)送數(shù)據(jù)多項(xiàng)式，k為生成多項(xiàng)式的最高冪值。

將除以生成多項(xiàng)式，得到：為余數(shù)多項(xiàng)式。將作為整體，從發(fā)送端通過(guò)通信信道傳送到接收端。CRC校驗(yàn)基本工作原理接收端對(duì)接收整體除以生成多項(xiàng)式如果可以整除，則認(rèn)定傳輸正確，如果不能整除，認(rèn)定傳輸錯(cuò)誤。CRC校驗(yàn)基本工作原理CRC校驗(yàn)碼生成過(guò)程舉例：

發(fā)送數(shù)據(jù)比特序列為：110011（6比特）

生成多項(xiàng)式比特序列11001（5比特，K=4）將發(fā)送的數(shù)據(jù)比特序列乘以24，產(chǎn)生的乘積為：1100110000CRC校驗(yàn)基本工作原理將乘積用生成多項(xiàng)式比特序列去除，得到：模二算法減法不借位加法不進(jìn)位CRC校驗(yàn)基本工作原理將余數(shù)序列加到乘積當(dāng)中得到1100111001如果在數(shù)據(jù)傳輸過(guò)程中沒(méi)有發(fā)生傳輸錯(cuò)誤，則接收端收到的帶有CRC校驗(yàn)碼的接收數(shù)據(jù)比特序列能被相同的生成多項(xiàng)式整除，即：CRC校驗(yàn)基本工作原理CRCFIELDgenerator-polynomial生成多項(xiàng)式ACKFIELD(StandardFormataswellasExtendedFormat)ENDOFFRAME(StandardFormataswellasExtendedFormat)REMOTEFRAMEERRORFRAMEERRORFLAGERRORDELIMITEROVERLOADFRAMEOVERLOADFRAME發(fā)出時(shí)刻O(píng)VERLOADFLAGOVERLOADDELIMITERINTERFRAMESPACINGINTERFRAMESPACINGINTERFRAMESPACINGINTERMISSIONINTERFRAMESPACINGBUSIDLEINTERFRAMESPACINGSUSPENDTRANSMISSIONINTERFRAMESPACINGDefinitionof

TRANSMITTER/RECEIVERTRANSMITTERRECEIVERMESSAGEFILTERINGMessagefilteringisbaseduponthewholeIdentifiermaskregistersMESSAGEFILTERINGMESSAGEFILTERINGMESSAGEVALIDATIONThepointoftimeatwhichamessageistakentobevalid,isdifferentforthetransmitterandthereceiversofthemessage.Transmitter:Receivers:CODINGArbitrationERRORHANDLINGThereare5differenterrortypesBITERRORSTUFFERRORCRCERRORFORMERRORACKNOWLEDGMENTERRORBITERRORmonitorsthebusdifferentfromthebitvalueSTUFFERROR6thconsecutiveequalbitlevelCRCERRORnotthesameasthatreceivedintheCRCsequenceFORMERRORoneormoreillegalbitsinafixed-formbitfieldExceptionACKNOWLEDGMENT

ERRORdoesnotmonitora’dominant’bitduringtheACKSLOTErrorSignalling誰(shuí)來(lái)發(fā)送類型時(shí)刻FAULTCONFINEMENTWithrespecttofaultconfinementaunitmaybeinoneofthreestates:erroractiveerrorpassivebusoffHowtoconfine（界定）Forfaultconfinementtwocountsareimplementedineverybusunit:1)TRANSMITERRORCOUNT2)RECEIVEERRORCOUNT1.WhenaRECEIVERdetectsanerror,theRECEIVEERRORCOUNTwillbeincreasedby1,2.WhenaRECEIVERdetectsa’dominant’bitasthefirstbitaftersendinganERRORFLAGtheRECEIVEERRORCOUNTwillbeincreasedby8.3.WhenaTRANSMITTERsendsanERRORFLAGtheTRANSMITERRORCOUNTisincreasedby8.4.IfanTRANSMITTERdetectsaBITERRORwhilesendinganACTIVEERRORFLAGoranOVERLOADFLAGtheTRANSMITERRORCOUNTisincreasedby8.5.IfanRECEIVERdetectsaBITERRORwhilesendinganACTIVEERRORFLAGoranOVERLOADFLAGtheRECEIVEERRORCOUNTisincreasedby8.6.Anynodetoleratesupto7consecutive’dominant’bitsaftersendinganACTIVEERRORFLAG,PASSIVEERRORFLAGorOVERLOADFLAG.Afterdetectingthe14thconsecutive’dominant’bit(incaseofanACTIVEERRORFLAGoranOVERLOADFLAG)orafterdetectingthe8thconsecutive’dominant’bitfollowingaPASSIVEERRORFLAGandaftereachsequenceofadditionaleightconsecutive’dominant’bitseveryTRANSMITTERincreasesitsTRANSMITERRORCOUNTby8andeveryRECEIVERincreasesitsRECEIVEERRORCOUNTby8.7.Afterthesuccessfultransmissionofamessage(gettingACKandnoerroruntilENDOFFRAMEisfinished)theTRANSMITERRORCOUNTisdecreasedby1unlessitwasalready0.8.Afterthesuccessfulreceptionofamessage(receptionwithouterroruptotheACKSLOTandthesuccessfulsendingoftheACKbit),theRECEIVEERRORCOUNTisdecreasedby1,9.Anodeis’errorpassive’whentheTRANSMITERRORCOUNTequalsorexceeds128,orwhentheRECEIVEERRORCOUNTequalsorexceeds128.10.Anodeis’busoff’whentheTRANSMITERRORCOUNTisgreaterthanorequalto256.11.An’errorpassive’nodebecomes’erroractive’againwhenboththeTRANSMITERRORCOUNTandtheRECEIVEERRORCOUNTarelessthanorequalto127.12.Annodewhichis’busoff’ispermittedtobecome’erroractive’(nolonger’busoff’)withitserrorcountersbothsetto0after128occurrenceof11consecutive’recessive’bitshavebeenmonitoredonthebus.OSCILLATOR

TOLERANCEAmaximumoscillatortoleranceof1.58%isgiventhereforetheuseofaceramicresonatoratabusspeedofupto125Kbits/sasaruleofthumb;ForthefullbusspeedrangeoftheCANprotocol,aquartzoscillatorisrequired.BITTIMING

REQUIREMENTSNOMINALBITRATETheNominalBitRateisthenumberofbitspersecondtransmittedintheabsenceofresynchronizationbyanidealtransmitter.NOMINALBITTIMENOMINALBITTIME=1/NOMINALBITRATEBITTIMING

REQUIREMENTSTheNominalBitTimecanbethoughtofasbeingdividedintoseparatenon-overlappingtimesegments.ThesesegmentsBITTIMING

REQUIREMENTS-SYNCHRONIZATIONSEGMENT(SYNC_SEG)-PROPAGATIONTIMESEGMENT(PROP_SEG)-PHASEBUFFERSEGMENT1(PHASE_SEG1)-PHASEBUFFERSEGMENT2(PHASE_SEG2)BITTIMING

REQUIREMENTSSYNCSEGThispartofthebittimeisusedtosynchronizethevariousnodesonthebus.Anedgeisexpectedtoliewithinthissegment.PROPSEGThispartofthebittimeisusedtocompensateforthephysicaldelaytimeswithinthenetwork.Itistwicethesumofthesignal’spropagationtimeonthebusline,theinputcomparatordelay,andtheoutputdriverdelay.PHASESEG1

PHASESEG2ThesePhase-Buffer-Segmentsareusedtocompensateforedgephaseerrors.Thesesegmentscanbelengthenedorshortenedbyresynchronization.SAMPLEPOINTTheSAMPLEPOINTisthepointoftimeatwhichthebuslevelisreadandinterpretedasthevalueofthatrespectivebit.It’slocationisattheendofPHASE_SEG1.INFORMATION

PROCESSINGTIMETheINFORMATIONPROCESSINGTIMEisthetimesegmentstartingwiththeSAMPLEPOINTreservedforcalculationthesubsequent(這一位)bitlevel.TIMEQUANTUMTheTIMEQUANTUMisafixedunitoftimederivedfromtheoscillatorperiod.Lengthof

TimeSegmentsSYNC_SEGis1TIMEQUANTUMlong.PROP_SEGisprogrammabletobe1,2,...,8TIMEQUANTAlong.PHASE_SEG1isprogrammabletobe1,2,...,8TIMEQUANTAlong.PHASE_SEG2isthemaximumofPHASE_SEG1andtheINFORMATIONPROCESSINGTIMELengthof

TimeSegmentsTheINFORMATIONPROCESSINGTIMEislessthanorequalto2TIMEQUANTAlong.ThetotalnumberofTIMEQUANTAinabittimehastobeprogrammableatleastfrom8to25.Lengthof

TimeSegmentssamplepointThepositionofthesamplepoint,however,shouldbeselectedincommonforallnodes.ThereforethebittimingofSLIOdevicesmustbecompatibletothefollowingdefinitionofthebittime:同步指兩個(gè)或兩個(gè)以上隨時(shí)間變化的量在變化過(guò)程中保持一定的相對(duì)關(guān)系。當(dāng)兩個(gè)設(shè)備一起工作并對(duì)時(shí)間有精確要求的時(shí)候，就需要在它們之間進(jìn)行同步HARD

SYNCHRONIZATIONAfteraHARDSYNCHRONIZATIONtheinternalbittimeisrestartedwithSYNC_SEG.ThusHARDSYNCHRONIZATIONforcestheedgewhichhascausedtheHARDSYNCHRONIZATIONtoliewithintheSYNCHRONIZATIONSEGMENToftherestartedbittime.RESYNCHRONIZATION

JUMPWIDTHAsaresultofRESYNCHRONIZATIONPHASE_SEG1maybelengthenedorPHASE_SEG2maybeshortened.TheamountoflengtheningorshorteningofthePHASEBUFFERSEGMENTshasanupperboundgivenbytheRESYNCHRONIZATIONJUMPWIDTH.TheRESYNCHRONIZATIONJUMPWIDTHshallbeprogrammablebetween1andmin(4,PHASE_SEG1).RESYNCHRONIZATION

JUMPWIDTHClockinginformationmaybederivedfromtransitionsfromonebitvaluetotheother.Thepropertythatonlyafixedmaximumnumberofsuccessivebitshavethesamevalueprovidesthepossibilityofresynchronizingabusunittothebitstreamduringaframe.Themaximumlengthbetweentwotransitionswhichcanbeusedforresynchronizationis29bittimes.PHASEERROR

ofanedgeThePHASEERRORofanedgeisgivenbythepositionoftheedgerelativetoSYNC_SEG,measuredinTIMEQUANTA.ThesignofPHASEERRORisdefinedasfollows:PHASEERROR

ofanedgee=0iftheedgelieswithinSYNC_SEG.e>0iftheedgeliesbeforetheSAMPLEPOINT.e<0iftheedgeliesaftertheSAMPLEPOINTofthepreviousbit.RESYNCHRONIZATIONTheeffectofaRESYNCHRONIZATIONisthesameasthatofaHARDSYNCHRONIZATION,whenthemagnitudeofthePHASEERRORoftheedgewhichcausestheRESYNCHRONIZATIONislessthanorequaltotheprogrammedvalueoftheRESYNCHRONIZATIONJUMPWIDTH.RESYNCHRONIZATIONWhenthemagnitudeofthePHASEERRORislargerthantheRESYNCHRONIZATIONJUMPWIDTH,andifthePHASEERRORispositive,thenPHASE_SEG1islengthenedbyanamountequaltotheRESYNCHRONIZATIONJUMPWIDTH.andifthePHASEERRORisnegative,thenPHASE_SEG2isshortenedbyanamountequaltotheRESYNCHRONIZATIONJUMPWIDTH.RESYNCHRONIZATIONRESYNCHRONIZATIONSYNCHRONIZATION

RULESHARDSYNCHRONIZATIONandRESYNCHRONIZATIONarethetwoformsofSYNCHRONIZATION.Theyobeythefollowingrules:1.OnlyoneSYNCHRONIZATIONwithinonebittimeisallowed.SYNCHRONIZATION

RULES2