ece 558/658 : lecture 20 interconnect design (chapter 9) clock distribution (chapter 10.3.3)

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ECE 558/658 : ECE 558/658 : Lecture 20Lecture 20

Interconnect DesignInterconnect Design(Chapter 9)(Chapter 9)

Clock distributionClock distribution(Chapter 10.3.3)(Chapter 10.3.3)

Atul Maheshwari

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The Wire – Chapter 4 – Lecture 15The Wire – Chapter 4 – Lecture 15

transmitters receivers

schematics physical

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Capacitance: The Parallel Plate ModelCapacitance: The Parallel Plate Model

Dielectric

Substrate

L

W

H

tdi

Electrical-field lines

Current flow

WLt

cdi

diint

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Fringing CapacitanceFringing Capacitance

W - H/2H

+

(a)

(b)

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Complete Capacitance PictureComplete Capacitance Picture

fringing parallel

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Wire Resistance Wire Resistance

W

LH

R = H W

L

Sheet ResistanceRo

R1 R2

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Impact of Interconnect ParasiticsImpact of Interconnect Parasitics

• Reduce Robustness

• Affect Performance• Increase delay• Increase power dissipation

Classes of Parasitics

• Capacitive

• Resistive

• InductiveImpact of Parasitics

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INTERCONNECTINTERCONNECT

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Capacitive Cross TalkCapacitive Cross Talk

X

YVX

CXY

CY

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Capacitive Cross TalkCapacitive Cross TalkDynamic NodeDynamic Node

3 x 1 m overlap: 0.19 V disturbance

CY

CXY

VDD

PDN

CLK

CLK

In1In2In3

Y

X

2.5 V

0 V

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Capacitive Cross TalkCapacitive Cross TalkDriven NodeDriven Node

XY = RY(CXY+CY)

Keep time-constant smaller than rise time

0

0.5

0.450.4

0.35

0.3

0.25

0.2

0.150.1

0.05

010.80.6

t (nsec)0.40.2

X

YVX

RYCXY

CY

tr↑

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Dealing with Capacitive Cross Dealing with Capacitive Cross TalkTalk

Avoid floating nodes Protect sensitive nodes Make rise and fall times as large as possible Differential signaling Do not run wires together for a long distance Use shielding wires Use shielding layers

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ShieldingShielding

GND

GND

Shieldingwire

Substrate (GND )

Shieldinglayer

VDD

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Cross Talk and PerformanceCross Talk and Performance

Cc

- When neighboring lines switch in opposite direction of victim line, delay increases

DELAY DEPENDENT UPON ACTIVITY IN NEIGHBORING WIRES

Miller EffectMiller Effect

- Both terminals of capacitor are switched in opposite directions (0 Vdd, Vdd 0)

- Effective voltage is doubled and additional charge is needed (from Q=CV)

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Impact of Cross Talk on Delay Impact of Cross Talk on Delay

r is ratio between capacitance to GND and to neighbor

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Structured Predictable InterconnectStructured Predictable Interconnect

S

S SV V S

G

SSV

G

VS

S SV V S

G

SSV

G

VExample: Dense Wire Fabric ([Sunil Kathri])Trade-off:• Cross-coupling capacitance 40x lower, 2% delay variation• Increase in area and overall capacitance

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Encoding Data Avoids Worst-CaseEncoding Data Avoids Worst-CaseConditionsConditions

Encoder

Decoder

Bus

In

Out

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Driving Large CapacitancesDriving Large Capacitances

V in Vout

CL

VDD

• Transistor Sizing• Cascaded Buffers

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Using Cascaded BuffersUsing Cascaded Buffers

CL = 20 pF

In Out

1 2 N

0.25 m processCin = 2.5 fFtp0 = 30 ps

F = CL/Cin = 8000fopt = 3.6 N = 7tp = 0.76 ns

(See Chapter 5)

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How to Design Large TransistorsHow to Design Large Transistors

G(ate)

S(ource)

D(rain)

MultipleContacts

small transistors in parallel

Reduces diffusion capacitanceReduces gate resistance

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Bonding Pad DesignBonding Pad DesignBonding Pad

Out

InVDD GND

100 m

GND

Out

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Tristate BuffersTristate Buffers

InEn

En

VDD

Out

Out = In.En + Z.En

VDD

In

En

EnOut

Increased output drive

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INTERCONNECTINTERCONNECT

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Impact of ResistanceImpact of Resistance Impact on performance - We have already

learned how to drive RC interconnect Impact of resistance is commonly seen in

power supply distribution: IR drop Voltage variations

Power supply is distributed to minimize the IR drop and the change in current due to switching of gates

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Using BypassesUsing BypassesDriver

Polysilicon word line

Polysilicon word line

Metal word line

Metal bypass

Driving a word line from both sides

Using a metal bypass

WL

WL K cells

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Resistivity and PerformanceResistivity and Performance

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50

0.5

1

1.5

2

2.5

time (nsec)

volta

ge (V

)

x= L/10

x = L/4

x = L/2

x= L

Diffused signal Diffused signal propagationpropagation

Delay ~ RCDelay ~ RCDelay ~ LDelay ~ L22

CN-1 CNC2

R1 R2

C1

Tr

Vin

RN-1 RN

The distributed rc-lineThe distributed rc-line

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RepeatersRepeaters

Repeaters are buffers / inverters inserted at regular intervals.

Makes Delay linearly proportional to the wire length.

Questions to be answered – Where and how big the repeaters should be ?

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Reducing RC-delay – Repeater insertionReducing RC-delay – Repeater insertion

Repeater

(chapter 5)

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Repeater InsertionRepeater Insertion

Taking the repeater loading into account

For a given technology and a given interconnect layer, there exists For a given technology and a given interconnect layer, there exists an optimal length of the wire segments between repeaters. The an optimal length of the wire segments between repeaters. The delay of these wire segments is delay of these wire segments is independent of the routing layer!independent of the routing layer!

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Repeater Design LimitationsRepeater Design Limitations Delay-optimal repeaters are area and power

hungry – use of sub-optimal insertion Optimal placement requires accurate

modeling of interconnect. Optimal placement not always possible. Performance limited due to significant

interconnect resistance. Source of noise – Supply and Substrate

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Advanced techniques - Reducing the Advanced techniques - Reducing the swingswing

tpHL = CL Vswing/2

Iav

Reducing the swing potentially yields linear reduction in delay Also results in reduction in power dissipation Delay penalty is paid by the receiver Requires use of “sense amplifier” to restore signal level Frequently designed differentially (e.g. LVDS)

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Single-Ended Static Driver and Single-Ended Static Driver and ReceiverReceiver

CL

VDD

VDD VDD

driver receiver

VDDL

VDDLIn

OutOut

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Dynamic Reduced Swing NetworkDynamic Reduced Swing Network

fIn2.fIn1.

f M2

M1 M3

M4

Cbus Cout

Bus Out

VDD VDD

f

VbusVasym

Vsym

2 4 6time (ns)

8 10 120

0.5

1

1.5

2

2.5

0

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RI Introduced NoiseRI Introduced Noise

M1

X

I

R̀

RV

f pre

V

VDD

VDD - V`

I

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Power DistributionPower Distribution Low-level distribution is in Metal 1 Power has to be ‘strapped’ in higher layers of

metal. The spacing is set by IR drop,

electromigration, inductive effects Always use multiple contacts on straps

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Power and Ground DistributionPower and Ground Distribution

GND

VDD

Logic

GND

VDD

Logic

GND

VDD

(a) Finger-shaped network (b) Network with multiple supply pins

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3 Metal Layer Approach (EV4)3 Metal Layer Approach (EV4)3rd “coarse and thick” metal layer added to the

technology for EV4 designPower supplied from two sides of the die via 3rd metal layer

2nd metal layer used to form power grid90% of 3rd metal layer used for power/clock routing

Metal 3

Metal 2

Metal 1

Courtesy Compaq

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4 Metal Layers Approach (EV5)4 Metal Layers Approach (EV5)4th “coarse and thick” metal layer added to the

technology for EV5 designPower supplied from four sides of the dieGrid strapping done all in coarse metal

90% of 3rd and 4th metals used for power/clock routing

Metal 3

Metal 2

Metal 1

Metal 4

Courtesy Compaq

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2 reference plane metal layers added to thetechnology for EV6 designSolid planes dedicated to Vdd/Vss

Significantly lowers resistance of gridLowers on-chip inductance

6 Metal Layer Approach – EV66 Metal Layer Approach – EV6

Metal 4

Metal 2Metal 1

RP2/Vdd

RP1/Vss

Metal 3

Courtesy Compaq

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Electromigration (1)Electromigration (1)

Limits dc-current to 1 mA/m

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Electromigration (2)Electromigration (2)

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Interconnect Projections:Interconnect Projections:GeometryGeometry

# of metal layers is steadily increasing due to:• Increasing die size and device count: we need more wires and longer wires to connect everything

• Rising need for a hierarchical wiring network; local wires with high density and global wires with low RC

substrate poly

M1

M2

M3

M4

M5

M6

Tins

H

W S

= 2.2 -cm

0.25 m wiring stack

Minimum Widths (Relative)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

M5M4M3M2M1Poly

Minimum Spacing (Relative)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

M5M4M3M2M1Poly

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Interconnect Projections :Interconnect Projections :Low-k dielectricsLow-k dielectrics

Both delay and power are reduced by dropping interconnect capacitance

Types of low-k materials include: inorganic (SiO2), organic (Polyimides) and aerogels (ultra low-k)

The numbers below are on the conservative side of the NRTS roadmap

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Interconnect Projections: CopperInterconnect Projections: Copper Copper is planned in full sub-0.25

m process flows and large-scale designs (IBM, Motorola, IEDM97)

With cladding and other effects, Cu ~ 2.2 -cm vs. 3.5 for Al(Cu) 40% reduction in resistance

Electromigration improvement; 100X longer lifetime (IBM, IEDM97) Electromigration is a limiting factor

beyond 0.18 m if Al is used (HP, IEDM95)

Vias

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Diagonal WiringDiagonal Wiring

y

x

destination

Manhattan

source

diagonal

• 20+% Interconnect length reduction• Clock speed Signal integrity Power integrity • 15+% Smaller chips plus 30+% via reduction

Courtesy Cadence X-initiative

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Clock distributionClock distribution

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Clock DistributionClock Distribution

CLK

Clock is distributed in a tree-like fashion

H-tree

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More realistic H-treeMore realistic H-tree

[Restle98]

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The Grid SystemThe Grid SystemD r iv e r

D r iv e r

Dri

ver

Driv

er

G C LK G C LK

G CL K

G CL K

•No rc-matching•Large power

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Example: DEC Alpha 21164Example: DEC Alpha 21164Clock Frequency: 300 MHz - 9.3 Million Transistors

Total Clock Load: 3.75 nF

Power in Clock Distribution network : 20 W (out of 50)

Uses Two Level Clock Distribution:

• Single 6-stage driver at center of chip• Secondary buffers drive left and right side

clock grid in Metal3 and Metal4Total driver size: 58 cm!

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21164 Clocking21164 Clocking 2 phase single wire clock,

distributed globally 2 distributed driver channels

Reduced RC delay/skew Improved thermal distribution 3.75nF clock load 58 cm final driver width

Local inverters for latching Conditional clocks in caches to

reduce power More complex race checking Device variation

trise = 0.35ns tskew = 150ps

tcycle= 3.3ns

Clock waveform

Location of clockdriver on die

pre-driver

final drivers

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Clock Drivers

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Clock Skew in Alpha ProcessorClock Skew in Alpha Processor

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2 Phase, with multiple conditional buffered clocks 2.8 nF clock load 40 cm final driver width

Local clocks can be gated “off” to save power

Reduced load/skew Reduced thermal issues Multiple clocks complicate race

checking

trise = 0.35ns tskew = 50ps

tcycle= 1.67ns

EV6 (Alpha 21264) ClockingEV6 (Alpha 21264) Clocking600 MHz – 0.35 micron CMOS600 MHz – 0.35 micron CMOS

Global clock waveform

PLL

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21264 Clocking21264 Clocking

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EV6 Clock ResultsEV6 Clock Results

GCLK Skew(at Vdd/2 Crossings)

ps5

101520253035404550

ps300305310315320325330335340345

GCLK Rise Times(20% to 80% Extrapolated to 0% to 100%)

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EV7 Clock HierarchyEV7 Clock Hierarchy

GCLK(CPU Core)L2

L_CL

K(L

2 Ca

che)

L2R_

CLK

(L2

Cach

e)

NCLK(Mem Ctrl)

DLL

PLL

SYSCLK

DLL

DLL

+ widely dispersed drivers

+ DLLs compensate static and low-frequency variation

+ divides design and verification effort

- DLL design and verification is added work

+ tailored clocks

Active Skew Management and Multiple Clock Domains