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Os motores de busca de Datasheet de Componentes eletrônicos |
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Os motores de busca de Datasheet de Componentes eletrônicos |
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HT68FB240 Folha de dados(PDF) 49 Page - Holtek Semiconductor Inc |
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HT68FB240 Folha de dados(HTML) 49 Page - Holtek Semiconductor Inc |
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49 / 133 page  Rev. 1.00 �� ���i� 1�� �01� Rev. 1.00 �9 ���i� 1�� �01� HT68FB240 USB Low Speed 8-Bit Flash MCU Inadditiontothepower-onreset,situationsmayarisewhereitisnecessarytoforcefullyapplya resetconditionwhenthemicrocontrollerisrunning.Oneexampleofthisiswhereafterpowerhas beenappliedandthemicrocontrollerisalreadyrunning,theRESlineisforcefullypulledlow.In suchacase,knownasanormaloperationreset,someoftheregistersremainunchangedallowing themicrocontrollertoproceedwithnormaloperationaftertheresetlineisallowedtoreturnhigh. AnothertypeofresetiswhentheWatchdogTimeroverflowsandresetsthemicrocontroller.All typesofresetoperationsresultindifferentregisterconditionsbeingsetup.Anotherresetexistsin theformofaLowVoltageReset,LVR,whereafullreset,similartotheRESresetisimplementedin situationswherethepowersupplyvoltagefallsbelowacertainthreshold. Reset Functions Thereareseveralwaysinwhicharesetcanoccur,througheventsoccurringbothinternallyand externally: Power-on Reset Themostfundamentalandunavoidableresetistheonethatoccursafterpowerisfirstappliedto themicrocontroller.AswellasensuringthattheProgramMemorybeginsexecutionfromthefirst memoryaddress,apower-onresetalsoensuresthatcertainotherregistersarepresettoknown conditions.AlltheI/Oportandportcontrolregisterswillpowerupinahighconditionensuringthat allI/Oportswillbefirstsettoinputs. RE S VD D In te r nal R es et 0. 9 V DD t RS TD + t SST t RS TD + t SST Power-On Reset Timing Chart RES Pin AlthoughthemicrocontrollerhasaninternalRCresetfunction,iftheVDDpowersupplyrisetime isnotfastenoughordoesnotstabilisequicklyatpower-on,theinternalresetfunctionmaybe incapableofprovidingproperresetoperation.Forthisreasonitisrecommendedthatanexternal RCnetworkisconnectedtotheRESpin,whoseadditionaltimedelaywillensurethattheRESpin remainslowforanextendedperiodtoallowthepowersupplytostabilise.Duringthistimedelay, normaloperationofthemicrocontrollerwillbeinhibited.AftertheRESlinereachesacertain voltagevalue,theresetdelaytimetRSTDisinvokedtoprovideanextradelaytimeafterwhichthe microcontrollerwillbeginnormaloperation.TheabbreviationSSTinthefiguresstandsforSystem Start-upTimer. FormostapplicationsaresistorconnectedbetweenVDDandtheRESpinandacapacitorconnected betweenVSSandtheRESpinwillprovideasuitableexternalresetcircuit.Anywiringconnectedto theRESpinshouldbekeptasshortaspossibletominimiseanystraynoiseinterference. ForapplicationsthatoperatewithinanenvironmentwheremorenoiseispresenttheEnhancedReset Circuitshownisrecommended. |
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