LOW-LEAKAGE REPEATERS LOW-LEAKAGE REPEATERS FOR FOR

NETWORK-ON-CHIP INTERCONNECTSNETWORK-ON-CHIP INTERCONNECTS

Arkadiy Morgenshtein, Israel Cidon, Avinoam Kolodny, Ran Arkadiy Morgenshtein, Israel Cidon, Avinoam Kolodny, Ran GinosarGinosar

Technion – Israel Institute of Technology

QNoC

Research

Group

QNoC Research GroupElectrical Engineering Department

Technion – Israel Institute of TechnologyHaifa, Israel

22 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

HighlightsHighlights

• Selecting the Repeater Type

• Optimizing Repeater Insertion

• Utilization-Oriented Analysis

• Leakage in NoC links with repeaters

33 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

Networks-on-Chip (NoC)Networks-on-Chip (NoC)

Router

Module

Repeater

NoC characteristics

• Packet-based data routing

• Multiple Quality-of-Service levels

Physical layer of NoC

• Low link utilizationLow link utilization Most links idle most of the time!

Leakage power is importantLeakage power is important

44 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

Leakage Reduction in LogicLeakage Reduction in Logic

• Sleep Transistors

• Dual Threshold

• more…

Subthreshold leakage is dominant at high temperatures

Solutions:

55 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

Leakage Reduction in Leakage Reduction in RepeatersRepeaters

??Lw

Lw/n Lw/n Lw/n1 2 n-1

large sizes

no transistor stack

very high wire loads

specific solutions neededunique characteristics

Solutions:

66 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

Existing Repeater TypesExisting Repeater Types

LVT – Low-Vt RepeatersLVT – Low-Vt Repeaters

SVT - Staggered-VtSVT - Staggered-Vt

wire wireLH

LH L

HInidle 0=

wire wireHInidle 0= H H

wire wireL L L

[16] Sylvester et al.

HVT – High-Vt RepeatersHVT – High-Vt Repeaters

• fast• high leakage

• slow• low leakage

• fast (In 01)• slow (In 10)• low leakage (idle)

77 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

Research OutlineResearch Outline

Network-on-ChipNetwork-on-Chip

Selecting the Repeater Type

Utilization-Oriented Analysis

Optimizing Repeater Insertion

Utilization-Dependant Optimal Number of

Repeaters

SR – Sleep Repeaters

DTD – Dual-Vt Domino Repeaters

Low & Varying Utilization

&

88 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

Dual-Threshold Domino (DTD) Dual-Threshold Domino (DTD) RepeatersRepeaters

wire wire wire wire

wire wire

L

H

H

L

wire wire

L

H

H

L

wire wire

L

H

H

L

wire wire

L

H

H

L

Da

ta [

1:n

]

Clock

synchronized Clk link

High-Vt Evaluation Transistors

Low-Vt Pre-charge Transistors

99 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

DTD Repeaters OperationDTD Repeaters Operation

• Precharge transistors disconnected

• CLK line is synchronized with Data

• Evaluation by HVT transistors – slower but tolerant to Vt fluctuations

• Each Evaluation transistor drives only one transistor at next stage – faster and can be down-sized

Data

Clock

1 0

EvalStbyP

re-c

har

ge

Tsetup

Eval

Pre

-cha

rgeX

X

X

X

X

X

X

X

wire wire wire wire

wire wire

L

H

H

L

wire wire

L

H

H

L

Da

ta

Clock

argprech esetup PD linksT t t

1010 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

DTD Repeaters OperationDTD Repeaters Operation

X

X

X

X

X

X

X

X• Evaluation transistors disconnected

• Precharge to low-leakage mode

• Precharge by LVT transistors - fast

wire wire wire wire

wire wire

L

H

H

L

wire wire

L

H

H

L

Da

ta

Clock

Data

Clock

1 0

EvalStbyP

re-c

har

ge

Tsetup

Eval

Pre

-cha

rge

1111 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

wire wire wire wire

wire wire

L

H

H

L

wire wire

L

H

H

L

Da

ta

Clock

DTD Repeaters OperationDTD Repeaters Operation

X

X

X

X

0 1

1

1 0

0X

X

X

X

0 1

1

1 0

0

• HVT transistors are “off” – low leakage

Data

Clock

1 0

EvalStbyP

re-c

har

ge

Tsetup

Eval

Pre

-cha

rge

1212 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

DTD Repeaters OperationDTD Repeaters Operation

X

X

X

X

X

X

X

X

1

1 0

0 1

1 0

0X

X

X

X

X

X

X

X

• For Data=‘0’ - no transition occurs

wire wire wire wire

wire wire

L

H

H

L

wire wire

L

H

H

L

Da

ta

Clock

Data

Clock

1 0

EvalStbyP

re-c

har

ge

Tsetup

Eval

Pre

-cha

rge

1313 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

DTD Repeaters OperationDTD Repeaters Operation

X

X

X

X

X

X

X

X

wire wire wire wire

wire wire

L

H

H

L

wire wire

L

H

H

L

Da

ta

Clock

Data

Clock

1 0

EvalStbyP

re-c

har

ge

Tsetup

Eval

Pre

-cha

rge

1414 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

DTD HighlightsDTD Highlights

Application of domino and double-Vt techniques to low-leakage repeaters

Benefits

+ Effective leakage reduction during standby

+ Reduced load on each repeater allowing downscaling and area reduction

+ Tolerance to VT fluctuations by using HVT evaluation transistors

Drawbacks

- Increased dynamic power consumption due to signaling in domino protocol

- Overhead of clock line and pre-charge wiring

1515 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

Sleep Transistors in Sleep Transistors in RepeatersRepeaters

MTCMOSMTCMOS

SRSR

LogicLogic

RepeatersRepeaters

Evolution

1616 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

MTCMOS in RepeatersMTCMOS in Repeaters

• Common sleep transistors insertion + Both NMOS and PMOS are used

- All stages enter and exit “sleep” mode simultaneously

- LARGE sleep transistors

- High routing complexity and wiring overhead

1717 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

Repeaters with Repeaters with Per-Stage Sleep TransistorPer-Stage Sleep Transistor

• Distributed sleep transistors along the link • Each stage of repeaters has a separate pair of sleep transistors

+ One stage of repeaters is active - others are in low-leakage standby

- Sleep Transistor is heavily loaded and has to be scaled with link width

clk

1818 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

Repeaters with Repeaters with Per-Stage Sleep TransistorPer-Stage Sleep Transistor

• Distributed sleep transistors along the link • Each stage of repeaters has a separate pair of sleep transistors

+ One stage of repeaters is active - others are in low-leakage standby

- Sleep Transistor is heavily loaded and has to be scaled with link width

clk

active sleep sleep

1919 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

Repeaters with Repeaters with Per-Stage Sleep TransistorPer-Stage Sleep Transistor

• Distributed sleep transistors along the link • Each stage of repeaters has a separate pair of sleep transistors

+ One stage of repeaters is active - others are in low-leakage standby

- Sleep Transistor is heavily loaded and has to be scaled with link width

clk

activesleep sleep

2020 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

Repeaters with Repeaters with Per-Stage Sleep TransistorPer-Stage Sleep Transistor

• Distributed sleep transistors along the link • Each stage of repeaters has a separate pair of sleep transistors

+ One stage of repeaters is active - others are in low-leakage standby

- Sleep Transistor is heavily loaded and has to be scaled with link width

clk

activesleep sleep

2121 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

SR – Sleep RepeatersSR – Sleep Repeaters

Parallel link using individual zigzag sleep transistors:

• One sleep transistor per repeater + Smaller sleep transistors

+ Simpler routing

• Zigzag connection:• Only to transistors that are off during sleep

+ Number of sleep transistors is reduced by 50%

clk

X1/32 X1/32

X1/32

X1/32 X1/32

X1/32

01

0

01

0

2222 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

Sleep Repeater HighlightsSleep Repeater Highlights

Novel: Efficient sleep transistors for repeaters

Benefits

+ Effective leakage reduction during standby

+ Optimized structure according to specifics of repeater insertion

Drawbacks

- Area overhead

- Increased dynamic power consumption due to additional transistors

2323 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

Simulation SetupSimulation Setup

• 65nm BPTM models for transistors and interconnect

• 32-bit link operating at 105°C temperature

• LVT design was used as baseline for repeater insertion:

Scaling factor was adjusted for SVT, DTD and SR to meet the delay target equal to LVT

• Area, delay and energy were obtained for each of the compared techniques

20.4,

0.7inv

inv inv inv

int tint intrep rep

t t t int

C RC R Lk h a

R C C R

3π 3π 3πDriver

2424 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

Total Repeater AreaTotal Repeater Area

+ DTD smallest area

- SR largest area

2525 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

Energy vs. UtilizationEnergy vs. Utilization

• SVT: Least energy at high utilization

+ SR: Least energy at low utilization

8mm link

2626 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

Set Target Delay (D<Dopt)

Optimal K for minimal Leakage Power - Kleak

Find for which utilization rates k_leak or k_dyn is optimal

Optimal K for minimal Dynamic Power - Kdyn

Repeaters Sizing for (1<K<n)

!=

Calculate Ratio of Total Power for Kdyn and Kleak vs. Utilization

Utilization-Dependant Utilization-Dependant Optimal Number of RepeatersOptimal Number of Repeaters

2727 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

Optimal Number of RepeatersOptimal Number of Repeaters

• For each k a suitable sizing factor h is found to meet the target delay

• Optimal k for minimal leakage is kleak=4

• Optimal k for minimal dynamic power is kdyn=6

Power vs. k for target D=309ps (instead of Dmin=280ps), L=10mm

19

20

21

22

23

24

25

26

3 4 5 6 7 8 9 10

K (number of repeaters)

Pd

yn

[m

W]

2

3

4

5

6

7

8

9

10

Ple

ak

[m

W]

Pdyn

Pleak

kleak kdyn

example

2828 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

Number of Repeaters vs. Number of Repeaters vs. UtilizationUtilization

• Total power as function of utilization for Kdyn and Kleak

• Power ratio is calculated for Kdyn and Kleak

+ Break-even point is at 40% utilization

+ The results of Kleak are up-to 17% better at low utilization rates

Power ratio of Kdyn vs. Kleak

Prefer Kdyn

Prefer Kleak

example

2929 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

SummarySummary

DTD (Dynamic Dual-Threshold) Repeaters

SR (Sleep Repeaters)

• Zig-zag structure

SR least power at low utilization

• Thanks to low leakage

Optimal number of repeaters depends on link utilization

3030 Low-Leakage Repeaters for NoC CommunicationsLow-Leakage Repeaters for NoC Communications ISCAS 2005

Questions?Questions?