Holtek HT66F0175 - Internal Hi�H Speed RC Oscillator - HIRC; External 3�.�6� Khz Crystal Oscillator - LXT

Rev. 1.50 42 August 28, 2017 Rev. 1.50 43 August 28, 2017
HT66F0175/HT66F0185
A/D Flash MCU with EEPROM
HT66F0175/HT66F0185
A/D Flash MCU with EEPROM
Internal High Speed RC Oscillator – HIRC
TheinternalRCoscillatorisafullyintegratedsystemoscillatorrequiringnoexternalcomponents.
TheinternalRCoscillatorhasthreexedfrequenciesof8MHz,12MHz,16MHz.Devicetrimming
duringthemanufacturingprocessandtheinclusionofinternalfrequencycompensationcircuitsare
usedtoensurethattheinuenceofthepowersupplyvoltage,temperatureandprocessvariationson
theoscillationfrequencyareminimised.Asaresult,atapowersupplyofeither3Vor5Vandata
temperatureof25°Cdegrees,thexedoscillationfrequencyof8MHz,12MHzor16MHzwillhave
atolerancewithin2%.Notethatifthisinternalsystemclockoptionisselected,asitrequiresno
externalpinsforitsoperation,I/OpinsarefreeforuseasnormalI/Opins.
External 32.768 kHz Crystal Oscillator – LXT
TheExternal32.768kHzCrystalSystemOscillatorisoneofthelowfrequencyoscillatorchoices,
whichisselectedviacongurationoption.Thisclocksourcehasaxedfrequencyof32.768kHz
andrequiresa32.768kHzcrystaltobeconnectedbetweenpinsXT1andXT2.Theexternalresistor
andcapacitorcomponentsconnectedtothe32.768kHzcrystalarenecessarytoprovideoscillation.
Forapplicationswhereprecisefrequenciesareessential,thesecomponentsmayberequiredto
providefrequencycompensationduetodifferentcrystalmanufacturingtolerances.Duringpower-up
thereisatimedelayassociatedwiththeLXToscillatorwaitingforittostart-up.
WhenthemicrocontrollerenterstheSLEEPorIDLEMode,thesystemclockisswitchedofftostop
microcontrolleractivityandtoconservepower.However,inmanymicrocontrollerapplications
itmaybenecessarytokeeptheinternaltimersoperationalevenwhenthemicrocontrollerisin
theSLEEPorIDLEMode.Todothis,anotherclock,independentofthesystemclock,mustbe
provided.
However,forsomecrystals,toensureoscillationandaccuratefrequencygeneration,itisnecessary
toaddtwosmallvalueexternalcapacitors,C1andC2.TheexactvaluesofC1andC2should
beselectedinconsultationwiththecrystalorresonatormanufacturerspecification.Theexternal
parallelfeedbackresistor,R
P
,isrequired.
SomecongurationoptionsdetermineiftheXT1/XT2pinsareusedfortheLXToscillatororasI/O
orotherpin-sharedfunctionalpins.
• IftheLXToscillatorisnotusedforanyclocksource,theXT1/XT2pinscanbeusedasnormalI/
Oorotherpin-sharedfunctionalpins.
• IftheLXToscillatorisusedforanyclocksource,the32.768kHzcrystalshouldbeconnectedto
theXT1/XT2pins.
Foroscillatorstabilityandtominimisetheeffectsofnoiseandcrosstalk,itisimportanttoensure
thatthecrystalandanyassociatedresistorsandcapacitorsalongwithinterconnectinglinesareall
locatedasclosetotheMCUaspossible.

 
   
 
 



     
    
   
    
   

 


External LXT Oscillator