OPTIMIZATION OF POWER REDUCTION

IN FPGA INTERCONNECT BY

CHARGE RECYCLING

Deepa Soman, HyunSuk Nam, Rekha Srinivasaraghavan, Shashank Sivakumar

Agenda

Day 2 Power Reduction T

echniques (Conti) Charge Recycling Our Project Discussions

Day 1 Intro Power Consumpti

on Techniques Power Reduction T

echniques Discussions

Motivation Achilles’ Heel Logic flexibility & re-programmability -

longer wires (7-14 X) higher than asics

Introduction

Power Consumption

Dynamic Power -  power consumed while the inputs are active

Static power - power consumed even when there is no circuit activity !!!

fCVP dddynamic2

KTqV

DSdd

leakageddsub

th

eIV

IVP

0

.

Why Panic about Power?

Why Static Power??

Low Power Opportunities

Hardware Techniques

• Voltage Scaling Dual Vdd

• Frequency Scaling• Clock Gating

9

Voltage Scaling

Selecting core voltage based on performance requirements

How to Choose? – From Timing Analysis

Types: 1) Static Voltage Scaling 2) Dynamic Voltage Scaling

10

1. Static Voltage Scaling

Selected core voltage only Realized using on chip Low-Dropout

regulator(LDO) Voltage controlled by configuration bit

stream  0.8-V - minimum dynamic and leakage

power 1.0-V - overall highest performance

[1]"A FPGA Prototype Design Emphasis on Low Power Technique" Xu, Jian

1.0v

0.8v

LDO

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2. Dynamic Voltage Scaling

Provides different voltage levels Realized using voltage controlling unit

Can be level shifter or DC-DC converter

DVS implementation

(LDMC – Logic Delay Measurement Unit) Delay error

 ”Dynamic Voltage Scaling for Commercial FPGAs”, C.T. Chow1, L.S.M. Tsui1, P.H.W.

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Dual Supply Voltage (Vdd)

Separate voltage supplies for configuration SRAM and other elements

Purpose: To support sleep mode Shutdown most logic except SRAM using

LDO

“A Dual-VDD Low Power FPGA Architecture” A. Gayasen1, K. Lee1, N. Vijaykrishnan1, M. Kandemir1, M.J. Irwin1, and T. Tuan2

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Performance

Static voltage scaling techniques leads to nearly 53% power reduction. Dynamic(upto 54%). Dual Vdd- 14%

Merits: SVS - Simple hardware DVS - Self adaptive Dual Vdd – eliminate speed penalty

Demerits: SVS - Voltage is fixed DVS - design complexity Dual Vdd - area overhead

[1]"A FPGA Prototype Design Emphasis on Low Power Technique" Xu, Jian[2]”A 90-nm Low-Power FPGA for Battery-Powered Applications”,Tuan, Das, Steve, Sean

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Frequency Scaling

fCVP dddynamic2 f : frequency of switching

Dynamic Clock Management Implementations

(a)Simple dynamic clock management circuit

(b) Using Feedback, PLL circuit can reduce skew; lock time

(c) dynamic clock division

Merits:• Can subsequently reduce voltageDemerits:• Increased Latency

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Benefits of Frequency Scaling

As frequency decreases, power consumption also decreases

"Dynamic Clock Management for Low Power Applications in FPGAs", Lan, zilic

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Clock Gating

Controlling the clock flow Purpose: To temporarily disable blocks Can be realized in hardware using clock enable

signals minimizes power dissipation in clock

circuits/network

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Clock Gating - Performance

industry-a,b,c,d, are DSP circuits, while the remaining circuits are collected from customers and are of unknown function

Over 20% power reductions are observed for the DSP circuits

Clock Power Reduction for Virtex-5 FPGAs

Eliminates unnecessary toggling on outputs, gates of FFs and clock signals

Demerits:Clock skew

"Clock Power Reduction for Virtex-5 FPGAs",Wang, Gupta, Anderson

A

• System Level: • Algorithm

Modification• CAD Tools :

• Logic Partitioning

• Mapping,• Clustering • Placement &

Routing

Software

Techniques

Low Power FFT Implementation Architecture

Matrix multiplication ->1D array low power dissipation than 2D array

Module Disabling – Clock gating to disable modules eg: twiddle factor calculation

dynamic memory activation Multiple time multiplexed Pipeline uP Parallel Processing Algorithm : Block Matrix Multiplication

FFT implementation Results

17% to 26% power reduction

"High throughput energy efficient multi-FFTarchitecture on FPGAs" , Chen , Park, Prasanna

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Energy Reduction Contributions of CAD Stages

Clustering contributes to the major share !

"On the interaction between power aware FPGA CAD algorithms" , Julien , Steven

Power Aware Clustering

Power Aware TV pack How?? Cost function Modification to include

power

Results: Power Aware clustering

“Netlength Based Routability Driven Power Aware Clustering" , Akoglu, Easwaran

Power Aware Placement

Results

"On the interaction between power aware FPGA CAD algorithms" , Julien , Steven

Temperature Aware Routing

leakage current increases exponentially with temperature

Switching capacitance

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Algorithm

By discouraging routing algorithm to form connections that cross hotspot regions

Cost Function Modification:

Power Savings Range between 30 – 63 %

"A Temperature-Aware Placement and Routing targeting 3D FPGAs", Kostas, Soudris

Power-Aware FPGA Design Flow

Step 2• Power Aware

Packing• or Clustering

CAD• Power Aware

Placement

Tools• Power Aware

Routing

Step 1

• Power Based Architectural

• (High level modelling)

RTL

• Voltage scaling, Dual Vdd

• Freq Scaling, Clock gating

Main/Baseline Paper

Problem Addressed Power consumption in FPGAs

is dominated by interconnect(62%)

Proposed idea Charge recycling for

power reduction in FPGA interconnect

Charge Recycling (CR)

Charge Recycling in FPGAs

How?? “Unused routing resources “ as reservoirs

Reduces charge drawn from Vdd25% reduction in energy

1. 2. 3.

4.

5. 6. 7.

Unused/Reservoir

Unused/Reservoir

Unused w/o friends !!

CR-Capable FPGA Interconnect

Analysis Four components

SRAM Cell• Produce signals CR and TS :

control a switch (Normal, CR, tri-state )

Delay Line• Transition between VIN and

DLOUT

CR Circuit• Perform the charge sharing

between the load and reservoir Input Stage

Experiments/Methodology

VPR6.0

Baseline : Island style, Unidirectional, Wilton (K=6 ,N=4)

Router – Path Finder - Cost Function Modification

Post Routing CR mode

VPR place/route tool helps in finding % increase in area

VPR Cost Function

Cost Function – Path Finder

Modified Cost Function

Post - Routing

Mixed Integer Linear Program

Tries to maximize the number of nodes to be put into CR mode

Constraint: Critical delay of the circuit

Results

Dynamic power in the FPGA interconnect is reduced by up to ∼15-18.4%

Results Continued…

Number of min-width transistors as the area metric

Reductions in power savings are not directly proportional to the reduction in CR-capable switches (area)

What we propose new?

Not all unused wires become friends Unused wires connected to constant voltage

“URekha” --- Unused wires Tri-stated “further power savings!!”

~6% savings

Thank you