英语翻译

Port3:Port3isan8-bitbidirectionalIOportwithinternalpullups.ThePort3outputbufferscandriveLSTTLinputs.Port3pinsthathave1’swrittentothemarepulledhighbytheinternalpullupsandinthatstatecanbeusedasinputs.Asinputs,Port3pinsthatareexternallypulledlowwillsourcecurrent(Ionthedatasheet)becauseofthepullups.Port3alsoservesthefunctionsofvariousspecialfeaturesoftheMCS-51Family,aslistedbelow:

Port3alsoreceivessomecontrolsignalsforEPROMprogrammingandprogramverification.

RST:Resetinput.Ahighonthispinfortwomachinecycleswhiletheoscillatorisrunningresetsthedevice.TheportpinswillbedriventotheirresetconditionwhenaminimumVIH1voltageisappliedwhethertheoscillatorisrunningornot.Aninternalpulldownresistorpermitsapower-onresetwithonlyacapacitorconnectedtoVcc.

ALEPROG:AddressLatchEnableoutputsignalforlatchingthelowbyteoftheaddressduringaccessestoexternalmemory.Thispinisalsotheprogrampulseinput(PROG)duringEPROMprogrammingforthe87C51.Ifdesired,ALEoperationcanbedisabledbysettingbit0ofSFRlocation8EH.Withthisbitsetthepinisweaklypulledhigh.HowevertheALEdisablefeaturewillbesuspendedduringaMOVXorMOVCinstruction,idlemodepowerdownmodeandICEmodeTheALEdisablefeaturewillbeterminatedbyreset.

WhentheALEdisablefeatureissuspendedorterminated,theALEpinwillnolongerbepulledupweakly.SettingtheALE-disablebithasnoeffectifthemicrocontrollerisinexternalexecutionmode.

InnormaloperationALEisemittedataconstantrateof16theoscillatorfrequency,andmaybe

usedforexternaltimingorclockingpurposes.Note,however,thatoneALEpulseisskippedduringeachaccesstoexternalDataMemory.

PSEN:ProgramStoreEnableistheReadstrobetoExternalProgramMemory.Whenthe87C51BHisexecutingfromInternalProgramMemory,PSENisinactive(high).WhenthedeviceisexecutingcodefromExternalProgramMemory,PSENisactivatedtwiceeachmachinecycle,exceptthattwoPSENactivationsareskippedduringeachaccesstoExternalDataMemory.

EA/Vpp:ExternalAccessenable.EAmustbestrappedtoVSSinordertoenablethe87C51BHto