HT66F20 datasheet(71/298 Pages) HOLTEK

71 / 298 page

Rev. 1.90

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Rev. 1.90

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HT66F20/HT66F30/HT66F40/HT66F50/HT66F60

HT66FU30/HT66FU40/HT66FU50/HT66FU60

Enhanced A/D Flash Type 8-Bit MCU with EEPROM

HT66F20/HT66F30/HT66F40/HT66F50/HT66F60

HT66FU30/HT66FU40/HT66FU50/HT66FU60

Enhanced A/D Flash Type 8-Bit MCU with EEPROM

8- st age Di vi der

f S

WD T Ti me - out

(2 8 /f S ~ 2 15 /f S )

f S /2 8

WD T Pr e sca le r

8- to -1 M U X

Cl ear W DT T yp e

C onf i gur at i on Op ti on

CL R WD T1 F la g

CL R WD T2 F la g

1 or 2 I ns tr uc ti ons

CL R

f SU B

WS 2~ WS 0

( f S /2 8 ~ f S /2 15 )

f SY S /4

LX T

LI RC

C onf i gur at io n

O p ti on

C onf i gur at io n

Op ti on

Watchdog Timer

Reset and Initialisation

Aresetfunctionisafundamentalpartofanymicrocontrollerensuringthatthedevicecanbeset

tosomepredeterminedconditionirrespectiveofoutsideparameters.Themostimportantreset

conditionisafterpowerisfirstappliedtothemicrocontroller.Inthiscase,internalcircuitrywill

ensurethatthemicrocontroller,afterashortdelay,willbeinawelldefinedstateandreadyto

executethefirstprograminstruction.Afterthispower-onreset,certainimportantinternalregisters

willbesettodefinedstatesbeforetheprogramcommences.OneoftheseregistersistheProgram

Counter,whichwillberesettozeroforcingthemicrocontrollertobeginprogramexecutionfromthe

lowestProgramMemoryaddress.

Inadditiontothepower-onreset,situationsmayarisewhereitisnecessarytoforcefullyapply

aresetconditionwhenthemicrocontrollerisrunning.Oneexampleofthisiswhereafterpower

hasbeenappliedandthemicrocontrollerisalreadyrunning,theRESlineisforcefullypulledlow.

Insuchacase,knownasanormaloperationreset,someofthemicrocontrollerregistersremain

unchangedallowingthemicrocontrollertoproceedwithnormaloperationaftertheresetlineis

allowedtoreturnhigh.

AnothertypeofresetiswhentheWatchdogTimeroverflowsandresetsthemicrocontroller.All

typesofresetoperationsresultindifferentregisterconditionsbeingsetup.Anotherresetexistsin

theformofaLowVoltageReset,LVR,whereafullreset,similartotheRESresetisimplementedin

situationswherethepowersupplyvoltagefallsbelowacertainthreshold.

Reset Functions

Therearefivewaysinwhichamicrocontrollerresetcanoccur,througheventsoccurringboth

internallyandexternally:

• Power-onReset

Themostfundamentalandunavoidableresetistheonethatoccursafterpowerisfirstappliedto

themicrocontroller.AswellasensuringthattheProgramMemorybeginsexecutionfromthefirst

memoryaddress,apower-onresetalsoensuresthatcertainotherregistersarepresettoknown

conditions.AlltheI/Oportandportcontrolregisterswillpowerupinahighconditionensuring

thatallpinswillbefirstsettoinputs.

RE S

VD D

In te r nal R es et

0. 9 V DD

t RS TD + t SST

Note:tRSTDispower-ondelay,typicaltime=100ms

Power-on Reset Timing Chart

HT66F20 Datasheet(PDF) 71 Page - Holtek Semiconductor Inc

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