Controller Area Network Physical Layer Requirements

1 ApplicationReportSLLA270 (CAN),ISO11898:1993,isfindingwidespreadu seinbuildingautomation,processcontrol, , (bittimingrequirements).. ( ).. LayerData-LinkLayerLogic Link ControlMedium Access ControlPhysicalLayerPhysical SignalingPhysical Medium AttachmentMedium Dependant InterfaceEmbeddedCANC ontrollerCANT ransceiverCAN BusLine Electricalspecifications:transceivers,co nnectors, cable CANcontroller,embedded shighreliability, ,hardwareinterfacecards, :1993 StandardArchitectureFigure1displaystheIS O11898standardarchitectureusingthebottom twolayersoftheOSImodel, ,andaftersendingamessage, , sand0 sintoelectricalpulsesleavinganode, , , 1 CANNODE 2 CANNODE 3 RECEIVE-ONLYNODE Bytes DataCCRRCCACKEOFEOFEOFFISIIDDEErr00 DDLLCC11- bitI dentifierRTRSOF11- bitI dentifi erRTRIDEr 0D L C0 ..8C R CA C Kr 118-bitI dentifierSRRSOFB ytes , ,onlythosenodesrequiringupdateddataallow themessagetopassthroughafilterthatissetb ythenetworkdesigner ,messagesfromcertainnodescanpass, ,muchofanode's ,thatasystemdesignercanusetoidentifythec ontentofamessage.

2 29-BitIdentifierCAN' , , C wins arbitrationACK bitACK bitACK bitNODEANODEBNODECCANBus4 PhysicalLayerRequirementsPhysicalLayerRe quirementsAllnodesonabusparticipateineac hbit , , ,NodesBandCthenbeginarbitration ' , , ,whicharenormallytransparenttoasystemope rator,areincludedinanycontrollerthatimpl ementstheCANprotocolsuchastheTexasInstru mentsTMS320F2812 (thebus)isthenimplementedthroughalinetra nsceiversuchasTI Bus-LineRLCANC ontroller(Node #1)DSP or C CANT ransceiverCANC ontroller(Node #n)DSP or C CANT ransceiverCANC ontroller(Node #2)DSP or C CANT ransceiverCANC ontroller(Node #3)DSP or C characteristicimpedance(ZO). resistors, , (m)(Mbps) ,themaximumbuslengthisdeterminedby, , ,whichdegradethesignaltransitiontimeandi ntroduceinter-symbolinterference(ISI), ,thesignalingrateisalsodeterminedfromthe totalsystemdelay downandbackbetweenthetwomostdistantnodes ofasystemandthesumofthedelaysintoandouto fthenodesonabuswiththetypical5 , ,skineffects,proximitytoothercircuitry,d ielectricloss, , (Mbps) BusLength(m) 50 SLLA270 , , srecommended30nodesmayrequiretheuseofhig hercablequality,aCANbusrepeater,andtight eroperatingtolerancessuchasa5% ,however,almostanytypeofcableworkstoacer taindegree, cableisusedinmanyapplications, , s 2 Vto7 Vcommon-moderangeoftolerablegroundoff-se t, ,theyaremoreeasilyinstalledandverified,a ndmay,therefore, , , , , (SLLS629)

3 , RLoneachendofthebus,orthesplitterminatio nusingtwo60 , ,abus-lineshort-circuittoa24 Vsupplylinewouldgenerateatransceiver SingleorSplitTerminationSLLA270 - pass filter withp2 RCL1fc=60W60W60 Wnode 1node nnode bus pin3V+Optional or 5-V power supply fortransceivers and digital isolators if required4 Reserved5 CAN_SHLD Optional shield6V-Ground return path/ 0V7 CANHCANH bus pin8 Reserved9V+Optional or 5-V power supply fortransceivers and digital isolators if requiredPhysicalLayerRequirementsNotetha ttransceiverssuchastheSN65 HVD1050haveaVreforVsplitpinshowninFigure 9thatisspecificallydesignedtostabilizeth ecommon-modebusvoltageduringcommunicatio n(thisalsohelpsreduceradiatedemissions). TheHVD1050 ~60 1% , , ,whilenotspecifiedbytheStandard,shouldha veacharacteristicimpedancematchingthatof thebuslineandterminators, , ,Multipole,RJ10,RJ45,M12,the5-pinmini-st yleinFigure12andmoremicro-styleconnector sintheCiAspecificationdocumentDR303-1, Optional shield2V+Optional or 5-V power supplyfor transceivers and digital isolatorsif required3V-Ground return path/ 0V4 CANHCANH bus pin5 CANLCANL bus ( )Bus-linesandagroundplanecanformaloopfor inductivelycouplednoisesignalsanddependi nguponthetopology, , (T) ,signalreflectionscandevelopinastubthatd rivesignallevelsbackandforthacrossarecei ver'sinputthresholds, , ,stub-linelengthshouldnotexceedone-third (1/3)oftheline' , (tprop(total))ofasignalthroughalineequal sthetransitiontime(tt)ofasignal(thegreat eroftheriseorfalltimes).

4 NetworkCriticalLength=tt=tprop(total)The refore,atypicalCANdrivermayhavea50nstran sitiontime,andwhenconsideringatypicaltwi sted-pairtransmissionlinepropdelayof5ns/ m,thedown-and-backdelayforonemeterbecome s10 (50ns/10ns/m=5m),andthemaxun-terminateds tublengthforthenetworkis1/3rdofthecritic allength,or5/3m( ).SLLA270 # 1 Node # 2 Node # 3 Node # nMost Distant Nodesdown & backdownbackTransceiversIsolatorsControl lersmt= t+ t+ ttt#n tBUDGETISOLATORTRANSCEIVERBUS_PROPTRANSC EIVERISOLATORCONTROLLER+++t+ t+ tttISOLATORTRANSCEIVERBUS_PROPTRANSCEIVE RISOLATOR++PhysicalLayerRequirementsWhen criticallengthistakenintoconsideration, ,butwithslewratecontrol,reducedsignaling rate,andcarefuldesign, ,ifa10k resistorisappliedforslope-controlattheRs pin(pin8)oftheHVD230 CANtransceiver,a160nsdrivertransitiontim eincreasesthemaximumstublengthto16/3mor5 1 , ,galvanicisolators, :CANcontroller(50nsto75ns),digitalisolat or(17nsforaTIisolatorandupto140nsforanop tocoupler),transceiver(100nsto250ns),and cable(about5ns/m).

5 , :atransmittedbitfromFigure14'sNode#1's C isolatordelay,driverdelay,10mbusdelay,re ceiverdelay,isolatordelay,controllerdela y,isolatordelay,driverdelay,busdelay,rec eiverdelay,isolatordelay receiveitbackto#1's ,17nsfortheTIisolator,100nsfortransceive rsdriverand100nsforthetransceiver sreceiver, ,inFigure14,thetotalpropdelayis:1sttrans mitbitfrom C isolatordelay(17ns),driverdelay(100ns),b usdelay[10m(5ns/m)],receiverdelay(100ns) ,isolatordelay(17ns),controllerdelay(50n s),isolatordelay(17ns),driverdelay(100ns ),busdelay[10m(5ns/m)],receiverdelay(100 ns),isolatordelay(17ns)= ,allowancesforoscillatorvariations,opera tingvariables,etcmustbeaccountedforbefor easignalingrateisselected, (bittimingrequirements)Hardsynchronizati onforces rising edgein first segmentCompensates forpropagationdelaysSample PointSEG 1 may be lengthenedor SEG 2 may be shortenedfor resynchronizationNominal Bit Length( Unit Interval )

6 SYNC _SEGPROP _SEGPHASE _SEG 1 PHASE _SEG =C,whereListheinductanceperunitlengthand CistheVS120Wt = 0S 1 LoadLoadLoadLoadd120WZ = 120 OWPhysicalLayerRequirementsEachCANbitisd ividedintothefoursegmentsinFigure15,with asamplepointtypicallylocatedatthe75% ,thesynchronizationsegment(SYNC_SEG), ,thepropagationtimesegment(PROP_SEG), ,bothphasebuffersegments(PHASE_SEG1andPH ASE_SEG2), (1) ,connectors,terminators, , ,intheformofdevicesandtheirinterconnecti on,thebusimpedanceisloweredtoZ'andwhenth ebusimpedanceislowered, (1)Capacitancehereisdefinedasdifferentia l, L VJ1where,ZZZZL+ - = > 3 + (-3) + (-3) > = (1)0L00000Z - Z => + ZZ - Z > (Z + Z )Z (1 + ) > Z ( )Z > Z (2)LZ =(C+C ) whereC ,C, ,Lcanbesolvedfromtherelationship0LZ =CasL=200Z CCZ == Z(C+C )C+C' (3) ,theCANdriverdifferentialoutputvoltage,V S, , (2)media, , ,anattenuation(oramplification) +VJ1+VR1,whereVL0istheinitialdominantdif ferentialvoltage,VJ1istheinputsignalrece ssivedifferentialvoltage, +VJ1+ L ,aCANdrivercreatesahigh-to-lowdifferenti alvoltagechangefromthestandardmaximumVLO of3 Vto0V,oraVJ1of ,VL1, ,Now,solvingforZ , ,theadditionofdevices, ,theloaded-busimpedancecanbeapproximated bycapacitanceofthemediawereknown,Z , , andsimplifying,C isthedistributeddevicecapacitance,CL,div idedbythedistance,d,betweendevicesorC = ,(2)Electricallylongisdefinedhereas >(t10% 90%)/3,where istheone-waytimedelayacrossthebusand(t10 % 90%),isthe10%to90% ( )( )( )0L20L2L0L20CZ = ZCC+dCZ'=ZCC +dCZC= C +Z'dCd =ZC-1Z' (4)L200 LCd >ZC-1meters (if C is pF/m) or feet (if C is pF/ft).

7 > (5)Minimum Distance between CAN Distributed Capacitance - pF/mC = 40 pFLC = 30 pFLC = 50 pFLC = 20 pFLC = 10 pFLDistance - mPhysicalLayerRequirementsNowsubstitutin gourminimumZ , ,CL,includescontributionsfromtheCANbuspi ns,connectorcontacts,printed-circuitboar dtraces,protectiondevices, ,suchastheSN65 HVD251, ,suchastheSN65 HVD230, ,suchasRS-485,RS-422,orM-LVDS, ,upto64nodesmaybeconnectedtoaDeviceNetbu s, ,itisrecommendedthatatransceiverwithahig hbus-inputimpedance, ,ithastosinkorsourcealloftheleakagecurre ntonthebus, , , , , 's1 Mbps, January2008 SubmitDocumentationFeedbackIMPORTANT NOTICET exas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements,and other changes to its products and services at any time and to discontinue any product or service without notice.

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