PLL抖动及其在CAN协议中的影响

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1、TB078PLLJitterandItsEffectsintheCANProtocolAuthor:CaioGübelEXTERNALCLOCK,INTERNALMicrochipTechnologyInc.CLOCKANDMEASURABLEJITTERThemicrocontrollerclockfrequencygeneratedfromaINTRODUCTIONPLLcircuitissubjecttoajitter,alsodefinedasPhaseJitterorPhaseSkew.ForitsPIC18Enhancedmicro-Phase

2、LockedLoop(PLL)circuitsareincreasinglycontrollers,MicrochipspecifiesPhaseJitter(Pjitter)asusedinmicrocontrollerstoachievehigherinternalbeing2%(Gaussiandistribution,within3standardclockfrequencies.Thisallowsbetterperformancedeviations)andTotalJitter(Tjitter)asbeing2*Pjitter.whilere

3、ducingoverallnoise.SeveralofMicrochip’sPIC18microcontrollersfeature4xPLLsintheirclockTheCANprotocolusesabitstuffingtechniquethatgenerationcircuits.Thismakesitpossibletogenerateinsertsabitofagivenpolarity,followingfivebitswithaninternal40MHzclockfromanexternal10MHztheoppositepolari

4、ty.Thisgivesatotalof10bitscrystal.transmittedwithoutresynchronization(compensationforjitterorphaseerror).OnedrawbackintheuseofPLLcircuitsisthattheycreateasmall,butstillmeasurableleveloftransientGiventherandomnatureofthejittererroradded,itcanphaseshifts,orjitter.ThisTechnicalBriefs

5、howstheinflu-beshownthatthetotalerrorcausedbythejittertendsenceofPLLjitteronMicrochip’sPIC18microcontrollers,tocancelitselfovertime.Foraperiodof10bits,itishowitaffectstheoverallclockofthemicrocontrollerandnecessarytoaddonlytwojitterintervalstocorrectforhowthecombinedeffectsofjitte

6、randcrystaldriftarewelljitterinducederror:oneintervalinthebeginningofthebelowtheCAN2.0specification.10-bitperiodandanotherattheend.TheoveralleffectisshowninFigure1.FIGURE1:EFFECTSOFPHASEJITTERONTHEMICROCONTROLLERCLOCKANDCANBITTIMENominalClockClockwithJitterPhaseSkew(Jitter)CANBitT

7、imewithJitterCANBitJitter2004MicrochipTechnologyInc.DS91078A-page1TB078Oncetheseconsiderationsaretakenintoaccount,itisTable1showstherelationbetweentheclockgener-possibletoshowthattherelationbetweenthejitterandatedbythePLLandthefrequencyerrorfromjitterthetotalfrequencyerrorcanbede

8、finedas:(measuredjitterinducederr