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CMOS VLSI Design Lecture 1 3 : Design for Low Power. 11/27/2007, UTPA (adopted from Harris’ lecture notes). Outline. Why low power design? Power and Energy Dynamic Power Static Power Low Power Design. Needs for Low Power VLSI Chips. Power dissipation was neglected due to

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## CMOS VLSI Design Lecture 1 3 : Design for Low Power

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**CMOS VLSIDesignLecture 13: Design for Low Power**11/27/2007, UTPA (adopted from Harris’ lecture notes)**Outline**• Why low power design? • Power and Energy • Dynamic Power • Static Power • Low Power Design 13: Design for Low Power**Needs for Low Power VLSI Chips**• Power dissipation was neglected due to • Low device density • Low operating frequency • Now it is important issue due to • High device density • High operating frequency • Proliferation of portable consumer electronics • Concerns on Environments and energy sources 13: Design for Low Power**Moore’s Law**• In 1965, Gordon Moore noted that the number of transistors on a chip doubled every 18 to 24 months. • He made a prediction that semiconductor technology will double its effectiveness every 18 months 13: Design for Low Power**Moore’s Law**Electronics, April 19, 1965. 13: Design for Low Power**Evolution in Complexity**13: Design for Low Power**Transistor Counts**1 Billion Transistors K 1,000,000 100,000 Pentium® III 10,000 Pentium® II Pentium® Pro 1,000 Pentium® i486 i386 100 80286 8086 10 Source: Intel 1 1975 1980 1985 1990 1995 2000 2005 2010 Projected 13: Design for Low Power Courtesy, Intel**Moore’s law in Microprocessors**1000 2X growth in 1.96 years! 100 10 P6 Pentium® proc Transistors (MT) 486 1 386 0.1 286 8086 8085 0.01 8080 8008 4004 0.001 1970 1980 1990 2000 2010 Year Transistors on Lead Microprocessors double every 2 years 13: Design for Low Power Courtesy, Intel**Die Size Growth**100 P6 Pentium ® proc 486 Die size (mm) 10 386 286 8080 8086 ~7% growth per year 8085 8008 ~2X growth in 10 years 4004 1 1970 1980 1990 2000 2010 Year Die size grows by 14% to satisfy Moore’s Law 13: Design for Low Power Courtesy, Intel**Frequency**10000 Doubles every2 years 1000 P6 100 Pentium ® proc Frequency (Mhz) 486 386 10 8085 286 8086 8080 1 8008 4004 0.1 1970 1980 1990 2000 2010 Year Lead Microprocessors frequency doubles every 2 years 13: Design for Low Power Courtesy, Intel**Power Dissipation**100 P6 Pentium ® proc 10 486 286 8086 Power (Watts) 386 8085 1 8080 8008 4004 0.1 1971 1974 1978 1985 1992 2000 Year Lead Microprocessors power continues to increase 13: Design for Low Power Courtesy, Intel**Power will be a major problem**100000 18KW 5KW 10000 1.5KW 500W 1000 Pentium® proc Power (Watts) 100 286 486 8086 10 386 8085 8080 8008 1 4004 0.1 1971 1974 1978 1985 1992 2000 2004 2008 Year Power delivery and dissipation will be prohibitive 13: Design for Low Power Courtesy, Intel**Rocket**Nozzle Nuclear Reactor Hot Plate Power density Sun Surface 10000 1000 Power Density (W/cm2) 100 8086 10 4004 P6 8008 Pentium® proc 8085 386 286 486 8080 1 1970 1980 1990 2000 2010 Year Power density too high to keep junctions at low temp 13: Design for Low Power Courtesy, Intel**Small**Signal RF Power RF Power Management 1996 1997 1998 1999 2000 Units48M 86M 162M 260M 435M Analog Baseband Digital Baseband (DSP + MCU) Not Only Microprocessors CellPhone Digital Cellular Market (Phones Shipped) (data from Texas Instruments) 13: Design for Low Power**Battery**• Portable consumer electronics powered by battery • Battery is heavy and big • Energy density barely doubles in several years • Safety concern: the energy density is approaching that of explosive chemicals. The battery technology alone will not solve the low power problem 13: Design for Low Power**Reliability and Cooling Costs**• High power dissipation high temperature malfunction • High performance microprocessors: ~50 Watts (a hand-held soldering iron) • Packaging cost and cooling cost: fans • Power supply rails: high transient current (e.g. 3A). 13: Design for Low Power**Environmental Concerns**• Office automation equipment • 5% of total US commercial energy in 1993 • 10% of total US commercial energy in 2000 • Electricity generation air pollution and consumption of energy sources 13: Design for Low Power**Power and Energy**• Power is drawn from a voltage source attached to the VDD pin(s) of a chip. • Instantaneous Power: • Energy: • Average Power: 13: Design for Low Power**Dynamic Power**• Dynamic power is required to charge and discharge load capacitances when transistors switch. • One cycle involves a rising and falling output. • On rising output, charge Q = CVDD is required • On falling output, charge is dumped to GND • This repeats Tfsw times over an interval of T 13: Design for Low Power**Dynamic Power Cont.**13: Design for Low Power**Dynamic Power Cont.**13: Design for Low Power**Activity Factor**• Suppose the system clock frequency = f • Let fsw = af, where a = activity factor • If the signal is a clock, a = 1 • If the signal switches once per cycle, a = ½ • Dynamic gates: • Switch either 0 or 2 times per cycle, a = ½ • Static gates: • Depends on design, but typically a = 0.1 • Dynamic power: 13: Design for Low Power**Short Circuit Current**• When transistors switch, both nMOS and pMOS networks may be momentarily ON at once • Leads to a blip of “short circuit” current. • < 10% of dynamic power if rise/fall times are comparable for input and output 13: Design for Low Power**Example**• 200 Mtransistor chip • 20M logic transistors • Average width: 12 l • 180M memory transistors • Average width: 4 l • 1.2 V 100 nm process • Cg = 2 fF/mm 13: Design for Low Power**Dynamic Example**• Static CMOS logic gates: activity factor = 0.1 • Memory arrays: activity factor = 0.05 (many banks!) • Estimate dynamic power consumption per MHz. Neglect wire capacitance and short-circuit current. 13: Design for Low Power**Dynamic Example**• Static CMOS logic gates: activity factor = 0.1 • Memory arrays: activity factor = 0.05 (many banks!) • Estimate dynamic power consumption per MHz. Neglect wire capacitance. 13: Design for Low Power**Static Power**• Static power is consumed even when chip is quiescent. • Ratioed circuits burn power in fight between ON transistors • Leakage draws power from nominally OFF devices 13: Design for Low Power**Leakage Example**• The process has two threshold voltages and two oxide thicknesses. • Subthreshold leakage: • 20 nA/mm for low Vt • 0.02 nA/mm for high Vt • Gate leakage: • 3 nA/mm for thin oxide • 0.002 nA/mm for thick oxide • Memories use low-leakage transistors everywhere • Gates use low-leakage transistors on 80% of logic 13: Design for Low Power**Leakage Example Cont.**• Estimate static power: 13: Design for Low Power**Leakage Example Cont.**• Estimate static power: • High leakage: • Low leakage: 13: Design for Low Power**Leakage Example Cont.**• Estimate static power: • High leakage: • Low leakage: • If no low leakage devices, Pstatic = 749 mW (!) 13: Design for Low Power**Low Power Design**• Reduce dynamic power • a: • C: • VDD: • f: • Reduce static power 13: Design for Low Power**Low Power Design**• Reduce dynamic power • a: clock gating, sleep mode • C: • VDD: • f: • Reduce static power 13: Design for Low Power**Low Power Design**• Reduce dynamic power • a: clock gating, sleep mode • C: small transistors (esp. on clock), short wires • VDD: • f: • Reduce static power 13: Design for Low Power**Low Power Design**• Reduce dynamic power • a: clock gating, sleep mode • C: small transistors (esp. on clock), short wires • VDD: lowest suitable voltage • f: • Reduce static power 13: Design for Low Power**Low Power Design**• Reduce dynamic power • a: clock gating, sleep mode • C: small transistors (esp. on clock), short wires • VDD: lowest suitable voltage • f: lowest suitable frequency • Reduce static power 13: Design for Low Power**Low Power Design**• Reduce dynamic power • a: clock gating, sleep mode • C: small transistors (esp. on clock), short wires • VDD: lowest suitable voltage • f: lowest suitable frequency • Reduce static power • Selectively use ratioed circuits • Selectively use low Vt devices • Leakage reduction: stacked devices, body bias, low temperature 13: Design for Low Power

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