1Spring 2003 EECS150 – Lec25-powerPage 1EECS150 - Digital DesignLecture 25 - PowerApril 22, 2003John WawrzynekSpring 2003 EECS150 – Lec25-powerPage 2Outline• Motivation for design constraints of power consumption• Power metrics• Power consumption analysis in CMOS• How can a logic designer control power?2Spring 2003 EECS150 – Lec25-powerPage 3Is Power Consumption Important?“The internet and wireless services are getting married”, Simon SegarsSpring 2003 EECS150 – Lec25-powerPage 4Motivation• Portable devices: – handhelds, laptops, phones, MP3 players, cameras, … all need to run for extended periods on small batteries without recharging – Devices that need regular recharging or large heavy batteries will lose out to those that don’t.• Power consumption important even in “tethered” devices. – System cost tracks power consumption:• power supplies, distribution, heat removal– power conservation, environmental concerns• In a span of 10 years we have gone from designing without concern for power consumption to (in many cases) designing with power consumption as the primary design constraint!Why should a digital designer care about power consumption?3Spring 2003 EECS150 – Lec25-powerPage 5Battery Technology• Battery technology has moved very slowly– Moore’s law does not seem to apply• Li-Ion and NiMh still the dominate technologies• Batteries still contribute significant to the weight of mobile devicesToshiba Portege3110 laptop - 20%Handspring PDA - 10%Nokia 61xx -33%Spring 2003 EECS150 – Lec25-powerPage 6Basics• Power supply provides energy for charging and discharging wires and transistor gates. The energy supplied is stored and dissipated as heat.• If a differential amount of charge dq is given a differential increase in energy dw, the potential of the charge is increased by: • By definition of current:dqdwV /=dtdqI /=dtdwP /≡Power: Rate of work being done w.r.t time.Rate of energy being used.IVPdtdqdqdwdtdw ×==×=/∫∞−=tPdtwtotal energyUnits:tEP ∆=Watts = Joules/secondsA very practicalformulation!If we would liketo know total energy4Spring 2003 EECS150 – Lec25-powerPage 7Basics• Warning! In everyday language, the term “power” is used incorrectly in place of “energy.”•Power is not energy.•Power is not something you can run out of.• Power can not be lost or used up.•It is not a thing, it is merely a rate.• It can not be put into a battery any more than velocity can be put in the gas tank of a car.Spring 2003 EECS150 – Lec25-powerPage 8Metrics• One popular metric for microprocessors is: MIPS/watt– MIPS, millions of instructions per second.• Typical modern value?– Watt, standard unit of power consumption.• Typical value for modern processor?– MIPS/watt is reflective of the tradeoff between performance and power. Increasing performance requires increasing power.– Problem with “MIPS/watt”• MIPS/watt values are typically not independent of MIPS– techniques exist to achieve very high MIPS/watt values, but at very low absolute MIPS (used in watches)• Metric only relevant for comparing processors with a similar performance.– One solution, MIPS2/watt. Puts more weight on performance.How do we measure and compare power consumption?5Spring 2003 EECS150 – Lec25-powerPage 9Metrics• How does MIPS/watt relate to energy?• Average power consumption = energy / timeMIPS/watt = instructions/sec / joules/sec = instructions/joule– therefore an equivalent metric (reciprocal) is energy per operation (E/op)• E/op is more general - applies to more that processors– also, usually more relevant, as batteries life is limited by total energy draw.– This metric gives us a measure to use to compare two alternativeimplementations of a particular function.Spring 2003 EECS150 – Lec25-powerPage 10Power in CMOSCpullupnetworkpulldownnetworkVddGND10i(t)v(t)t0 t1v(t)VddSwitching Energy:energy used toswitch a nodeEnergy supplied Energy dissipatedEnergy storedCalculate energy dissipated in pullup:2222121)()()()()(1010101010ddttttddddddttddttddttswcVcVcVdvvcdvcVdtdtdvcvVdttivVdttPE=−=⋅−==⋅−=⋅−==∫∫∫∫∫An equal amount of energy is dissipated on pulldown.6Spring 2003 EECS150 – Lec25-powerPage 11Switching Power• Gate power consumption:– Assume a gate output is switching its output at a rate of:1/fPavgclock ff⋅αswavgErateswitchingtEP⋅=∆= 221ddavgcVfP ⋅⋅=α221ddavgavgavgVcfnP ⋅⋅⋅=α• Chip/circuit power consumption:activity factor clock rateTherefore:number of nodes (or gates)(probability of switching on any particular clock period)Spring 2003 EECS150 – Lec25-powerPage 12Other Sources of Energy Consumption• “Short Circuit” Current:• Junction Diode Leakage:• Device Ids Leakage:Vo u tVi nVi nIIVo u tVi nIVDiodeCharacteristicIoffVout=VddVi n =0IdsVgsVth10-20% of total chip power~1nWatt/gatefew mWatts/chipTransistor drain regions“leak” charge to substrate.Transistor s/d conductancenever turns off all the way.~3pWatts/transistor. ~1mWatt/chipLow voltage processes much worse.7Spring 2003 EECS150 – Lec25-powerPage 13Controlling Energy Consumption• Largest contributing component to CMOS power consumption is switching power:• Factors influencing power consumption:n: total number of nodes in circuitα: activity factor (probability of each node switching)f: clock frequency (does this effect energy consumption?)Vdd: power supply voltage• What control do you have over each factor? • How does each effect the total Energy? What control do you have as a designer?221ddavgavgavgVcfnP ⋅⋅⋅=αIn EECS150 design projects, we will not optimize for power consumption.Spring 2003 EECS150 – Lec25-powerPage 14Power / Cost / Performance• In lecture 23 we discussed using parallelism to trade cost for performance. As we trade cost for performance what happens to energy?4 EMUL+ 3 EADD + EWIRES2 EMUL+ 3 EADD + EWIRES2 EMUL+ 3 EADD + EMUXES + ECNTL+ EWIRES• The lowest energy consumer is the solution that minimizes cost without time multiplexing operations.xx xx+++0.2mt10.2 mt2 0.4 proj0.2 mt3gradex++0.2mt1 mt20.4 projmt3gradex+acc1 = mt1 + mt2;acc1 = acc1 + mt3;acc1 = 0.2 x acc1;acc2 = 0.4 x proj;grade = acc1 + acc2;controllerALUmt1 mt1mt3
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