signal integrity issues capacitive coupling, resistance, inductance cross talk

ELEN654 ATM

Signal Integrity Issues»Capacitive Coupling, Resistance, Inductance»Cross talk

Routability design, Coding, and other Design Measures for protectionI/O DesignPackaging

Routing Considerations

ELEN654 ATM

Impact of Interconnect Parasitics

• Reduce Robustness

• Affect Performance• Increase delay• Increase power dissipation

Classes of Parasitics

• Capacitive

• Resistive

• Inductive

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Capacitive Cross Talk

X

YVX

CXY

CY

ELEN654 ATM

Capacitive Cross TalkDynamic Node

3 x 1 m overlap: 0.19 V disturbance

CY

CXY

VDD

PDN

CLK

CLK

In1

In2

In3

Y

X

2.5 V

0 V

ELEN654 ATM

Capacitive Cross TalkDriven Node

XY = RY(CXY+CY)

Keep time-constant smaller than rise time

0

0.5

0.45

0.4

0.35

0.3

0.25

0.2

0.15

0.1

0.05

010.80.6

t (nsec)

0.40.2

X

YVXRY

CXY

CY

tr↑

ELEN654 ATM

Dealing with Capacitive Cross Talk

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

ELEN654 ATM

Shielding

GND

GND

Shieldingwire

Substrate (GND )

Shieldinglayer

VDD

ELEN654 ATM

Cross 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)

ELEN654 ATM

Impact of Cross Talk on Delay

r is ratio between capacitance to GND and to neighbor

ELEN654 ATM

Structured Predictable Interconnect

S

S SV V S

G

S

SV

G

VS

S SV V S

G

S

SV

G

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

ELEN654 ATM

Interconnect ProjectionsLow-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

ELEN654 ATM

Encoding Data Avoids Worst-Case

Conditions

Encoder

Decoder

Bus

In

Out

ELEN654 ATM

Information Theoretic Approach to Address Delay and Reliability in Long On-chip Interconnects

Interconnects

ELEN654 ATM

Overview

ELEN654 ATM

Sources of Error on Interconnects

Capacitive Coupling Inductive Coupling Process Variations Power Noise

ELEN654 ATM

Signal Integrity

Tradition:» Protect the signal on every single wire.» Design the clock period to be greater than the

worst case delay.

Questions asked?» Is it an overkill?» Can some errors be tolerated?» Can this be optimized?

ELEN654 ATM

Capacitive Coupling

Signals are influenced by the signals in the adjacent wires due to coupling capacitance

Phenomenon known as “Crosstalk” Results in “Deterministic” Delay Variations

ELEN654 ATM

Modeling Interconnects

I(s) G(s) O(s)

O(s) = G(s).I(s)

ELEN654 ATM

Modeling Interconnects

I(s) G(s) O(s)

Capacitive Coupling

Structure

ELEN654 ATM

Modeling Interconnects

I(s) G(s) O(s)

Inductive Coupling

Structure

ELEN654 ATM

Modeling Interconnects

I(s) G(s) O(s)

Power Noise

Randomness

ELEN654 ATM

Modeling Interconnects

I(s) G(s) O(s)

Process Variations

Randomness

ELEN654 ATM

Transfer Function

O(s) = F(s).I(s) F(s) = (1 + L(s)C(s))-

1

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Bus Delay

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Bus Delay

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Waveforms

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Problems in Interconnects

Delay Power Reliability

ELEN654 ATM

Binary Symmetric Channel

Inputs, Outputs Є {0,1} Crossover Probability pe

0

1

pe

1-pe1

0

pe

1-pe

ELEN654 ATM

Self Information(of an event)

Defined as » - log2(p)

– p is the probability of occurrence.

Example » if 1 occurs with p = ½, then every time it

occurs -log2(1/2) = 1 bits of information is conveyed.

ELEN654 ATM

Entropy

Entropy of a system of random events is the measure of uncertainty

Defined as» H(S)= -Σ p.log2(p)

– p is the probability of occurrence of each event in the system.

Example: » For a random binary system

– H = - p1.log2(p1) - p0.log2(p0)– If p1 = p0 = ½, H = 1 bit.

ELEN654 ATM

Conditional Entropy

The uncertainty in one system, given the outcome of the second system.

Defined as» H(S1|S2) = -Σ Σ pjk.log2(pjk/pk)

– J and k are events in systems 1 and 2 respectively.– The equation represents the entropy of system1

conditioned upon the outcome of system 2.

ELEN654 ATM

Channel Capacity

reduction in uncertainty about the input given the output

C = H(Si) – H(Si|So)

For p1 = p0 = ½

» C = 1 + pe.log2(pe) + (1-pe).log2(1-pe)

ELEN654 ATM

Channels with memory

Have multiple states» A1, A2,…, An

Each state has a probability of occurrence» p1,p2,…,pn

Each state has a probability of error» pe1,pe2,…,pen

Capacity of each state» Ci = 1 + pei log(pei) + (1+pei) log(1+pei)

Capacity of channel» C=Σ pi Ci

ELEN654 ATM

The States

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Capacity

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Capacity

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Concluding Remark» Non of the simple ad-hoc codes are

approaching the capacity

Current/Future work» Capacity-approaching bus codes.. and bus

design

ELEN654 ATM

Driving Large Capacitances

V in Vout

CL

VDD

• Transistor Sizing• Cascaded Buffers

ELEN654 ATM

Using 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

ELEN654 ATM

Output Driver Design

Trade off Performance for Area and Energy

Given tpmax find N and f Area

Energy

minminmin12

1

1

1

1...1 A

f

FA

f

fAfffA

NN

driver

22212

11

1...1 DD

LDDiDDi

Ndriver V

f

CVC

f

FVCfffE

ELEN654 ATM

Delay as a Function of F and N

101 3 5 7

Number of buffer stages N

9 11

10,000

1000

100

F = 100F = 1000

F = 10,000t p

/tp

0

ELEN654 ATM

Output Driver Design

Transistor Sizes for optimally-sized cascaded buffer tp = 0.76 ns

Transistor Sizes of redesigned cascaded buffer tp = 1.8 ns

0.25 m process, CL = 20 pF

ELEN654 ATM

How to Design Large Transistors

G(ate)

S(ource)

D(rain)

Multiple

Contacts

small transistors in parallel

Reduces diffusion capacitanceReduces gate resistance

ELEN654 ATM

Bonding Pad DesignBonding Pad

Out

InVDD GND

100 m

GND

Out

ELEN654 ATM

ESD Protection

When a chip is connected to a board, there is unknown (potentially large) static voltage difference

Equalizing potentials requires (large) charge flow through the pads

Diodes sink this charge into the substrate – need guard rings to pick it up.

ELEN654 ATM

ESD Protection

Diode

PAD

VDD

R D1

D2

X

C

ELEN654 ATM

Chip Packaging

ChipL

L ´

Bonding wire

Mountingcavity

Leadframe

Pin

•Bond wires (~25m) are used to connect the package to the chip

• Pads are arranged in a frame around the chip

• Pads are relatively large (~100m in 0.25m technology),with large pitch (100m)

•Many chips areas are ‘pad limited’

ELEN654 ATM

Pad Frame

Layout Die Photo

ELEN654 ATM

Chip Packaging An alternative is ‘flip-chip’:

» Pads are distributed around the chip» The soldering balls are placed on pads» The chip is ‘flipped’ onto the package» Can have many more pads

ELEN654 ATM

Tristate Buffers

InEn

En

VDD

Out

Out = In.En + Z.En

VDD

In

En

En

Out

Increased output drive

ELEN654 ATM

Reducing the swing

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)

ELEN654 ATM

Single-Ended Static Driver and Receiver

CL

VDD

VDD VDD

driver receiver

VDDL

VDDLIn

OutOut

ELEN654 ATM

Dynamic Reduced Swing Network

fIn2.fIn1.

f M2

M1 M3

M4

Cbus Cout

Bus Out

VDD VDD

f

Vbus

Vasym

Vsym

2 4 6time (ns)

8 10 120

0.5

1

1.5

2

2.5

0

ELEN654 ATM

Impact of Resistance

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

ELEN654 ATM

RI Introduced Noise

M1

X

I

R9

RDV

fpre

DV

VDD

VDD 2 DV9

I

ELEN654 ATM19

ASP DAC 2000

Power Dissipation TrendsPower Dissipation Trends

Power consumption is increasingPower consumption is increasing Better cooling technology neededBetter cooling technology needed

Supply current is increasing faster!Supply current is increasing faster!

OnOn--chip signal integrity will be a major chip signal integrity will be a major issueissue

Power and current distribution are criticalPower and current distribution are critical

Opportunities to slow power growthOpportunities to slow power growth Accelerate Accelerate VddVdd scalingscaling

Low κ dielectrics & thinner (Cu) Low κ dielectrics & thinner (Cu) interconnectinterconnect

SOI circuit innovations SOI circuit innovations

Clock system designClock system design

micromicro--architecturearchitecture

Power Dissipation

020406080

100120140160

EV4 EV5 EV6 EV7 EV8

Po

we

r (W

)

0

0.5

1

1.5

2

2.5

3

3.5

Vol

tage

(V

)

Supply Current

0

20

40

60

80

100

120

140

EV4 EV5 EV6 EV7 EV8

Curr

ent (

A)

0

0.5

1

1.5

2

2.5

3

3.5

Vo

ltag

e (

V)

ELEN654 ATM

Resistance and the Power Distribution Problem

Source: Cadence

• Requires fast and accurate peak current predictionRequires fast and accurate peak current prediction• Heavily influenced by packaging technologyHeavily influenced by packaging technology

BeforeBefore AfterAfter

ELEN654 ATM

Power 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

ELEN654 ATM

Power and Ground Distribution

GND

VDD

Logic

GND

VDD

Logic

GND

VDD

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

ELEN654 ATM

3 Metal Layer Approach (EV4)

3rd “coarse and thick” metal layer added to thetechnology for EV4 design

Power supplied from two sides of the die via 3rd metal layer

2nd metal layer used to form power grid

90% of 3rd metal layer used for power/clock routing

Metal 3

Metal 2

Metal 1

Courtesy Compaq

ELEN654 ATM

4 Metal Layers Approach (EV5)

4th “coarse and thick” metal layer added to thetechnology for EV5 design

Power supplied from four sides of the die

Grid 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

ELEN654 ATM

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 – EV6

Metal 4

Metal 2Metal 1

RP2/Vdd

RP1/Vss

Metal 3

Courtesy Compaq

ELEN654 ATM

Electromigration (1)

Limits dc-current to 1 mA/m

ELEN654 ATM

Electromigration (2)

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Resistivity 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

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 ~ LDelay ~ L22

CN-1 CNC2

R1 R2

C1

Tr

Vin

RN-1 RN

The distributed rc-lineThe distributed rc-line

ELEN654 ATM

The Global Wire Problem

Challenges No further improvements to be expected after the

introduction of Copper (superconducting, optical?) Design solutions

» Use of fat wires» Insert repeaters — but might become prohibitive (power, area)» Efficient chip floorplanning

Towards “communication-based” design » How to deal with latency?» Is synchronicity an absolute necessity?

outwwdoutdwwd CRCRCRCRT 693.0377.0

ELEN654 ATM

Interconnect 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

ELEN654 ATM

Interconnect:# of Wiring Layers

# 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

WS

= 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

M5

M4

M3

M2

M1

Poly

Minimum Spacing (Relative)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

M5

M4

M3

M2

M1

Poly

ELEN654 ATM

Diagonal 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

ELEN654 ATM

Using 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

ELEN654 ATM

Reducing RC-delay

Repeater

ELEN654 ATM

Repeater Insertion (Revisited)

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!

ELEN654 ATM

Inductance IssuesL di/dt

Impact of inductance on supply voltages:• Change in current induces a change in voltage• Longer supply lines have larger LCL

V’DD

VDD

L i(t)

VoutVin

GND’

L

ELEN654 ATM

L di/dt: Simulation

0 0.5 1 1.5 2

x 10-9

0

0.5

1

1.5

2

2.5V

out (

V)

0 0.5 1 1.5 2

x 10-9

0

0.02

0.04

i L (A

)

0 0.5 1 1.5 2

x 10-9

0

0.5

1

VL (

V)

time (nsec)

0 0.5 1 1.5 2

x 10-9

0

0.5

1

1.5

2

2.5

0 0.5 1 1.5 2

x 10-9

0

0.02

0.04

0 0.5 1 1.5 2

x 10-9

0

0.5

1

time (nsec)

Input rise/fall time: 50 psec Input rise/fall time: 800 psec

decoupled

Without inductorsWith inductors

ELEN654 ATM

Dealing with Ldi/dt

Separate power pins for I/O pads and chip core. Multiple power and ground pins. Careful selection of the positions of the power

and ground pins on the package. Increase the rise and fall times of the off-chip

signals to the maximum extent allowable. Schedule current-consuming transitions. Use advanced packaging technologies. Add decoupling capacitances on the board. Add decoupling capacitances on the chip.

ELEN654 ATM

Choosing the Right Pin

ChipL

L ´

Bonding wire

Mountingcavity

Leadframe

Pin

ELEN654 ATM

Decoupling Capacitors

SUPPLY

Boardwiring

Bondingwire

Decouplingcapacitor

CHIPCd

1

2

Decoupling capacitors are added: • on the board (right under the supply pins)• on the chip (under the supply straps, near large buffers)

ELEN654 ATM

De-coupling Capacitor Ratios

EV4» total effective switching capacitance = 12.5nF» 128nF of de-coupling capacitance» de-coupling/switching capacitance ~ 10x

EV5» 13.9nF of switching capacitance » 160nF of de-coupling capacitance

EV6» 34nF of effective switching capacitance» 320nF of de-coupling capacitance -- not enough!

Source: B. Herrick (Compaq)

ELEN654 ATM

EV6 De-coupling Capacitance

Design for Idd= 25 A @ Vdd = 2.2 V, f = 600 MHz» 0.32-µF of on-chip de-coupling capacitance was

added– Under major busses and around major gridded clock

drivers– Occupies 15-20% of die area

» 1-µF 2-cm2 Wirebond Attached Chip Capacitor (WACC) significantly increases “Near-Chip” de-coupling

– 160 Vdd/Vss bondwire pairs on the WACC minimize inductance

Source: B. Herrick (Compaq)

ELEN654 ATM

EV6 WACC

587 IPGA

MicroprocessorWACC

Heat Slug

389 Signal - 198 VDD/VSS Pins389 Signal Bondwires

395 VDD/VSS Bondwires

320 VDD/VSS Bondwires

Source: B. Herrick (Compaq)

ELEN654 ATM

The Transmission Line

The Wave Equation

V in Voutr

g c

r r x

g c

r

g c g c

l l l l

ELEN654 ATM

Design Rules of Thumb

Transmission line effects should be considered when the rise or fall time of the input signal (tr, tf) is smaller than the time-of-flight of the transmission line (tflight).

tr (tf) << 2.5 tflight Transmission line effects should only be considered when the

total resistance of the wire is limited:R < 5 Z0

The transmission line is considered lossless when the total resistance is substantially smaller than the characteristic impedance,

R < Z0/2

ELEN654 ATM

Should we be worried?

Transmission line effects cause overshooting and non-monotonic behavior

Clock signals in 400 MHz IBM Microprocessor(measured using e-beam prober) [Restle98]

ELEN654 ATM

Matched Termination

Z 0 Z L

Z0

Series Source Termination

Z 0 Z 0

Z S

Parallel Destination Termination

ELEN654 ATM

Segmented Matched Line Driver

Z0

c1 c2

s0 s1 s2 sn

cn

ZL

GND

VDD

In

ELEN654 ATM

Parallel Termination─Transistors as Resistors

0.5 1

PMOS with-1V bias

NMOS-PMOS

PMOS only

NMOS only

1.5VR (Volt)

2 2.50

1.11

1.21.31.41.51.61.71.81.9

2

Out

M r

Vdd

Out

M r

Vdd

Vbb

Out

M rp M rn

Vdd

ELEN654 ATM

Output Driver with Varying Terminations

1 2 3 4

time (sec)

Revised design with matched driver impedance

Vs

Vd

Vin

5 6 7 80

1

0

1

2

3

4

1 2 3 4

Initial design

Vs

Vd

Vin

5 6 7 80

1

0

1

2

3

4

V in

L= 2.5 nH

VDD

V s V d

V DD

ClampingDiodes

CL= 5 pF CL

L = 2.5 nH

L = 2.5 nH Z 0 = 50

275

120

ELEN654 ATM

The “Network-on-a-Chip”

EmbeddedProcessorsEmbeddedProcessors

MemorySub-system

MemorySub-system

AccelatorsAccelatorsConfigurableAcceleratorsConfigurableAccelerators

PeripheralsPeripherals

Interconnect Backplane