HT45FH4M
Lithium Battery Backup Power ASSP MCU
TheꢀlowerꢀbyteꢀofꢀtheꢀProgramꢀCounter,ꢀknownꢀasꢀtheꢀProgramꢀCounterꢀLowꢀregisterꢀorꢀPCL,ꢀisꢀ
availableꢀforꢀprogramꢀcontrolꢀandꢀisꢀaꢀreadableꢀandꢀwriteableꢀregister.ꢀByꢀtransferringꢀdataꢀdirectlyꢀ
intoꢀthisꢀregister,ꢀaꢀshortꢀprogramꢀjumpꢀcanꢀbeꢀexecutedꢀdirectly,ꢀhowever,ꢀasꢀonlyꢀthisꢀlowꢀbyteꢀ
isꢀavailableꢀforꢀmanipulation,ꢀtheꢀjumpsꢀareꢀlimitedꢀtoꢀtheꢀpresentꢀpageꢀofꢀmemory,ꢀthatꢀisꢀ256ꢀ
locations.ꢀWhenꢀsuchꢀprogramꢀjumpsꢀareꢀexecutedꢀitꢀshouldꢀalsoꢀbeꢀnotedꢀthatꢀaꢀdummyꢀcycleꢀ
willꢀbeꢀinserted.ꢀManipulatingꢀtheꢀPCLꢀregisterꢀmayꢀcauseꢀprogramꢀbranching,ꢀsoꢀanꢀextraꢀcycleꢀisꢀ
neededꢀtoꢀpre-fetch.
Stack
ThisꢀisꢀaꢀspecialꢀpartꢀofꢀtheꢀmemoryꢀwhichꢀisꢀusedꢀtoꢀsaveꢀtheꢀcontentsꢀofꢀtheꢀProgramꢀCounterꢀ
only.ꢀTheꢀstackꢀisꢀneitherꢀpartꢀofꢀtheꢀdataꢀnorꢀpartꢀofꢀtheꢀprogramꢀspace,ꢀandꢀisꢀneitherꢀreadableꢀnorꢀ
writeable.ꢀTheꢀactivatedꢀlevelꢀisꢀindexedꢀbyꢀtheꢀStackꢀPointer,ꢀandꢀisꢀneitherꢀreadableꢀnorꢀwriteable.ꢀ
Atꢀaꢀsubroutineꢀcallꢀorꢀinterruptꢀacknowledgeꢀsignal,ꢀtheꢀcontentsꢀofꢀtheꢀProgramꢀCounterꢀareꢀpushedꢀ
ontoꢀtheꢀstack.ꢀAtꢀtheꢀendꢀofꢀaꢀsubroutineꢀorꢀanꢀinterruptꢀroutine,ꢀsignaledꢀbyꢀaꢀreturnꢀinstruction,ꢀ
RETꢀorꢀRETI,ꢀtheꢀProgramꢀCounterꢀisꢀrestoredꢀtoꢀitsꢀpreviousꢀvalueꢀfromꢀtheꢀstack.ꢀAfterꢀaꢀdeviceꢀ
reset,ꢀtheꢀStackꢀPointerꢀwillꢀpointꢀtoꢀtheꢀtopꢀofꢀtheꢀstack.
Ifꢀtheꢀstackꢀisꢀfullꢀandꢀanꢀenabledꢀinterruptꢀtakesꢀplace,ꢀtheꢀinterruptꢀrequestꢀflagꢀwillꢀbeꢀrecordedꢀ
butꢀtheꢀacknowledgeꢀsignalꢀwillꢀbeꢀinhibited.ꢀWhenꢀtheꢀStackꢀPointerꢀisꢀdecremented,ꢀbyꢀRETꢀorꢀ
RETI,ꢀtheꢀinterruptꢀwillꢀbeꢀserviced.ꢀThisꢀfeatureꢀpreventsꢀstackꢀoverflowꢀallowingꢀtheꢀprogrammerꢀ
toꢀuseꢀtheꢀstructureꢀmoreꢀeasily.ꢀHowever,ꢀwhenꢀtheꢀstackꢀisꢀfull,ꢀaꢀCALLꢀsubroutineꢀinstructionꢀcanꢀ
stillꢀbeꢀexecutedꢀwhichꢀwillꢀresultꢀinꢀaꢀstackꢀoverflow.ꢀPrecautionsꢀshouldꢀbeꢀtakenꢀtoꢀavoidꢀsuchꢀ
casesꢀwhichꢀmightꢀcauseꢀunpredictableꢀprogramꢀbranching.ꢀIfꢀtheꢀstackꢀisꢀoverflow,ꢀtheꢀfirstꢀProgramꢀ
Counterꢀsaveꢀinꢀtheꢀstackꢀwillꢀbeꢀlost.
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Arithmetic and Logic Unit – ALU
Theꢀarithmetic-logicꢀunitꢀorꢀALUꢀisꢀaꢀcriticalꢀareaꢀofꢀtheꢀmicrocontrollerꢀthatꢀcarriesꢀoutꢀarithmeticꢀ
andꢀlogicꢀoperationsꢀofꢀtheꢀinstructionꢀset.ꢀConnectedꢀtoꢀtheꢀmainꢀmicrocontrollerꢀdataꢀbus,ꢀtheꢀALUꢀ
receivesꢀrelatedꢀinstructionꢀcodesꢀandꢀperformsꢀtheꢀrequiredꢀarithmeticꢀorꢀlogicalꢀoperationsꢀafterꢀ
whichꢀtheꢀresultꢀwillꢀbeꢀplacedꢀinꢀtheꢀspecifiedꢀregister.ꢀAsꢀtheseꢀALUꢀcalculationꢀorꢀoperationsꢀmayꢀ
resultꢀinꢀcarry,ꢀborrowꢀorꢀotherꢀstatusꢀchanges,ꢀtheꢀstatusꢀregisterꢀwillꢀbeꢀcorrespondinglyꢀupdatedꢀtoꢀ
reflectꢀtheseꢀchanges.ꢀTheꢀALUꢀsupportsꢀtheꢀfollowingꢀfunctions:
•ꢀ Arithmeticꢀoperations:ꢀADD,ꢀADDM,ꢀADC,ꢀADCM,ꢀSUB,ꢀSUBM,ꢀSBC,ꢀSBCM,ꢀDAA
•ꢀ Logicꢀoperations:ꢀAND,ꢀOR,ꢀXOR,ꢀANDM,ꢀORM,ꢀXORM,ꢀCPL,ꢀCPLA
•ꢀ RotationꢀRRA,ꢀRR,ꢀRRCA,ꢀRRC,ꢀRLA,ꢀRL,ꢀRLCA,ꢀRLC
•ꢀ IncrementꢀandꢀDecrementꢀINCA,ꢀINC,ꢀDECA,ꢀDEC
•ꢀ Branchꢀdecision,ꢀJMP,ꢀSZ,ꢀSZA,ꢀSNZ,ꢀSIZ,ꢀSDZ,ꢀSIZA,ꢀSDZA,ꢀCALL,ꢀRET,ꢀRETI
Rev. 1.10
16
�anꢀaꢁꢂ 1ꢃꢄ ꢅ01ꢆ
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
