cse140: components and design techniques for digital...
TRANSCRIPT
Sources: TSR, Katz, Boriello, Vahid
1
CSE140: Components and Design Techniques for Digital Systems
Power & Energy
Tajana Simunic Rosing
Sources: TSR, Katz, Boriello, Vahid
Overview
• Motivation for design constraints of power consumption• Power metrics• Power consumption analysis in CMOS• How can a logic designer control power?
Sources: TSR, Katz, Boriello, Vahid
In the Physical World: Sensor Devices
Sources: TSR, Katz, Boriello, Vahid
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
Sources: TSR, Katz, Boriello, Vahid
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
Sources: TSR, Katz, Boriello, Vahid
Important (Wireless) Technology Trends
“Spectral Efficiency”:More bits/m3
Rapidly decliningsystem cost
Rapidly increasingtransistor density
Sources: TSR, Katz, Boriello, Vahid
Important (Wireless)Technology Trends
Speed-Distance-CostTradeoffs
Rapid Growth: Machine-to-Machine Devices (mostly sensors)
Sources: TSR, Katz, Boriello, Vahid
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!
Sources: TSR, Katz, Boriello, Vahid
• 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
Sources: TSR, Katz, Boriello, Vahid
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
Sources: TSR, Katz, Boriello, Vahid
+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.
Sources: TSR, Katz, Boriello, Vahid
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 ...
Sources: TSR, Katz, Boriello, Vahid
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!
Sources: TSR, Katz, Boriello, Vahid
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)
Sources: TSR, Katz, Boriello, Vahid
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
Sources: TSR, Katz, Boriello, Vahid
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!
Sources: TSR, Katz, Boriello, Vahid
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 ...
Sources: TSR, Katz, Boriello, Vahid
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
Sources: TSR, Katz, Boriello, Vahid
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
Sources: TSR, Katz, Boriello, Vahid
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
Sources: TSR, Katz, Boriello, Vahid
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.
Sources: TSR, Katz, Boriello, Vahid
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
Sources: TSR, Katz, Boriello, Vahid
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)
Sources: TSR, Katz, Boriello, Vahid
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:
Sources: TSR, Katz, Boriello, Vahid
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
Sources: TSR, Katz, Boriello, Vahid
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
Sources: TSR, Katz, Boriello, Vahid
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
Sources: TSR, Katz, Boriello, Vahid
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
Sources: TSR, Katz, Boriello, Vahid
Customize processes for product types ...
From: “Facing the Hot Chips Challenge Again”, Bill Holt, Intel, presented at Hot Chips 17, 2005.
Sources: TSR, Katz, Boriello, Vahid
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!