CAN总线物理层.docx

CAN总线物理层.docx

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CAN总线物理层

CANPhysicalLayer

TheControllerAreaNetwork（CAN）protocoldefinesthedatalinklayerandpartofthephysicallayerintheOSImodel,whichconsistsofsevenlayers.TheInternationalStandardsOrganization（ISO）definedastandard,whichincorporatestheCANspecificationsaswellasapartofphysicallayer:

thephysicalsignaling,whichcomprisesbitencodinganddecoding（NonReturntoZero,NRZ）aswellasbit-timingandsynchronization.

Bitencoding

NRZcomparedwithManchesterbitrepresentation

InthechosenNonReturntoZero（NRZ）bitcodingthesignallevelremainsconstantoverthebittimeandthusjustonetimeslotisrequiredfortherepresentationofabit（othermethodsofbitencodingaree.g.ManchesterorPulse-width-modulation）.Thesignallevelcanremainconstantoveralongerperiodoftime;thereforemeasuresmustbetakentoensurethatthemaximumpermissibleintervalbetweentwosignaledgesisnotexceeded.Thisisimportantforsynchronizationpurposes.Bitstuffingisappliedbyinsertingacomplementarybitafterfivebitsofequalvalue.Ofcoursethereceiverhastoun-stuffthestuff-bitssothattheoriginaldatacontentisprocessed.

 Bit-timingandsynchronization

Nominalbit-time

Onthebit-level（OSIlayer1,physicallayer）CANusessynchronousbittransmission.Thisenhancesthetransmittingcapacitybutalsomeansthatasophisticatedmethodofbitsynchronizationisrequired.Whilebitsynchronizationinacharacter-orientedtransmission（asynchronous）isperformeduponthereceptionofthestartbitavailablewitheachcharacter,asynchronoustransmissionprotocolthereisjustonestartbitavailableatthebeginningofaframe.Toenablethereceivertocorrectlyreadthemessages,continuousresynchronizationisrequired.Phasebuffersegmentsarethereforeinsertedbeforeandafterthenominalsamplepointwithinabitinterval.

TheCANprotocolregulatesbusaccessbybit-wisearbitration.Thesignalpropagationfromsendertoreceiverandbacktothesendermustbecompletedwithinonebit-time.Forsynchronizationpurposesafurthertimesegment,thepropagationdelaysegment,isneededinadditiontothetimereservedforsynchronization,thephasebuffersegments.Thepropagationdelaysegmenttakesintoaccountthesignalpropagationonthebusaswellassignaldelayscausedbytransmittingandreceivingnodes.

Twotypesofsynchronizationaredistinguished:

hardsynchronizationatthestartofaframeandresynchronizationwithinaframe.

∙Afterahardsynchronizationthebittimeisrestartedattheendofthesyncsegment.Thereforetheedge,whichcausedthehardsynchronization,lieswithinthesyncsegmentoftherestartedbittime.

∙Resynchronizationshortensorlengthensthebittimesothatthesamplepointisshiftedaccordingtothedetectededge

Thedevicedesignermayprogramthebit-timingparametersintheCANcontrollerbymeansoftheappropriateregisters.

Interdependencyofdatarateandbuslength

Dependingonthesizeofthepropagationdelaysegmentthemaximumpossiblebuslengthataspecificdatarate（orthemaximumpossibledatarateataspecificbuslength）canbedetermined.Thesignalpropagationisdeterminedbythetwonodeswithinthesystemthatarefarthestapartfromeachother.Itisthetimethatittakesasignaltotravelfromonenodetotheonefarthestapart（takingintoaccountthedelaycausedbythetransmittingandreceivingnode）,synchronizationandthesignalfromthesecondnodetotravelbacktothefirstone.Onlythencanthefirstnodedecidewhetheritsownsignallevel（recessiveinthiscase）istheactuallevelonthebusorwhetherithasbeenreplacedbythedominantlevelbyanothernode.Thisfactisimportantforbusarbitration.

Somemoderntransceiverssupportnolowdatarates.Thereforeonacquisitionoftransceiversthemaximalrequirednetworklengthmustbeconsidered.

Physicalmedia

Thisclauseismostinterestingforsystemdesigners.

ThebasisfortransmittingCANmessagesandforcompetingforbusaccessistheabilitytorepresentadominantandarecessivebitvalue.Thisispossibleforelectricalandopticalmediasofar.Alsopowerlineandwirelesstransmissionispossible.

ForelectricalmediathedifferentialoutputbusvoltagesaredefinedinISO11898-2andISO11898-3,inSAEJ2411,andISO11992（seebelow）.

Withopticalmediatherecessivelevelisrepresentedby"dark"andthedominantlevelby"light".

ThephysicalmediamostcommonlyusedtoimplementCANnetworksisadifferentiallydrivenpairofwiredwithcommonreturn.Forvehiclebodyelectronicssinglewirebuslinesarealsoused.SomeeffortshavebeenmadetodevelopasolutionforthetransmissionofCANsignalsonthesamelineasthepowersupply.

Theparametersoftheelectricalmediumbecomeimportantwhenthebuslengthisincreased.Signalpropagation,thelineresistanceandwirecrosssectionsarefactorswhendimensioninganetwork.Inordertoachievethehighestpossiblebitrateatagivenlength,ahighsignalspeedisrequired.Forlongbuslinesthevoltagedropsoverthelengthofthebusline.Thewirecrosssectionnecessaryiscalculatedbythepermissiblevoltagedropofthesignallevelbetweenthetwonodesfarthestapartinthesystemandtheoverallinputresistanceofallconnectedreceivers.Thepermissiblevoltagedropmustbesuchthatthesignallevelcanbereliablyinterpretedatanyreceivingnode.

Theconsiderationofelectromagneticcompatibilityandchoiceofcablesandconnectorsbelongsalsotothetasksofasystemintegrator.

assumedlinelength100m

Specificsignalpropagationtime（ns/m）

Maximumbitrate（kbit/s）

5.0

80

5.5

73

6.0

67

6.5

62

7.0

58

Networktopology

Thisclauseismostinterestingforsystemdesigners.

Electricalsignalsonthebusarereflectedattheendsoftheelectricallineunlessmeasuresagainstthathavebeentaken.Forthenodetoreadthebuslevelcorrectlyitisimportantthatsignalreflectionsareavoided.Thisisdonebyterminatingthebuslinewithaterminationresistoratbothendsofthebusandbyavoidingunnecessarilylongstubslinesofthebus.Thehighestpossibleproductoftransmissionrateandbuslengthlineisachievedbykeepingascloseaspossibletoasinglelinestructureandbyterminatingbothendsoftheline.Specificrecommendationsforthiscanbefoundintheaccordingstandards（i.e.ISO11898-2and-3）.

Itispossibletoovercomethelimitationsofthebasiclinetopologybyusingrepeaters,bridgesorgateways.Arepeatertransfersanelectricalsignalfromonephysicalbussegmenttoanothersegment.Thesignalisonlyrefreshedandtherepeatercanberegardedasapassivecomponentcomparabletoacable.Therepeaterdividesabusintotwophysicallyindependentsegments.Thiscausesanadditionalsignalpropagationtime.However,itislogicallyjustonebussystem.

Abridgeconnectstwologicallyseparatednetworksonthedatalinklayer（OSIlayer2）.ThisissothattheCANidentifiersareuniqueineachofthetwobussystems.Bridgesimplementastoragefunctionandcanforwardmessagesorpartsthereofinanindependenttime-delayedtransmission.Bridgesdifferfromrepeaterssincetheyforwardmessages,whicharenotlocal,whilerepeatersforwardallelectricalsignalsincludingtheCANidentifier.

Agatewayprovidestheconnectionofnetworkswithdifferenthigher-layerprotocols.Itthereforeperformsthetranslationofprotocoldatabetweentwocommunicationsystems.Thistranslationtakesplaceontheapplicationlayer（OSIlayer7）.

Busaccess

TheconnectionbetweenaCANcontrollerchipandatwo-wiredifferentialbusavarietyofCANtransceiverchipsaccordingtodifferentphysicallayerstandardsareavailable（seebelowISO11898-2and-3,etc.）.

Thisinterfacebasicallyconsistsofatransmittingamplifierandareceivingamplifier（transceiver=transmitandreceive）.Asidefromtheadaptationofthesignalrepresentationbetweenchipandbusmediumthetransceiverhastomeetaseriesofadditionalrequirements.Asatransmitteritprovidessufficientdriveroutputcapacityandprotectstheon-controller-chipdriveragainstoverloading.Italsoreduceselectromagneticradiation.AsareceivertheCANtransceiverprovidesadefinedrecessivesignallevelandprotectstheon-controller-chipinputcomparatoragainstover-voltagesonthebuslines.ItalsoextendsthecommonmoderangeoftheinputcomparatorintheCANcontrollerandprovidessufficientinputsensitivity.Furthermoreitdetectsbuserrorssuchaslinebreakage,shortcircuits,shortstoground,etc.AfurtherfunctionofthetransceivercanalsobethegalvanicisolationofaCANnodeandthebusline.

Physicallayerstandards

ISO11898-2highspeed

ISO11898-2isthemostusedphysicallayerstandardforCANnetworks.Itdescribesthebusaccessunit（implementedasCANhigh-speedtransceiver）functionsaswellassomemedium-dependentinterfacefeatures.

Inthisstandardthedatarateisdefinedupto1Mbit/swithatheoreticallypossiblebuslengthof40mat1Mbit/s.Thehigh-speedstandardspecifiesatwo-wiredifferentialbuswherebythenumberofnodesislimitedbytheelectricalbusload.Thecharacteristiclineimpedanceis120Ohm,thecommonmodevoltagerangesfrom-2VonCAN_Lto+7VonCAN_H.Thenominalspecificpropagationdelayofthetwo-wirebuslineisspecifiedat5ns/m.Allthesefiguresarevalidonlyfora1Mbit/stransferrateandamaximumnetworklengthof40m.

Inordertoachievephysicalcompatibilityallnodesinthenetworkmustusethesameorasimilarbit-timing.ForautomotiveapplicationstheSAEpublishedtheSAEJ2284specification.Forindustrialandothernon-automotiveappl