CSE140: Components and Design Techniques for Digital Systems

Transcription

1 CSE140: Components and Design Techniques for Digital Systems Tajana Simunic Rosing 1

2 Announcements and Outline Check webct grades, make sure everything is there and is correct Pick up graded homework at TA s or my assistant s office Final exam Tuesday, June 10 th, at 3pm, same location as the class Everything covered in lectures, whole book & all handouts Format: Problems similar to HW and previous exams Multiple choice and/or T/F questions on the assigned reading Discussion session will go over the previous year s final Today s topic: Power and Energy 2

3 CSE140: Components and Design Techniques for Digital Systems Power & Energy Tajana Simunic Rosing 3

4 Overview Motivation for design constraints of power consumption Power metrics Power consumption analysis in CMOS How can a logic designer control power?

5 Phone + Messenger + PDA Blackberry 8310 Quad-band GSM/GPRS/EDGE, wi-fi, Bluetooth megapixel camera s with 5x zoom and built-in flash MicroSD memory card slot , IM, SMS, Media player Docs: Word, Excel, PDF and JPEG 4 hours talk time, 17 days standby

6 Phone + Messenger + PDA iphone Quad-band GSM phone; b/g, EDGE & Bluetooth v2.0+edr View PDF, JPEG, Word and Excel docs Chat-style SMS text messaging 2.0 megapixel camera Screen Resolution: 480 x 320 pixels (163 ppi) Talk time: Up to 8 hours Standby time: Up to 250 hours Internet use: Up to 6 hours Video playback: Up to 7 hours Audio playback: Up to 24 hours

7 Important (Wireless) Technology Trends Spectral Efficiency : i More bits/m 3 Rapidly increasing transistor density Rapidly declining system cost

8 In the Physical World: Sensor Devices

9 Important (Wireless) Technology Trends Speed-Distance-Cost Tradeoffs Rapid Growth: Machine-to- Machine Devices (mostly sensors)

10 Portable devices: Why Worry About Power? 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 10 years, have gone from minimal consideration of power consumption to (designing with power consumption as a primary design constraint!

11 Power and Energy Basics Power supply provides energy for charging and discharging wires and transistor gates. The energy supplied is stored & then dissipated as heat. P dw / dt Power: Rate of work being done over time Rate of energy being used Watts = Joules/seconds P = E Δ t If a differential amount of charge dq is given a differential increase in energy dw, the potential of the charge is increased by Given that current: Power is work done over time: Energy is: w = t V = dw / dq I = dq / dt dw dq dw / dt = = P = V I dq dt Pdt

12 Basics Warning! In everyday language, the term power is used incorrectly in place of energy Power is not energy Power is not something gyou 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

13 This is how electric tea pots work... Heats 1 gram of water 0.24 degree C 0.24 Calories per Second 1A 1 Joule of Heat Energy per Second 1V Ohm Resistor 20 W rating: Maximum power the package is able to transfer to the air. Exceed rating and resistor burns.

14 Cooling an ipod nano... Like a resistor, ipod relies on passive transfer of heat from case to the air Why? Users don t want fans in their pocket... To stay cool to the touch via passive cooling, power budget of 5 W If ipod nano used 5W all the time, its battery would last 15 minutes...

15 Powering an ipod nano Battery has W-hour rating: Can supply 1.2 W of power for 1 hour 1.2 W / 5 W = 15 minutes More W-hours require bigger battery and thus bigger form factor -- it wouldn t be nano anymore! Real specs for ipod nano 05 : 14 hours for music, 4 hours for slide shows 85 mw for music 300 mw for slides

16 0.55 ounces 12 hour battery life 1 GB

17 20 hour battery life for audio, 6.5 hours for movies (80GB version) 24 hour battery life for audio 5 hour battery life for photos 12 hour battery life CS Spring 2007 Lec #28 P 17

18 8.9 in Notebooks... now most of the PC market Apple MacBook -- Weighs 5.2 lbs 1 in 12.8 in Performance: Must be close enough to desktop performance... many people no longer own a desktop Size and Weight: Ideal: paper notebook Heat: No longer laptops p -- top may get warm, bottom hot. Quiet fans OK

19 Battery: Set by size and weight limits... Battery rating: 55 W-hour 46x energy than ipod nano. ipod lets you listen to music for 14 hours! At 2.3 GHz, Intel Core Duo CPU consumes 31 W running a heavy load - under 2 hours battery life! And, just for CPU! Almost full 1 inch depth. Width and height set by available space, weight. At 1 GHz, CPU consumes 13 Watts. En Energy saver option uses this mode...

20 Battery Technology Battery technology has developed slowly Li-Ion and NiMh still the dominate technologies Batteries still contribute significantly to the weight of mobile devices Nokia 61xx - 33% Handspring PDA - 10% Toshiba Portege 3110 laptop - 20%

21 55 W-hour battery stores the energy of 1/2 a stick of dynamite. If battery short-circuits, catastrophe is possible... CS Spring 2007 Lec #28 P 21

22 CPU Only Part of Power Budget Notebook running a full workload. GPU other LCD Backlight CPU If our CPU took no power at all to run, that would only double battery life! LCD

23 Servers: Total Cost of Ownership (TCO) Machine rooms are expensive removing heat dictates how many servers to put in a machine room. Reliability: running computers hot makes them fail more often Electric bill adds up! Powering the servers + powering the air conditioners is a big part of TCO

24 How Do We Measure and Compare Power Consumption? 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 reflects 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, MIPS 2 /watt. Puts more weight on performance

25 Metrics How does MIPS/watt relate to energy? Average power consumption = energy / time MIPS/watt = instructions/sec / joules/sec = instructions/joule 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 alternative implementations ti of a particular function.

26 Power in CMOS Switching Energy: energy used to switch a node Vdd pullup network i(t) Vdd v(t) pulldown network 0 C 1 v(t) t0 t1 Energy dissipated in pullup: GND t 1 t 1 t 1 E = P( t) dt = ( V v) i( t) dt = ( V v) c ( dv dt) dt = sw t 0 t = dd t 0 t dd 2 dd 2 dd 1 1 cv dv c v dv = cv 1 2cV = 1 t 0 0 t t 0 dd 2cV 2 dd Energy supplied Energy stored Energy dissipated An equal amount of energy is dissipated on pulldown

27 Switching Power Gate power consumption: Assume a gate output is switching its output at a rate of: α f activity factor clock rate 1/f (probability of switching on any particular clock period) P = E Δt = switching rate avg E sw Therefore: P P avg = n α = α Chip/circuit power consumption: avg number of nodes (or gates) avg f f 1 2cV dd 2 Pavg c V avg dd clock f

28 Other Sources of Energy Consumption Short Circuit Current: Junction diode leakage: Vout I Vin Vout I Vin Transistor drain regions leak charge to substrate % of total chip power Vin I Diode Characteristic V ~1nWatt/gate few mwatts/chip

29 Other Sources of Energy Consumption Consumption caused by DC leakage current (Ids leakage): Vin=0 Vout=Vdd Ids Ioff Transistor s/d conductance never turns off all the way Low voltage processes much worse Vth Vgs This source of power consumption is becoming increasing significant as process technology scales down For 90nm chips around 10-20% of total power consumption Estimates put it at up to 50% for 65nm

30 Controlling Energy Consumption: What Control Do You Have as a Designer? Largest contributing component to CMOS power consumption is switching power: P avg = n α avg f 2 1 2cavg Vdd Factors influencing power consumption: n: total number of nodes in circuit α: 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? Our design projects do not optimize for power consumption

31 Scaling Switching Energy per Gate Moore s Law at work Due to reduced V and C (length and width of Cs decrease, but plate distance gets smaller) Recent slope reduced because V is scaled less aggressively From: Facing the Hot Chips Challenge Again, Bill Holt, Intel, presented at Hot Chips 17, 2005.

32 Device Engineers Trade Speed and Power We can reduce CV 2 (Pactive) by lowering Vdd We can increase speed by raising Vdd and lowering Vt We can reduce leakage (Pstandby) by raising Vt From: Silicon Device Scaling to the Sub-10-nm Regime Meikei Ieong, 1* Bruce Doris, 2 Jakub Kedzierski, 1 Ken Rim, 1 Min Yang 1

33 Customize processes for product types... From: Facing the Hot Chips Challenge Again, Bill Holt, Intel, presented at Hot Chips 17, 2005.

34 Intel: Comparing 2 CPU Generations... Find enough tricks, and you can afford to raise Vdd a little so that you can raise the clock speed! Clock speed unchanged... Lower Vdd, lower C, but more leakage Design tricks: architecture & circuits

35 Switching Energy: Fundamental Physics Every logic transition dissipates s energy V dd V dd C 1 E C 2 0- = 2 Vdd Strong result: Independent >1 of technology 1 E C 2 1- = 2 Vdd >0 How can we limit switching energy? (1) Slow down clock (fewer transitions). But we like speed... (2) Reduce Vdd. But lowering Vdd lowers the clock speed... (3) Fewer circuits. But more transistors can do more work. (4) Reduce C per node. One reason why we scale processes.

36 Second Factor: Leakage Currents Even when a logic gate isn t switching, it burns power Isub: Even when this nfet is off, it passes an Ioff leakage current. 0V = We can engineer any Ioff we like, but a lower Ioff also results in a lower Ion, and thus the lower the clock speed. Intel s current processor designs, leakage vs switching power Igate: Ideal capacitors have zero DC current. But modern transistor gates are a few atoms thick, and are not ideal. Bill Holt, Intel, Hot Chips 17. A lot of work was done to get a ratio this good... 50/50 is common.

37 Engineering On Current at 25 nm... V d We can increase Ion by raising Vdd and/or lowering Vt. I ds V g I ds V s 1.2 ma = Ion 0.25 V t I off = 0??? 0.7 = V dd

38 Plot on a Log Scale to See Off Current V d We can decrease Ioff by raising Vt -but that lowers Ion I I V ds ds 1.2 ma = Ion g V s 0.25 V t I off 10 na 0.7 = V dd