Holtek BS82B12A-3 - LXT Oscillato� Low Powe� F�Nction; Operating Modes and System Clocks; S�Stem Clocks

Rev. 1.20 48 January 23, 2015 Rev. 1.20 49 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
LXT Oscillator Low Power Function
TheLXToscillatorcanfunctioninoneoftwomodes,theQuickStartModeandtheLowPower
Mode.ThemodeselectionisexecutedusingtheLXTLPbitintheCTRLregister.
LXTLP Bit LXT Mode
0
Quick Start
1 Low-power
Afterpoweron,theLXTLPbitwillbeautomaticallyclearedtozeroensuringthattheLXToscillator
isintheQuickStartoperatingmode.IntheQuickStartModetheLXToscillatorwillpowerup
andstabilisequickly.However,aftertheLXToscillatorhasfullypoweredupitcanbeplaced
intotheLow-powermodebysettingtheLXTLPbithigh.Theoscillatorwillcontinuetorunbut
withreducedcurrentconsumption,asthehighercurrentconsumptionisonlyrequiredduringthe
LXToscillatorstart-up.Inpowersensitiveapplications,suchasbatteryapplications,wherepower
consumptionmustbekepttoaminimum,itisthereforerecommendedthattheapplicationprogram
setstheLXTLPbithighabout2secondsafterpower-on.
Itshouldbenotedthat,nomatterwhatconditiontheLXTLPbitissetto,theLXToscillatorwill
alwaysfunctionnormally,theonlydifferenceisthatitwilltakemoretimetostartupifintheLow-
powermode.
Operating Modes and System Clocks
Presentdayapplicationsrequirethattheirmicrocontrollershavehighperformancebutoftenstill
demandthattheyconsumeaslittlepoweraspossible,conictingrequirementsthatareespecially
trueinbatterypoweredportableapplications.Thefastclocksrequiredforhighperformancewill
bytheirnatureincreasecurrentconsumptionandofcoursevice-versa,lowerspeedclocksreduce
currentconsumption.AsHoltekhasprovidedthesedeviceswithbothhighandlowspeedclock
sourcesandthemeanstoswitchbetweenthemdynamically,theusercanoptimisetheoperationof
theirmicrocontrollertoachievethebestperformance/powerratio.
System Clocks
Themainsystemclock,cancomefromeitherahighfrequency,f
H
,orlowfrequency,f
SUB
,source,
andisselectedusingtheHLCLKbitandCKS2~CKS0bitsintheSMODregister.Thehighspeed
systemclockissourcedfromtheHIRCoscillator.Thelowspeedsystemclocksourcecanbesourced
frominternalclockf
SUB
.Dependingonthedevices,iff
SUB
isselectedthenitissourcedbytheLIRC
oscillatororcanbesourcedbyeithertheLXTorLIRCoscillators,selectedviaaconfiguration
option.Theotherchoice,whichisadividedversionofthehighspeedsystemoscillatorhasarange
off
H
/2~f
H
/64.