Holtek BS82B12A-3 - Exte�Nal �2.768 Khz C

Rev. 1.20 46 January 23, 2015 Rev. 1.20 47 January 23, 2015
BS82B12A-3/BS82C16A-3/BS82D20A-3
Touch Key 8-Bit Flash MCU with LED/LCD Driver
BS82B12A-3/BS82C16A-3/BS82D20A-3
Touch Key 8-Bit Flash MCU with LED/LCD Driver
External 32.768kHz Crystal Oscillator – LXT
FortheBS82C16A-3andBS82D20A-3devices,theExternal32.768kHzCrystalSystemOscillator
isoneofthelowfrequencyoscillatorchoices,whichisselectedviaconfiguration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manufacturerspecication.Theexternalparallel
feedbackresistor,Rp,isrequired.Notethatthewireconnectedbetweenthe32.768kHzcrystaland
theXT1/XT2pinsshouldbekeptasshortaspossibletominimizethestraynoiseinterface.
ThecongurationoptiondeterminesiftheXT1/XT2pinsareusedfortheLXToscillatororasI/O
pinsorotherpin-sharedfunctions.
• IftheLXToscillatorisnotusedforanyclocksource,theXT1/XT2pinscanbeusedasnormalI/
Opinsorotherpin-sharedfunctions.
• 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minimize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
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