CS 150 - Spring 2007 – Lec #28 – Power - 1

Power

! Motivation for design constraints of power consumption

! Power metrics

! Power consumption analysis in CMOS

! How can a logic designer control power?

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Automobiles700 Million

Telephones4 Billion

Electronic Chips60 Billion

X-Internet

“X-Internet” Beyond the PC

Forrester Research, May 2001Revised 2007

500Million

1.5 Billion

Internet Computers

Internet Users

Today’s Internet

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“X-Internet” Beyond the PC

Forrester Research, May 2001

Millions

Year

XInternet

PCInternet

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Cell Phones

Siemens SL45i Ericsson T68

! Phone w/voice command,voice dialing, intelligenttext for short msgs

! MP3 player + headset,digital voice recorder

! “Mobile Internet” with abuilt-in WAP Browser

! Java-enabled, over the airprogrammable

! Bluetooth + GPRS

! Enhanced displays +embedded cameras

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Shape of Things! Phone + Messenger + PDA Combinations

" E.g., Blackberry 5810 Wireless Phone/Handheld# Integration of PDA + Telephone

# PLUS Gateway to Internet and Enterprise applications

# 1900 MHz GSM/GPRS (Euroversion at 900 Mhz)

# SMS Messaging, Internet access

# QWERTY Keyboard, 20 line display

# JAVA applications capable

# 8 MB flash + 1 MB SRAM

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Shape of Things to Come

! Danger “Hiptop”" Full-featured mobile phone w/Internet Access" Email + attachments/instant messaging + PIM" Digital camera accessory" End-to-end integration of voice + data apps" Media-rich UI for graphics + sound" Large screen + QWERTY keyboard" Data nav: keyboard or push wheel" Affordable (under $200)" MIDI synthesizer for quality sound" Multi-tasking of user actions" Customizable ring tones and alerts

to personalize hiptop experience

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Important (Wireless)Technology Trends

“Spectral Efficiency”:More bits/m3

Rapidly decliningsystem cost

Rapidly increasingtransistor density

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In the Physical World: Sensor Devices

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Important (Wireless)Technology Trends

Speed-Distance-CostTradeoffs

Rapid Growth: Machine-to-Machine Devices (mostly sensors)

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Why Worry About Power?

! 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 tothose 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 consumptionto (designing with power consumption as a primary design constraint!

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! Power supply provides energy for charging and discharging wires andtransistor gates. The energy supplied is stored & then dissipated asheat.

! If a differential amount of charge dq is given a differential increasein energy dw, the potential of the charge is increased by:

! By definition of current: dqdwV /=dtdqI /=

dtdwP /!Power: Rate of work being done wrt time

Rate of energy being used

IVPdt

dq

dq

dwdtdw !==!=/

!"#

=

t

Pdtw total energy

Units: tEP != Watts = Joules/seconds

A very practical formulation!

If we would liketo know total energy

Basics

CS 150 - Spring 2007 – Lec #28 – Power - 12

Basics

! Warning! In everyday language, the term “power” isused 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 thanvelocity can be put in the gas tank of a car

CS 150 - Spring 2007 – Lec #28 – Power - 13

+1V -

1 OhmResistor

1A

0.24 Calories per Second

Heats 1 gram of water0.24 degree C

This is how electric tea pots work ...

1 Joule of HeatEnergy per Second

1 Watt

20 W rating: Maximum powerthe package is able to transferto the air. Exceed rating andresistor burns.

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Cooling an iPod nano ...Like a resistor, iPod relieson passive transfer of heatfrom case to the air

Why? Users don’t wantfans 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 last15 minutes ...

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Powering an iPod nano (2005 edition)

Battery has 1.2 W-hourrating: Can supply1.2 W of power for 1 hour

1.2 W / 5 W = 15 minutes

Real specs for iPod nano :14 hours for music,4 hours for slide shows

85 mW for music

300 mW for slides

More W-hours require bigger batteryand thus bigger “form factor” --it wouldn’t be “nano” anymore!

CS 150 - Spring 2007 – Lec #28 – Power - 16

0.55 ounces

12 hourbattery life

$79.00

1 GB

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12 hourbattery life

24 hourbattery lifefor audio

5 hourbattery lifefor photos

20 hour battery life for audio,6.5 hours for movies (80GB version)

Up from 14hours for 2005

iPod nano

Up from 4hours for 2005

iPod nano

Thinner than 2005 iPod nano

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Notebooks ... now most of the PC market

Performance: Must be “close enough” to desktopperformance ... many people no longer own a desktop

Heat: No longer “laptops” -- top may get “warm”,bottom “hot”. Quiet fans OK

Size and Weight: Ideal: paper notebook

1 in

8.9 in

12.8 in

Apple MacBook -- Weighs 5.2 lbs

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Battery: Set by size and weight limits ...

Almost full 1 inchdepth. Width andheight set by availablespace, weight.

Battery rating:55 W-hour

At 2.3 GHz,Intel Core DuoCPU consumes 31W running aheavy load -under 2 hoursbattery life! And,just for CPU!

At 1 GHz, CPU consumes13 Watts. “Energy saver”option uses this mode ...

46x energy than iPod nano.iPod lets you listen to musicfor 14 hours!

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Battery Technology

! Battery technology has developed slowly

! Li-Ion and NiMh still the dominate technologies

! Batteries still contribute significantly to the weightof mobile devices

Toshiba Portege

3110 laptop - 20%

Handspring

PDA - 10%Nokia 61xx -

33%

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55 W-hour batterystores the energy of

1/2 a stick of dynamite.

If battery short-circuits,catastrophe is possible ...

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CPU Only Part of Power Budget

2004-era notebook running afull workload.

If our CPU took no powerat all to run, that wouldonly double battery life!CPULCD

Backlight

“other”

LCD

GPU

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Servers: Total Cost of Ownership (TCO)

Machine roomsare expensive …removing heatdictates howmany servers toput in a machineroom.

Electric bill addsup! Powering theservers +powering the airconditioners is abig part of TCO

Reliability: running computers hotmakes them fail more often

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Thermal Image of Typical Cluster Rack

RackSwitch

M. K. Patterson, A. Pratt, P. Kumar,

“From UPS to Silicon: an end-to-end evaluation of datacenter efficiency”, Intel Corporation

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How Do We Measure and ComparePower 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 atvery low absolute MIPS (used in watches)

# Metric only relevant for comparing processors with a similarperformance

" One solution, MIPS2/watt. Puts more weight on performance

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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 of a particular function.

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Power in CMOS

C

pullupnetwork

pulldownnetwork

Vdd

GND

10

i(t)

v(t)t0 t1

v(t)

VddSwitching Energy: energy used to switch a node

Energy supplied Energy dissipatedEnergy stored

Calculate energydissipated in pullup:

2222121

)()()()()(

1

0

1

0

1

0

1

0

1

0

dd

t

t

t

tdddddd

t

tdd

t

tdd

t

tsw

cVcVcVdvvcdvcV

dtdtdvcvVdttivVdttPE

=!="!=

="!="!==

# #

###

An equal amount of energy is dissipated on pulldown

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Switching Power! Gate power consumption:

" Assume a gate output is switching its output at a rate of:

1/f

Pavg

clock f

f!"

swavg ErateswitchingtEP !="=

2

21 ddavg cVfP !!="

2

21 ddavgavgavg VcfnP !!!= "! Chip/circuit power consumption:

activity factor clock rate

Therefore:

number of nodes (or gates)

(probability of switching on any particular clock period)

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Other Sources of Energy Consumption! “Short Circuit” Current:

Vout

Vin

Vin

I

I

VoutVin

I

V

Diode

Characteristic10-20% of total chip power

~1nWatt/gatefew mWatts/chip

Transistor drain regions“leak” charge to substrate.

! Junction Diode Leakage :

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Other Sources of Energy Consumption! Consumption caused by “DC leakage current” (Ids leakage):

! This source of power consumption is becoming increasing significantas process technology scales down

! For 90nm chips around 10-20% of total power consumptionEstimates put it at up to 50% for 65nm

Ioff

Vout=VddVin=0

Ids

VgsVthTransistor s/d conductance

never turns off all the way

Low voltage processes much worse

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Controlling Energy Consumption: WhatControl Do You Have as a Designer?

! Largest contributing component to CMOS power consumption isswitching power:

! Factors influencing power consumption:" n: total number of nodes in circuit" !: 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?

2

21 ddavgavgavg VcfnP !!!= "

Our design projects do not optimize for power consumption

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Scaling Switching Energy per GateMoore’s Lawat work …

From: “Facing the Hot Chips Challenge Again”, Bill Holt, Intel, presented at Hot Chips 17, 2005.

Due to reducedV and C (lengthand width of Csdecrease, butplate distancegets smaller)

Recent slopereducedbecause V isscaled lessaggressively

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Device Engineers Trade Speed and Power

From: Silicon Device Scaling to the Sub-10-nm Regime

Meikei Ieong,1* Bruce Doris,2 Jakub Kedzierski,1 Ken Rim,1 Min Yang1

We can reduce leakage(Pstandby) by raising Vt

We can increase speedby raising Vdd andlowering Vt

We can reduce CV2 (Pactive)by lowering Vdd

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Customize processes for product types ...

From: “Facing the Hot Chips Challenge Again”, Bill Holt, Intel, presented at Hot Chips 17, 2005.

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Intel: Comparing 2 CPU Generations ...

Clock speedunchanged ... Lower Vdd, lower C,

but more leakage

Design tricks:architecture & circuits

Find enoughtricks, and youcan afford toraise Vdd alittle so thatyou can raisethe clockspeed!

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Switching Energy: Fundamental Physics

Vdd

C

1

2C V

ddE

1->0=

21

2C V

ddE

0->1=

2

Vdd

Every logic transition dissipates energy

Strong result: Independent of technology

How canwe limit

switchingenergy?

(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.

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0V =

Second Factor: Leakage CurrentsEven when a logic gate isn’t switching, it burns power …

Igate: Ideal capacitors havezero DC current. But moderntransistor gates are a few atomsthick, and are not ideal.

Isub: Even when this nFetis off, it passes an Ioffleakage current.

We can engineer any Ioffwe like, but a lower Ioff alsoresults in a lower Ion, and thusthe lower the clock speed.

Intel’s current processor designs,leakage vs switching power

A lot of work wasdone to get a ratiothis good ... 50/50is common.

Bill Holt, Intel, Hot Chips 17.

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Engineering “On” Current at 25 nm ...

I ds

Vs

Vd

V g

0.7 = Vdd

0.25 ! Vt

I ds

1.2 mA = Ion

Ioff

= 0 ???

We can increase Ion byraising Vdd and/or lowering Vt.

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Plot on a “Log” Scale to See “Off” Current

Ids

Vs

Vd

Vg I

ds

Ioff

! 10 nA

We can decrease Ioff byraising Vt - but that lowers Ion

0.25 ! Vt

1.2 mA = Ion

0.7 = Vdd