Holtek HT66F30 - External 32.�68 Khz Crystal Oscillator - LXT

Rev. 2.50 54 June 22, 2017 Rev. 2.50 55 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
External 32.768kHz 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32.768kHz
andrequiresa32.768kHzcrystaltobeconnectedbetweenpinsXT1andXT2.Theexternalresistor
andcapacitorcomponentsconnectedtothe32.768kHz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manufacturersspecification.Theexternal
parallelfeedbackresistor,Rp,isrequired.
SomecongurationoptionsdetermineiftheXT1/XT2pinsareusedfortheLXToscillatororasI/O
pins.
•
IftheLXToscillatorisnotusedforanyclocksource,theXT1/XT2pinscanbeusedasnormal
I/Opins.
• IftheLXToscillatorisusedforanyclocksource,the32.768kHz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
thatthecrystalandanyassociatedresistorsandcapacitorsalongwithinterconnectinglinesareall
locatedasclosetotheMCUaspossible.

 
   
 
 



     
    
   
    
   

 


External LXT Oscillator
LXT Oscillator C1 and C2 Values
Crystal Frequency C1 C2
32.768kHz 10pF 10pF
Note:
1. C1 and C2 values are for guidance only.
2. R
P
=5MΩ~10MΩ is recommended.
32.768kHz Crystal Recommended Capacitor Values