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ELEC 5770-001/6770-001 Fall 2010VLSI DesignLow

Power VLSI Design

Vishwani D. Agrawal James J. Danaher

Professor Dept. of Electrical and Computer

Engineering Auburn University, Auburn, AL

36849 vagrawal_at_eng.auburn.edu http//www.eng.aubu

rn.edu/vagrawal/COURSE/E6770_Fall10/VLSID_Fall201

0_LowPower.ppt

Power Consumption of VLSI Chips

Why is it a concern?

ISSCC, Feb. 2001, Keynote

Ten years from now, microprocessors will run at

10GHz to 30GHz and be capable of processing 1

trillion operations per second about the same

number of calculations that the world's fastest

supercomputer can perform now. Unfortunately,

if nothing changes these chips will produce as

much heat, for their proportional size, as a

nuclear reactor. . . .

Patrick P. Gelsinger Senior Vice

PresidentGeneral Manager Digital Enterprise

Group INTEL CORP.

VLSI Chip Power Density

Suns

Surface

Hot Plate

Source Intel?

Low-Power Design

- Design practices that reduce power consumption at

least by one order of magnitude in practice 50

reduction is often acceptable. - Low-power design methods
- Algorithms and architectures
- High-level and software techniques
- Gate and circuit-level methods
- Test power

Specific Topics in Low-Power

- Power dissipation in CMOS circuits
- Transistor-level methods
- Low-power CMOS technologies
- Energy recovery methods
- Ultra low power logic (subthreshold VDD)
- Circuit and gate level methods
- Logic synthesis
- Dynamic power reduction techniques
- Leakage power reduction
- System level methods
- Microprocessors
- Arithmetic circuits
- Low power memory technology
- Test Power
- Power estimation

CMOS Logic (Inverter)

VDD

No current flows from power supply! Where is

power consumed?

GND

F. M. Wanlass and C. T. Sah, Nanowatt Logic

using Field-Effect Metal-Oxide-Semiconductor

Triodes, IEEE International Solid-State Circuits

Conference Digest, vol. IV, February 1963, pp.

32-33.

Components of Power

- Dynamic, when output changes
- Signal transitions (major component)
- Logic activity
- Glitches
- Short-circuit (small)
- Static, when signal is in steady state
- Leakage (used to be small)

Ptotal Pdyn Pstat Ptran Psc Pstat

Power of a Transition Ptran

V

R Ron

i(t)

vi (t)

v(t)

Large resistance

C

Ground

C Total load capacitance for gate includes

transistor capacitances of driving gate

routing capacitance transistor capacitances of

driven gates obtained by layout analysis.

Charging of a Capacitor

R

t 0

v(t)

i(t)

C

V

Charge on capacitor, q(t) C v(t) Current,

i(t) dq(t)/dt C dv(t)/dt

i(t) C dv(t)/dt V v(t) /R

dv(t) dt ? ----- ? ---- V

v(t) RC

t ln V v(t) -- A RC

Initial condition, t 0, v(t) 0 ? A ln V

t v(t) V 1 exp(---) RC

t v(t) V 1 exp( -- ) RC

dv(t) V t i(t) C --- -- exp(

-- ) dt R RC

Total Energy Per Charging Transition from Power

Supply

8 8 V 2 t Etrans ? V i(t) dt ? --

exp( -- ) dt 0 0 R RC CV 2

Energy Dissipated Per Transition in Resistance

8 V 2 8 2t R ? i2(t) dt R --

? exp( -- ) dt 0 R2 0 RC 1

- CV 2 2

Energy Stored in Charged Capacitor

8 8 t V t ? v(t) i(t) dt

? V 1 exp( -- ) - exp( -- ) dt 0 0

RC R RC 1 - CV

2 2

Transition Power

- Gate output rising transition
- Energy dissipated in pMOS transistor CV 2/2
- Energy stored in capacitor CV 2/2
- Gate output falling transition
- Energy dissipated in nMOS transistor CV 2/2
- Energy dissipated per transition CV 2/2
- Power dissipation

Ptrans Etrans a fck a fck CV 2/2

a activity factor

fck clock frequency

Components of Power

- Dynamic
- Signal transitions
- Logic activity
- Glitches
- Short-circuit
- Static
- Leakage

Ptotal Pdyn Pstat Ptran Psc Pstat

Short Circuit Power of a Transition Psc

VDD

isc(t)

vi (t)

vo(t)

CL

Ground

Short-Circuit Power

- Increases with rise and fall times of input.
- Decreases for larger output load capacitance

large capacitor takes most of the current. - Small, about 5-10 of dynamic power dissipated in

charging and discharging of the output

capacitance. - Becomes zero when VDD Vthn Vthp

Components of Power

- Dynamic
- Signal transitions
- Logic activity
- Glitches
- Short-circuit
- Static
- Leakage

Static (Leakage) Power

- Leakage power as a fraction of the total power

increases as clock frequency drops. Turning

supply off in unused parts can save power. - For a gate it is a small fraction of the total

power it can be significant for very large

circuits. - Static power increases as feature size is scaled

down controlling leakage is an important aspect

of transistor design and semiconductor process

technology.

CMOS Gate Power

vi (t)

V

R Ron

i(t)

vi (t)

v(t)

i(t)

C

Large resistance

isc(t)

Ground

Leakage current

time

Some Examples

Energy Saving by Voltage Reduction

Battery size VDD 0.9V, 500MHz VDD 0.9V, 500MHz VDD 0.9V, 500MHz VDD 0.3V, 5MHz VDD 0.3V, 5MHz VDD 0.3V, 5MHz

Battery size Efficiency Battery lifetime Battery lifetime Efficiency Battery lifetime Battery lifetime

AHr Efficiency x103 seconds x10 11 cycles Efficiency x103 seconds x10 11 cycles

1.2 93 1.263 7.03 100 1234 48.60

3.6 103 4.198 22.80 100 3894 150.30

seven-times

- 70 million gate circuit, 45nm CMOS bulk PTM.
- Lithium-ion battery.
- Ref. M. Kulkarni and V. D. Agrawal, A Tutorial

on Battery Simulation Matching Power Source to

Electronic System, Proc. VLSI Design and Test

Symp., July 2010.

State Encoding for a Counter

- Two-bit binary counter
- State sequence, 00 ? 01 ? 10 ? 11 ? 00
- Six bit transitions in four clock cycles
- 6/4 1.5 transitions per clock
- Two-bit Gray-code counter
- State sequence, 00 ? 01 ? 11 ? 10 ? 00
- Four bit transitions in four clock cycles
- 4/4 1.0 transition per clock
- Gray-code counter is more power efficient.

G. K. Yeap, Practical Low Power Digital VLSI

Design, Boston Kluwer Academic Publishers (now

Springer), 1998.

Binary Counter Original Encoding

a b

Present state Present state Next state Next state

a b A B

0 0 0 1

0 1 1 0

1 0 1 1

1 1 0 0

A B

CK CLR

Binary Counter Gray Encoding

Present state Present state Next state Next state

a b A B

0 0 0 1

0 1 1 1

1 0 0 0

1 1 1 0

a b

A B

CK CLR

Three-Bit Counters

Binary Binary Gray-code Gray-code

State No. of toggles State No. of toggles

000 - 000 -

001 1 001 1

010 2 011 1

011 1 010 1

100 3 110 1

101 1 111 1

110 2 101 1

111 1 100 1

000 3 000 1

Av. Transitions/clock 1.75 Av. Transitions/clock 1.75 Av. Transitions/clock 1 Av. Transitions/clock 1

N-Bit Counter Toggles in Counting Cycle

- Binary counter T(binary) 2(2N 1)
- Gray-code counter T(gray) 2N
- T(gray)/T(binary) 2N-1/(2N 1) ? 0.5

Bits T(binary) T(gray) T(gray)/T(binary)

1 2 2 1.0

2 6 4 0.6667

3 14 8 0.5714

4 30 16 0.5333

5 62 32 0.5161

6 126 64 0.5079

8 - - 0.5000

FSM State Encoding

Transition probability based on PI statistics

0.6

0.6

11

01

0.3

0.3

0.1

0.1

0.4

0.4

01

00

11

00

0.1

0.1

0.9

0.9

0.6

0.6

Expected number of state-bit transitions

1(0.30.40.1) 2(0.1) 1.0

2(0.30.4) 1(0.10.1) 1.6

State encoding can be selected using a

power-based cost function.

FSM Clock-Gating

- Moore machine Outputs depend only on the state

variables. - If a state has a self-loop in the state

transition graph (STG), then clock can be stopped

whenever a self-loop is to be executed.

Xi/Zk

Si

Sk

Xk/Zk

Clock can be stopped when (Xk, Sk) combination

occurs.

Sj

Xj/Zk

Clock-Gating in Moore FSM

Combinational logic

PI

PO

Flip-flops

Clock activation logic

Latch

L. Benini and G. De Micheli, Dynamic Power

Management, Boston Springer, 1998.

CK

Bus Encoding for Reduced Power

- Example Four bit bus
- 0000 ? 1110 has three transitions.
- If bits of second pattern are inverted, then 0000

? 0001 will have only one transition. - Bit-inversion encoding for N-bit bus

N N/2 0

Number of bit transitions after inversion

encoding

0 N/2 N

Number of bit transitions

Bus-Inversion Encoding Logic

Sent data

Received data

Bus register

Polarity decision logic

M. Stan and W. Burleson, Bus-Invert Coding for

Low Power I/O, IEEE Trans. VLSI Systems, vol. 3,

no. 1, pp. 49-58, March 1995.

Polarity bit

Clock-Gating in Low-Power Flip-Flop

D

D Q

CK

S5378 with Gated-Clock FF

- 2958 gates, 179 flip-flops
- TSMC025 CMOS
- 1,000 random vectors, clock period 50ns
- Simulation by Powersim

Flip-flops used Power (microwatts) Power (microwatts) Power (microwatts) Power (microwatts) Power (microwatts) Power (microwatts)

Flip-flops used Combinational logic Combinational logic Combinational logic Clock Flip-flops Total

Flip-flops used Transitions Short-circuit Static (leakage) Clock Flip-flops Total

Normal 95.4 14.1 0.13 220.3 751.6 1,081.5

Gated 133.5 23.1 0.13 118.9 32.5 308.0

J. D. Alexander, Simulation Based Power

Estimation for Digital CMOS Technologies,

Masters Thesis, Auburn University, Dec. 2008.

Books on Low-Power Design (1)

- L. Benini and G. De Micheli, Dynamic Power

Management Design Techniques and CAD Tools,

Boston Springer, 1998. - T. D. Burd and R. A. Brodersen, Energy Efficient

Microprocessor Design, Boston Springer, 2002. - A. Chandrakasan and R. Brodersen, Low-Power

Digital CMOS Design, Boston Springer, 1995. - A. Chandrakasan and R. Brodersen, Low-Power CMOS

Design, New York IEEE Press, 1998. - J.-M. Chang and M. Pedram, Power Optimization and

Synthesis at Behavioral and System Levels using

Formal Methods, Boston Springer, 1999. - M. S. Elrabaa, I. S. Abu-Khater and M. I.

Elmasry, Advanced Low-Power Digital Circuit

Techniques, Boston Springer, 1997. - R. Graybill and R. Melhem, Power Aware Computing,

New York Plenum Publishers, 2002. - S. Iman and M. Pedram, Logic Synthesis for Low

Power VLSI Designs, Boston Springer, 1998. - J. B. Kuo and J.-H. Lou, Low-Voltage CMOS VLSI

Circuits, New York Wiley-Interscience, 1999. - J. Monteiro and S. Devadas, Computer-Aided Design

Techniques for Low Power Sequential Logic

Circuits, Boston Springer, 1997. - S. G. Narendra and A. Chandrakasan, Leakage in

Nanometer CMOS Technologies, Boston Springer,

2005. - W. Nebel and J. Mermet, Low Power Design in Deep

Submicron Electronics, Boston Springer, 1997.

Books on Low-Power Design (2)

- N. Nicolici and B. M. Al-Hashimi,

Power-Constrained Testing of VLSI Circuits,

Boston Springer, 2003. - V. G. Oklobdzija, V. M. Stojanovic, D. M.

Markovic and N. Nedovic, Digital System Clocking

High Performance and Low-Power Aspects,

Wiley-IEEE, 2005. - M. Pedram and J. M. Rabaey, Power Aware Design

Methodologies, Boston Springer, 2002. - C. Piguet, Low-Power Electronics Design, Boca

Raton Florida CRC Press, 2005. - J. M. Rabaey and M. Pedram, Low Power Design

Methodologies, Boston Springer, 1996. - S. Roudy, P. K. Wright and J. M. Rabaey, Energy

Scavenging for Wireless Sensor Networks, Boston

Springer, 2003. - K. Roy and S. C. Prasad, Low-Power CMOS VLSI

Circuit Design, New York Wiley-Interscience,

2000. - E. Sánchez-Sinencio and A. G. Andreaou,

Low-Voltage/Low-Power Integrated Circuits and

Systems Low-Voltage Mixed-Signal Circuits, New

York IEEE Press, 1999. - W. A. Serdijn, Low-Voltage Low-Power Analog

Integrated Circuits, BostonSpringer, 1995. - S. Sheng and R. W. Brodersen, Low-Power Wireless

Communications A Wideband CDMA System Design,

Boston Springer, 1998. - G. Verghese and J. M. Rabaey, Low-Energy FPGAs,

Boston springer, 2001. - G. K. Yeap, Practical Low Power Digital VLSI

Design, BostonSpringer, 1998. - K.-S. Yeo and K. Roy, Low-Voltage Low-Power

Subsystems, McGraw Hill, 2004.

Books Useful in Low-Power Design

- A. Chandrakasan, W. J. Bowhill and F. Fox, Design

of High-Performance Microprocessor Circuits, New

York IEEE Press, 2001. - R. C. Jaeger and T. N. Blalock, Microelectronic

Circuit Design, Third Edition, McGraw-Hill, 2006. - S. M. Kang and Y. Leblebici, CMOS Digital

Integrated Circuits, New York McGraw-Hill, 1996. - E. Larsson, Introduction to Advanced

System-on-Chip Test Design and Optimization,

Springer, 2005. - J. M. Rabaey, A. Chandrakasan and B. Nikolic,

Digital Integrated Circuits, Second Edition,

Upper Saddle River, New Jersey Prentice-Hall,

2003. - J. Segura and C. F. Hawkins, CMOS Electronics,

How It Works, How It Fails, New York IEEE Press,

2004. - N. H. E. Weste and D. Harris, CMOS VLSI Design,

Third Edition, Reading, Massachusetts

Addison-Wesley, 2005.

Problem Bus Encoding

A 1-hot encoding is to be used for reducing the

capacitive power consumption of an n-bit data

bus. All n bits are assumed to be independent and

random. Derive a formula for the ratio of power

consumptions on the encoded and the un-coded

buses. Show that n 4 is essential for the 1-hot

encoding to be beneficial. Reference A. P.

Chandrakasan and R. W. Brodersen, Low Power

Digital CMOS Design, Boston Kluwer Academic

Publishers, 1995, pp. 224-225. Hint You should

be able to solve this problem without the help of

the reference.

Solution Bus Encoding

Un-coded bus Two consecutive bits on a wire can

be 00, 01, 10 and 11, each with a probability

0.25. Considering only the 01 transition, which

draws energy from the supply, the probability of

a data pattern consuming CV 2 energy on a wire is

¼. Therefore, the average per pattern energy for

all n wires of the bus is CV 2n/4. Encoded bus

Encoded bus contains 2n wires. The 1-hot encoding

ensures that whenever there is a change in the

data pattern, exactly one wire will have a 01

transition, charging its capacitance and

consuming CV 2 energy. There can be 2n possible

data patterns and exactly one of these will match

the previous pattern and consume no energy. Thus,

the per pattern energy consumption of the bus is

0 with probability 2n, and CV 2 with probability

1 2n. The average per pattern energy for the

1-hot encoded bus is CV 2(1 2n).

Solution Bus Encoding (Cont.)

Power ratio Encoded bus power / un-coded bus

power 4(1 2n)/n ? 4/n for large n For the

encoding to be beneficial, the above power ratio

should be less than 1. That is, 4(1 2n)/n 1,

or 1 2n n/4, or n/4 1 (approximately) ? n

4. The following table shows 1-hot encoded bus

power ratio as a function of bus width

n 4(1 2n)/n n 4(1 2n)/n

1 2.0000 8 0.4981

2 1.5000 16 0.2500 1/4

3 1.1670 32 1/8

4 0.9375 64 1/16