cse140: components and design techniques for digital...

Sources: TSR, Katz, Boriello, Vahid

1

CSE140: Components and Design Techniques for Digital Systems

Power & Energy

Tajana Simunic Rosing


Overview

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


In the Physical World: Sensor Devices


Phone + Messenger + PDA

• Quad-band GSM/GPRS/EDGE, wi-fi, Bluetooth™ 2.0

• 2 megapixel camera s with 5x zoom and built-in flash

• MicroSD memory card slot• Email, IM, SMS, Media player• Docs: Word, Excel, PDF and

JPEG• 4 hours talk time, 17 days standby

Blackberry 8310


Phone + Messenger + PDA

• Quad-band GSM™ phone; 802.11 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

iPhone


Important (Wireless) Technology Trends

“Spectral Efficiency”:More bits/m3

Rapidly decliningsystem cost

Rapidly increasingtransistor density


Important (Wireless)Technology Trends

Speed-Distance-CostTradeoffs

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


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 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!


• Power supply provides energy for charging and discharging wires and transistor gates. The energy supplied is stored & then 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

• Given that current:• Power is work done over time:

• Energy is:

dqdwV /=dtdqI /=

dtdwP /≡ Power: Rate of work being done over timeRate of energy being used

Watts = Joules/seconds

IVPdtdq

dqdwdtdw ×==×=/

∫∞−

=t

Pdtw

tEP ∆=

Power and Energy Basics


Basics• 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


+1V -

1 Ohm Resistor

1A

0.24 Calories per Second

Heats 1 gram of water 0.24 degree C

This is how electric tea pots work ...

1 Joule of Heat Energy per Second

20 W rating: Maximum power the package is able to transfer to the air. Exceed rating and resistor burns.


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 ...


Powering an iPod nanoBattery has 1.2 W-hour rating:Can supply 1.2 W of power for 1 hour

1.2 W / 5 W = 15 minutes

Real specs for iPod nano ‘05 : 14 hours for music, 4 hours for slide shows

85 mW for music

300 mW for slides

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


0.55 ounces

12 hour battery life

1 GB

Sources: TSR, Katz, Boriello, VahidCS 150 - Spring 2007 – Lec #28 –P 15

12 hour battery life

24 hour battery life for audio

5 hour battery life for photos

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


Notebooks ... now most of the PC market

Performance: Must be “close enough” to desktop performance ... 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


Battery: Set by size and weight limits ...

Almost full 1 inch depth. Width and height set by available space, weight.

Battery rating: 55 W-hour

At 2.3 GHz, Intel Core Duo CPU consumes 31 W running a heavy load - under 2 hours battery life! And, just for CPU!

At 1 GHz, CPU consumes 13 Watts. “Energy saver” option uses this mode ...

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


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

Toshiba Portege3110 laptop - 20%

Handspring PDA - 10%Nokia 61xx -

33%

Sources: TSR, Katz, Boriello, VahidCS 150 - Spring 2007 – Lec #28 –P 19

55 W-hour battery stores the energy of

1/2 a stick of dynamite.

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


CPU Only Part of Power Budget

Notebook running a full workload.

If our CPU took no power at all to run, that would only double battery life!CPULCD

Backlight

“other”

LCD

GPU


Servers: Total Cost of Ownership (TCO)Machine rooms are expensive … removing heat dictates how many servers to put in a machine room.

Electric bill adds up! Powering the servers + powering the air conditioners is a big part of TCO

Reliability: running computers hot makes them fail more often


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, MIPS2/watt. Puts more weight on performance


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.


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

Energy dissipated in pullup:

222 2121

)()()()()(

1

0

1

0

1

0

1

0

1

0

dd

t

t

t

t dddddd

t

t dd

t

t dd

t

tsw

cVcVcVdvvcdvcV

dtdtdvcvVdttivVdttPE

=−=⋅−=

=⋅−=⋅−==

∫ ∫

∫∫∫

An equal amount of energy is dissipated on pulldown


Switching Power• Gate power consumption:

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

1/f

Pavg

clock f

f⋅α

swavg ErateswitchingtEP ⋅=∆=

221 ddavg cVfP ⋅⋅=α

221 ddavgavgavg VcfnP ⋅⋅⋅= αChip/circuit power consumption:

activity factor clock rate

Therefore:

number of nodes (or gates)

(probability of switching on any particular clock period)


Other Sources of Energy Consumption

• “Short Circuit” Current:

Vout

Vin

Vin

I

I

VoutVin

I

V

DiodeCharacteristic10-20% of total chip power

~1nWatt/gatefew mWatts/chip

Transistor drain regions“leak” charge to substrate.

• Junction diode leakage:


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

• 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

Ioff

Vout=VddVin=0Ids

VgsVthTransistor s/d conductance

never turns off all the way

Low voltage processes much worse


Controlling Energy Consumption: What Control Do You Have as a Designer?

• Largest contributing component to CMOS power consumption is switching power:

• 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?

221 ddavgavgavg VcfnP ⋅⋅⋅= α

Our design projects do not optimize for power consumption


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 reduced V and C (length and width of Cs decrease, but plate distance gets smaller)

Recent slope reduced because V is scaled less aggressively


Device Engineers Trade Speed and Power

From: Silicon Device Scaling to the Sub-10-nm RegimeMeikei Ieong,1* Bruce Doris,2 Jakub Kedzierski,1 Ken Rim,1 Min Yang1

We can reduce leakage (Pstandby) by raising Vt

We can increase speed by raising Vdd andlowering Vt

We can reduce CV2 (Pactive) by lowering Vdd


Customize processes for product types ...

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


Intel: Comparing 2 CPU Generations ...

Clock speed unchanged ... Lower Vdd, lower C,

but more leakage

Design tricks: architecture & circuits

Find enough tricks, and you can afford to raise Vdd a little so that you can raise the clock speed!

cse140: components and design techniques for digital...

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