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7.3ClockManagement
7.3.1ModuleClockON/OFF
7.3.2ModuleClockFrequencyScaling
7.3.3PLLBypassandPowerDown
7.4ARMandDSPSleepModeManagement
7.4.1ARMWait-For-InterruptSleepMode
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ClockManagement
Themoduleclockon/offfeatureallowssoftwaretodisableclockstomoduleindividually,inordertoreduce
themodule'sactivepowerconsumptionto0.TheDM646xDMSoCisdesignedinfullstaticCMOS;thus,
whenamoduleclockstops,themodule'sstateispreserved.Whentheclockisrestarted,themodule
resumesoperatingfromthestoppingpoint.
Note:Stoppingclockstoamoduleonlyaffectsactivepowerconsumption,itdoesnotaffect
leakagepowerconsumption.
Ifamodule'sclock(s)isstoppedwhiletheconfigurationbusortheEDMAbusisaccessingit,theaccess
maynotoccur,andcouldpotentiallylock-upthebus.Ensurethatallofthetransactionstothemoduleare
finishedpriortostoppingtheclocks.
Thepowerandsleepcontroller(PSC)controlsmoduleclockgating.Theproceduretoturnmoduleclocks
on/offisdescribedinChapter6.
ModuleclockfrequencyisscalablebyprogrammingthePLL'smultiplyanddivideparameters.Reducing
theclockfrequencyreducestheactiveswitchingpowerconsumptionlinearlywithfrequency.Ithasno
impactonleakagepowerconsumption.
Chapter5describeshowtoprogramthePLLfrequencyandthefrequencyconstraints.
ThePLLscanbebypassedintheDM646xDMSoC.BypassingthePLLssendsthePLLreferenceclockto
thepostdividersofthePLLCinsteadoftothePLLVCOoutput.ThePLLreferenceclockistypicallyat
27MHZ;therefore,thisbypassmodecanbeusedtoreducethecoreandmoduleclockfrequenciesto
verylowmaintenancelevelswithoutusingthePLLduringperiodsofverylowsystemactivity.
Furthermore,thePLLcanbepowereddowninbypassmodetosaveadditionalactivepower.
Chapter5describesPLLbypassandPLLpowerdowndetails.
TheARMmodulecannothaveitsclockgatedinthePSCmodule.However,theARMincludesaspecial
sleepmodecalled“wait-for-interrupt”.Whenthewait-for-interruptmodeisenabled,theclocktotheCPU
coreisshutoffandtheARM9iscompletelyinactiveandonlyresumesoperationafterreceivingan
interrupt.Thismodedoesnotaffectleakageconsumption.
Youcanenablethewait-for-interruptmodeviatheCP15register#7usingthefollowinginstruction:
•mcrp15,#0,rd,c7,c0,#4
Thefollowingsequenceexemplifieshowtoenterwait-for-interruptmode:
•Enableanyinterrupt(forexample,anexternalinterrupt).
•Enablewait-for-interruptmodeusingthefollowingCP15instruction:
–mcrp15,#0,rd,c7,c0,#4
SPRUEP9A–May2008PowerManagement81
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