Controller Area Network Physical Layer …

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.

2 8C R CA C Kr 118-bitI dentifierSRRSOFB ytes , ,onlythosenodesrequiringupdateddataallow themessagetopassthroughafilterthatissetb ythenetworkdesigner ,messagesfromcertainnodescanpass, ,muchofanode's ,thatasystemdesignercanusetoidentifythec ontentofamessage, , , :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).

3 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) , 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).

4 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).

5 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).

6 , :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 )

7 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% ( )( )( )

8 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). > (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)

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