Holtek HT66F20 - Reset and Initialisation; Reset F�Nctions

Rev. 2.50 70 June 22, 2017 Rev. 2.50 71 June 22, 2017
HT66F20/HT66F30/HT66F40/HT66F50/HT66F60
HT66FU30/HT66FU40/HT66FU50/HT66FU60
A/D Flash MCU with EEPROM
HT66F20/HT66F30/HT66F40/HT66F50/HT66F60
HT66FU30/HT66FU40/HT66FU50/HT66FU60
A/D Flash MCU with EEPROM
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Watchdog Timer
Reset and Initialisation
Aresetfunctionisafundamentalpartofanymicrocontrollerensuringthatthedevicecanbeset
tosomepredeterminedconditionirrespectiveofoutsideparameters.Themostimportantreset
conditionisafterpowerisrstappliedtothemicrocontroller.Inthiscase,internalcircuitrywill
ensurethatthemicrocontroller,afterashortdelay,willbeinawelldefinedstateandreadyto
executetherstprograminstruction.Afterthispower-onreset,certainimportantinternalregisters
willbesettodenedstatesbeforetheprogramcommences.OneoftheseregistersistheProgram
Counter,whichwillberesettozeroforcingthemicrocontrollertobeginprogramexecutionfromthe
lowestProgramMemoryaddress.
Inadditiontothepower-onreset,situationsmayarisewhereitisnecessarytoforcefullyapply
aresetconditionwhenthemicrocontrollerisrunning.Oneexampleofthisiswhereafterpower
hasbeenappliedandthemicrocontrollerisalreadyrunning,theRESlineisforcefullypulledlow.
Insuchacase,knownasanormaloperationreset,someofthemicrocontrollerregistersremain
unchangedallowingthemicrocontrollertoproceedwithnormaloperationaftertheresetlineis
allowedtoreturnhigh.
AnothertypeofresetiswhentheWatchdogTimeroverflowsandresetsthemicrocontroller.All
typesofresetoperationsresultindifferentregisterconditionsbeingsetup.Anotherresetexistsin
theformofaLowVoltageReset,LVR,whereafullreset,similartotheRESresetisimplementedin
situationswherethepowersupplyvoltagefallsbelowacertainthreshold.
Reset Functions
Therearefivewaysinwhichamicrocontrollerresetcanoccur,througheventsoccurringboth
internallyandexternally:
• Power-onReset
Themostfundamentalandunavoidableresetistheonethatoccursafterpowerisrstappliedto
themicrocontroller.AswellasensuringthattheProgramMemorybeginsexecutionfromtherst
memoryaddress,apower-onresetalsoensuresthatcertainotherregistersarepresettoknown
conditions.AlltheI/Oportandportcontrolregisterswillpowerupinahighconditionensuring
thatallpinswillberstsettoinputs.
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Note:t
RSTD
ispower-ondelay,typicaltime=100ms
Power-on Reset Timing Chart