Holtek HT66F002 - Program Counter

Rev. 1.71 20 April 11, 2017 Rev. 1.71 21 April 11, 2017
HT66F002/HT66F0025/HT66F003/HT66F004
Cost-Effective A/D Flash MCU with EEPROM
HT66F002/HT66F0025/HT66F003/HT66F004
Cost-Effective A/D Flash MCU with EEPROM
Forinstructionsinvolvingbranches,suchasjumporcallinstructions,twomachinecyclesare
requiredtocompleteinstructionexecution.Anextracycleisrequiredastheprogramtakesone
cycletorstobtaintheactualjumporcalladdressandthenanothercycletoactuallyexecutethe
branch.Therequirementforthisextracycleshouldbetakenintoaccountbyprogrammersintiming
sensitiveapplications.
    
  
 






 
  
 
  



 
   
      
   
Instruction Fetching
Program Counter
Duringprogramexecution,theProgramCounterisusedtokeeptrackoftheaddressofthe
nextinstructiontobeexecuted.Itisautomaticallyincrementedbyoneeachtimeaninstruction
isexecutedexceptforinstructions,suchas“JMP”or“CALL”thatdemandajumptoanon-
consecutiveProgramMemoryaddress.Onlythelower8bits,knownastheProgramCounterLow
Register,aredirectlyaddressablebytheapplicationprogram.
Whenexecutinginstructionsrequiringjumpstonon-consecutiveaddressessuchasajump
instruction,asubroutinecall,interruptorreset,etc.,themicrocontrollermanagesprogramcontrol
byloadingtherequiredaddressintotheProgramCounter.Forconditionalskipinstructions,once
theconditionhasbeenmet,thenextinstruction,whichhasalreadybeenfetchedduringthepresent
instructionexecution,isdiscardedandadummycycletakesitsplacewhilethecorrectinstructionis
obtained.
Device
Program Counter
Program Counter
High byte
PCL Register
HT66F002/HT66F003 PC9~PC8 PCL7~PCL0
HT66F0025/HT66F004 PC10~PC8 PCL7~PCL0
ThelowerbyteoftheProgramCounter,knownastheProgramCounterLowregisterorPCL,is
availableforprogramcontrolandisareadableandwriteableregister.Bytransferringdatadirectly
intothisregister,ashortprogramjumpcanbeexecuteddirectly,however,asonlythislowbyte
isavailableformanipulation,thejumpsarelimitedtothepresentpageofmemory,thatis256
locations.Whensuchprogramjumpsareexecuteditshouldalsobenotedthatadummycycle
willbeinserted.ManipulatingthePCLregistermaycauseprogrambranching,soanextracycleis
neededtopre-fetch.