HT67F488 datasheet(39/171 Pages) HOLTEK

39 / 171 page

Rev. 1.60

�ove��e� �� 016

HT67F488/HT67F489

TinyPowerTM A/D Flash MCU with LCD & EEPROM

External 32.768kHz Crystal Oscillator – LXT

TheExternal32.768kHzCrystalSystemOscillatorisoneofthelowfrequencyoscillatorchoices,

whichisselectedviatheFSUBCregister.Thisclocksourcehasafixedfrequencyof32.768kHzand

requiresa32.768kHzcrystaltobeconnectedbetweenpinsXT1andXT2.Theexternalresistorand

capacitorcomponentsconnectedtothe32.768kHzcrystalarenecessarytoprovideoscillation.For

applicationswhereprecisefrequenciesareessential,thesecomponentsmayberequiredtoprovide

frequencycompensationduetodifferentcrystalmanufacturingtolerances.Duringpower-upthereis

atimedelayassociatedwiththeLXToscillatorwaitingforittostart-up.

WhenthemicrocontrollerenterstheSLEEPorIDLEMode,thesystemclockisswitchedofftostop

microcontrolleractivityandtoconservepower.However,inmanymicrocontrollerapplications

itmaybenecessarytokeeptheinternaltimersoperationalevenwhenthemicrocontrollerisin

theSLEEPorIDLEMode.Todothis,anotherclock,independentofthesystemclock,mustbe

provided.

However,forsomecrystals,toensureoscillationandaccuratefrequencygeneration,itisnecessary

toaddtwosmallvalueexternalcapacitors,C1andC2.TheexactvaluesofC1andC2should

beselectedinconsultationwiththecrystalorresonatormanufacturerspecification.Theexternal

parallelfeedbackresistor,RP,isrequired.

TheFSUBCregisterdeterminesiftheXT1/XT2pinsareusedfortheLXToscillatororasI/Opins.

• IftheLXToscillatorisnotusedforanyclocksource,theXT1/XT2pinscanbeusedasnormalI/O

pins.

• IftheLXToscillatorisusedforanyclocksource,the32.768kHzcrystalshouldbeconnectedto

theXT1/XT2pins.

Foroscillatorstabilityandtominimisetheeffectsofnoiseandcrosstalk,itisimportanttoensure

thatthecrystalandanyassociatedresistorsandcapacitorsalongwithinterconnectinglinesareall

locatedasclosetotheMCUaspossible.

To i nt er na l

ci rc ui ts

In te rn al

O sc illa to r

Ci rc ui t

No te :1 . Rp , C1 a nd C2 a re r e qui re d.

2. A lt hough n ot s ho wn p in s ha ve a

p ar as it ic c apac it an ce o f ar ound 7pF .

In te r nal R C

Os c illa to r

32 .7 68

kH z

XT 1

XT 2

External LXT Oscillator

LXT Oscillator C1 and C2 Values

Crystal Frequency

3�.768kHz

10pF

�ote: 1. C1 and C� values a�e fo� guidance only.

�. RP=5M~10MΩ is recommended.

32.768kHz Crystal Recommended Capacitor Values

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