bit timing(can总线bt时间确定-用于确定波特率)

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1、Bittiming(can总线BT时间确定-用于确定波特率)CANhasadvancedfeaturesforcopingwiththetimedelaysfoundinlongbuslengths(incomparisontothebitrate)andcopingwithdifferencesinclockchrystalfrequenciesfornodesonthebus.Thechoiceofbittimingisveryimportantsinceitdecidesthebitrate,thesample

2、pointandtheabilitytoresynchronise.Bitsegments(asinBoschstandard)Eachbitisdividedintofoursegments-thesynchronisationsegment,thepropagationsegmentandthephasesegmentsoneandtwo.Eachsegmentconsistsofoneormoretimequanta.Atimequantumisafixedamountoftimewhichisderivedf

3、romtheCANcontrollerclockwithaprescalefactor.Synchronisationsegment(Synch_Seg)Thesynchronisationsegmentisusedtosynchronisethevariousnodesonthebus.Whenabitissentonthebus,theleadingedgeisexpectedtobewithinthissegment.Thissegmentisalwaysonetimequantumlong.Propagati

4、onsegment(Prop_Seg)ThePropagationSegmentisneededtocompensateforthedelayinthebuslines.Thesegmentsizeisprogrammablebetween1and8timequanta.PhaseSegment1(Phase_Seg1),PhaseSegment2(Phase_Seg2)Thesesegmentscanbeusedlengthenedorshortenedbyresynchronisation.Bitsegments

5、(asinimplementationslikeIntel527andC167CR)InmostimplementationsofCANcontrollersthesegmentsseemtobeimplementedinadifferentwaythandescribedinthestandard.Thesynchsegmentlooksasinthestandardandconsistsofonetimequantum.Thebigdifferenceisthatthepropagationsegmentandt

6、hephasesegment1inthestandardhavebeencombinedintoonesegment,TSEG1.Phasesegment2isleftuntouched,butisrenamedtoTSEG2.Normallythereisonlyonesamplepointforeachbit.Inthiscase,thesamplepointisintheedgebetweenTSEG1andTSEG2.However,someCANcontrollerscanalsosampleeachbit

7、threetimes.Inthiscase,thebitwillbesampledthreequantainarow,withthelastsamplebeingtakenintheedgebetweenTSEG1andTSEG2.Threesamplesshouldonlybeusedforrelativelyslowbaudrates.CalculationofbaudrateandsamplepointBaudrateThebaudrateofthebuscanbecalculatedfrom:Baudrate

8、=fcrystal/(2*n*(BRP+1))wherenisthenumberoftimequantaforonebitandisdefinedas:n=SYNCHSEG+TSEG1+TSEG2BRPisthevalueoftheBaudRatePrescaler.Warning:someCANcontrollers(likeIntel526