Low Power Processor Design

VLSI Systems Lab.

3 월 28 일박 봉 일

Introduction

• Processor power consumption

PowerPower

1 Watt

3-5 Watt

5-15 Watt

15+ Watt

heat sink,air flow

fan sink

exotic

none none

$1-5

$10-15

$50+

CostCostStrategyStrategy

LaptopComputer

Low powerprocessor

Power Reduction Technique

• Process level– low voltage

– low capacitance

• Circuit level– TR sizing

– adiabatic circuit

– low power arithmetic components

• Logic level– precomputation logic

– logic synthesis

– retiming

• System level– frequency reduction

– voltage reduction

– power management mode

System Level

0 20 40 60 80 100

Load/Store

Fixed Point

Floating Point

Special Register

SPECfp92SPECint92

• Execution unit idle time(PowerPC 603)

2.2

0.366

0.135

0.047

0

0.5

1

1.5

2

2.5

PowerPC 603

NormalDozeNapSleep

1.5

0.4

0

0.5

1

1.5

2

MIPS 4200

NormalReduced

0.9

0.02 0.00002

0

0.2

0.4

0.6

0.8

1

Strong ARM

Normal

Idle

Sleep

System Level

• Power management support

Power Estimation

• Simulation-based techniques– circuit simulation

– switch level simulation : IRSIM

– transistor level simulation : PowerMill

– gate level simulation

– Monte Carlo simulation

Power Estimation

• Probabilistic techniques– combinational circuits

• zero delay model

• real delay model

– sequential circuits

Logic Level

• Logic Synthesis– precomputation logic

– retiming

– state assignment

– path balancing

– technology mapping

– gate resizing

gg RR R

R1R1

R2R2AA R3R3

gg

Memory

• Architectural selection– select as little of the array as

possible

– dynamically powering up sense amp.

– Clocking only as needed

1/2Cell

Array

1/2Cell

Array

1/2Cell

Array

1/2Cell

Array

RowDe-

coder

RowDe-

coder

1/2 ColumnDecoder

1/2 ColumnDecoder

1/2 ColumnDecoder

1/2 ColumnDecoder

Split Array, half of columns active

W/R W/R DataData Addr

Memory Block Diagram

Clock

• Fast transition time and low skew– consume lots of power

– 10~20% of total chip power

• Clock power management– clock branches are segmented

and can be enabled as needed

PLLPLL

Datapath Signal Activity

• PowerPC 601

Instruction TypeInstruction Type # of instructions# of instructions # of 0 to 1# of 0 to 1 # of 1 to 0# of 1 to 0

ShiftShift 31753175 1.981.98 4.634.63

Switching FactorSwitching Factor

0.210.21

ADD/SUBADD/SUB 49374937 3.413.41 4.314.31 0.240.24

EA calculationEA calculation 1549615496 3.153.15 2.372.37 0.170.17

MUL/DIVMUL/DIV 10701070 1.501.50 1.561.56 0.100.10

Control RegisterControl Register 192192 1.441.44 2.262.26 0.120.12

CompareCompare 23492349 3.813.81 4.114.11 0.250.25

BranchBranch 5858 5.035.03 13.5013.50 0.580.58

TotalTotal 2727727277 3.053.05 3.133.13 0.190.19

Traditional Method

• Use enabling logic– Enable only the adder needed

– reduce the signal activities

• Minimizing temporal bit transition activity– gray coding

– bus inversion coding

AdderAdder AdderAdder

AdderAdder

MUXMUX

clkcontrol

clkcontrol

clkcontrol

clkcontrol

A B C D

A B C D

Datapath Components:Adder

• 특징– 다양한 구조에 따른 transition 의 변화가 심함

Adder Type(32 bit)

Adder Type(32 bit)

Delay(in gate units)

Delay(in gate units) # of gates# of gates # of transitions

(average)# of transitions

(average)

Ripple CarryRipple Carry 6868 288288 182182

Carry Skip(1)Carry Skip(1) 3333 304304 392392

Carry Skip(2)Carry Skip(2) 1919 350350 437437

Carry LookaheadCarry Lookahead 1414 401401 405405

Carry SelectCarry Select 1414 597597 711711

Conditional SumConditional Sum 1515 857857 13231323

Datapath Components: Multiplier

• 특징– 많은 transition

– transition 이 일어날 확률이 1 회 /1clock 인 노드가 50% 이상임

Multiplier Type(32 bit)

Multiplier Type(32 bit)

Delay(in gate units)

Delay(in gate units) # of gates# of gates # of transitions

(average)# of transitions

(average)

Modified ArrayModified Array 9898 24052405 73487348

Wallace/DaddaWallace/Dadda 5151 25692569 38743874

Future Works

• 저전력 프로세서 설계– Arithmetic component 에 대한 분석– 저전력을 위한 arithmetic component 의 제안

• 저전력 프로세서 구조의 제안– 다양한 구조에 대한 전력측면에서의 분석

Continued Story:

ACCENT_Light

VLSI Systems Lab. KAIST.

Mar. 28. 1998.

You-Sung Chang

Previous Work

• Dr. Bong has done!

• Everyone knows well now.

• Nothing to explain.

Feature of Accent

• Highly integrated CISC Processor-Core• 4-stage Pipelined Architecture• Configuration

– Pre-fetch Cache– Decode– Execution– Memory Management– Micro-code– External Interface– Embedded DRAM– . . .

Low Power in Accent

• Support Programmable Very Complex Code

• Micro-code based Stripe Power Control

• Pre-charging Biasing in Mask-ROM

• Inverse Data Store in Embedded DRAM

• Minimizing switching in BUS transfer

Very Complex Code

• Maximize the advantage of CISC micro-code approach

• Adaptive Programmable Micro-code – Program analyzer extract application specific instruction

– Compile micro-code ROM and decoder

• A small loop is translated into a complex instruction– Small code size

– Give more idle time to pre-fetch and decode units

– Enable low power from the small code size and the clock blocking for the induced idle time of pre-fetch and decode units

Stripe Power Control

• Clocking only as needed– Obvious!

– How?

• Cut data-path in strips

• Power control using micro-code field information

• Request enables clocking for peripheral units

Func1Func1

F/FF/F

Pass LatchPass LatchGatedclock 1

Gatedclock 2

Func2Func2

F/FF/F

Pass LatchPass LatchGatedclock 3

Gatedclock 4

Mask ROM

• Selective pre-charging/discharging for Micro-code ROM.

• Using the static statistics, assemble Micro-code ROM cell column by column.

• Simulation shows– Not so effective for Micro-code ROM

– Some potential for constant ROM

Embedded DRAM

• Full voltage pre-charging– Does not need half voltage generator

• Single-ended type

• Read/Write word by word

• To save power, minimize switching in the bit-line

• Store inverse data if ‘0’ is dominated with indicator. Sense AmplifierSense Amplifier

ReferencePre-charged(low cap.)

Pre-charged(high cap.)

BUS Transfer

• One-Hot Coding

• Gray Coding

• Bus Inversion Coding (1994 stan)

BUS

N+11: inversion indicator

N N

Self Evaluation

• Evaluation of Anticipation– Support Programmable Very Complex Code(H)

– Micro-code based Stripe Power Control (M)

– Pre-charging Biasing in Mask-ROM (M)

– Inverse Data Store in Embedded DRAM (H)

– Minimizing switching in BUS transfer (X)

H: highM: mediumL: lowX: X, its dedicated signification

Further Work

• Complete power estimation for each block– Functional Blocks in Data-path

– Pre-fetch and Decoder

• Inspect physical constraints in pre-charging biasing

• Estimate the power advantage of inverse data store

• Target : Workshop ~

• Task force : Caviar, Woosee, Bipark