SoC设计方法和实现第十一章-低功耗.ppt
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1、SoC设计方法与实现设计方法与实现郭炜郭炜 郭筝郭筝 谢憬谢憬第十一章第十一章 低功耗设计低功耗设计OutlineWhy low powerSources of power consumptionLow power design methodologyLow power techniques Power analysis and toolsTrends in the futureWhy Low PowerPotable system-Battery lifetimenExample:mobile phone,PDA,Digital cameraDesktops:high power consu
2、mptionnReliability and performancenNeed expensive chip package,cooling systemSeveral deleterious effectsnDecreased reliability and performancenIncreased cost:packaging cost and cooling systemnExceed power limits of the chip&systemPower,Cost and HeatComponent:silicon and packagenIncreased die size(wi
3、der power busses)nNeed better thermal capabilities(package material)nNeed better electrical capabilities System:Cooling and mechanicalsnLarger fansnOversized power suppliesPower limits to the walln1100W dc limit for 110V/20A plugChallenge of Design as Process ScalingOutlineWhy low powerSources of po
4、wer consumptionLow power design methodologyLow power techniques Power analysis and toolsTrends in the futureSource of Power ConsumptionDynamic power consumptionStatic power consumptionKey areas of power consumption in SOCSource of Power Dissipation in CMOS DevicesC =node capacitancesNsw=switching ac
5、tivities (number of gate transitions per clock cycle)F =frequency of operationVDD =supply voltageQsc =charge carried byshort circuit currentper transitionIleak =leakage currentStatic Power Consumption:Leakage currents:nSub-threshold current(I2)nGate leakage nGate tunnelling(I4)nGate induced drain le
6、akage(I3)npn-junction reverse current(I1)DC currentsnAnalog circuit:sense-amps,pull-upsnState dependentLeakage vs.ProcessWhat will be the dominated leakage current?Long Channel(L1um)Very small leakageShort channel(L180nm,tox30A)Subthreshold leakageVery short channel(L90nm,tox20A)subthreshold+gate le
7、akageNano-scaled(L90nm,Tox20A)Subthreshold+gate+junction leakageSub-threshold leakage current Has become quite important with technology scalingGate leakage currentIs becoming important with shrinking device dimensions PN junction leakage currentNegligible OutlineWhy low powerSources of power consum
8、ptionLow power design methodologyLow power techniques Low power analysis and toolsTrends in the futureLow Power Design MethodologyMust know your systemMaximize the performance while minimize the power consumptionMinimize the power consumption while maximize the performanceOpportunities for Power Sav
9、ingOutlineWhy low powerSources of power consumptionLow power design methodologyLow power techniques Power analysis and toolsTrends in the futureLow Power TechniquesLeakage power controlDynamic power controlArchitecture level power optimizationSystem level power optimizationLow Power TechniquesProces
10、s scaling nLow Vdd,Multi-thresholdVoltage scalingnSubstrate bias(200mv)nMulti-voltage(voltage island)nDynamic voltage scaling;multi-thresholdHW design techniquesnPre-computation,glitch minimization,Logic level,Physical level optimizationLow power System/SW nPower aware Operation System,compiler,SW d
11、esign etc.Low Power Techniques on Chip DesignLeakage PowernMulti Vt optimizationnPower gatingnSubstrate biasnPower gatingDynamic PowernMulti-voltage designnAdvanced clock-gatingnGate-level power optimizationTechniques for Reduce Leakage PowerUsing Multi-Vt LibrariesTiming and leakage tradeoffnLow Vt
12、 cell:faster speed,high leakagenHigh Vt cell:slower speed,lower leakagenPrinciple:low Vt for critical path and high Vt for non-critical pathsHigh Vt cell on Critical PathHints:You need to have dual Vt library1.You need to pay for the extra layer mask for multi-vt Using Multi-Vt Libraries cont.Synthe
13、sis Strategy:nUse high Vt cells first,then fix setup violation by replace the high Vt cells on the critical path to low Vt cellsnUse low Vt cells first,then swap to high Vt cells,fix setup violation by swap low Vt cells on the pathsNo area penalty nLibrary design for freely mix and match on SoC desi
14、gnPower Gating Also called Multi-Threshold CMOS(MTCOMS),logic sleep control,etc.Active mode:sleep control devices on,VDDV and GNDV act as virtual supplySleep mode:sleep control devices off,reduce leakagenHigh Vt transistors reducing both leakage and switching powerPower Gating cont.Sleep transistors
15、 used only on the supply rail or on both supply and ground railsnNot added on every gatePower gating retention registernActive modenHigh performance regular FF functionnSleep ModenCut-off VddnLow leakage stage saving latch functionBody BiasVariable threshold according to body biasingZero body bias i
16、n active mode(Low Vt)Reverse body bias in stand-by mode(High Vt)Tradeoff between the time on module turn-on and leakage currentTriple well structure CMOS InverterHint:Do you have the triple well structuredstandard cell lib?Techniques for Reduce Dynamic PowerMulti Voltage DesignBlock based approach i
17、n the design flowNeed to additional isolation cells and voltage level-shifter cells between voltage domainsClock Gating Technology Toggling consume power.Enable the module clock only when neededgated_clkEnableLogicGlobalClkComb.LogicDataRegClock Gating Cell DesignProblem with simple clock gating:nUn
18、completed cyclenGlitchclk_enclkgclkClock Gating with Latchclk_enclkgclkAdd a transparent-low latchMake sure the clk gating cells are placed tightly for correct function clk cell hardeningCommonly in SoC:make a“hardmacro”-clk gating cellRTL code for clk cell:always(clk or clk_en)if(!clk)ctrl_latch=cl
19、k_en;assign gclk=ctrl_latch&clkClock gating cells and a glitch free clock gatingDQGLatchclk_enclkgclkClock Gating With Integrated Test LogicAbility to let clk pass through in test mode (TEST=1)Gated Clock in Clock Tree DesignDisable clocking near the root of a clock tree,instead of at each FF.Specia
20、l care must be taken in clk tree synthesis to prevent the buffers inserted between clk root and the clk gating cellGate Level OptimizationTechnology independent optimization:nCircuit optimization:logic optimization,reduce redundant logicnTrimming for low power:reduce positive slacknGate resizingnPin
21、 swapping/reassignmentnRe-mappingnPhase assignmentnRe-factoringLow power driven technology mappingnlow power cellGate Level Optimization Gate SizingGate sizingnDown-size gates on fast paths to decrease their input capacitances for minimizing switching current in front driver nEnlarge heavily loaded
22、gates to increase their output slew rates for minimizing short-circuit currentDealing with GlitchesFor some type of data path circuits,up to 60%of the dynamic power is due to glitchesVery expensive calculationnNeed to propagate probabilistic waveformsExample:Glitch MinimizationHazardous transition o
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