Crosstalk-Aware Transmitter Pulse-Shapingfor Parallel Chip-to-Chip Links

Mike Bichan, Anthony Chan Carusone

Department of Electrical and Computer EngineeringUniversity of Toronto

ISCAS 2007

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Board-to-Board Channel

Connectors

Chips

Daughtercards

Board

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Characterize the Channel

Chip2Chip1

crosstalk

Rx

Rx

Rx

Tx

Tx

TxPSfrag replacements

Z0=50 Ω

0 1 2 3 4 5−0.2

0

0.2

0.4

0.6

0.8

1

throughcrosstalk1crosstalk2

PSfrag replacements

Vol

tage

(V)

Time (ns)

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Impulse and Frequency Responses

0 1 2 3 4 5−0.02

00.020.040.06

0 1 2 3 4 5−0.02

00.020.040.06

Vol

ts

0 1 2 3 4 5−0.02

00.020.040.06

Time (ns)

PSfrag replacements

through

crosstalk1

crosstalk2

107

108

109

−50

−40

−30

−20

−10

0

throughcrosstalk1crosstalk2

PSfrag replacements

Frequency (Hz)C

hann

elG

ain

(dB

)

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System Model

CrosstalkChip-to-Chip Channel

Transmit Filter

(many links in parallel)

Tx G RxH

C

RxHGTx

HGTx

C

Rx

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Conventional Solutions

Tx

Chip-to-Chip Channel

Transmit Filter

(many links in parallel)

Tx-1

-1C

CRx

Rx

RxH

H

HG

G

G

Tx

slew-rate limitingpro: simplecon: not goodwhen ISI issevere

crosstalkcancellation inaddition to transmitfilter G

pro: goodperformancecon: hardwarecost

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Example: Different Pulse Shapes

PSfrag replacements

1 UI

1 UI

1 UI

square pulse1 total tap1 tap per UI

pre-emphasis pulse2 total taps1 tap per UI

fractionally-spaced6 total taps3 taps per UI

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Example: Different Pulse Shapes

PSfrag replacements

1 UI

1 UI

1 UI

square pulse1 total tap1 tap per UI

pre-emphasis pulse2 total taps1 tap per UI

fractionally-spaced6 total taps3 taps per UI

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Example: Different Pulse Shapes

PSfrag replacements

1 UI

1 UI

1 UI

square pulse1 total tap1 tap per UI

pre-emphasis pulse2 total taps1 tap per UI

fractionally-spaced6 total taps3 taps per UI

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Optimize with Respect to a Figure of Merit

Definition

E2C =

crosstalk-free eye openingmaximum possible crosstalk

PSfrag replacementsCrosstalk-FreeEye Opening

1 Unit Interval (UI)

PSfrag replacements

MaximumPossibleCrosstalk

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Find E2C for each Pulse Shape

0 200 400 600

−0.6

−0.4

−0.2

0

0.2

0.4

0.6

485 mV

0 1 2 3 4 5−0.02

−0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

PSfrag replacements

∗ ∗ =Time (ps)

Vol

tage

(V)

Time (ns)

Vol

tage

(V)

PRBS7

0 200 400 600−0.2

−0.15

−0.1

−0.05

0

0.05

0.1

0.15

0.2

crosstalk = 157 mV 0 1 2 3 4 5−0.02

−0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

PSfrag replacements

∗ ∗ =Time (ps)

Vol

tage

(V)

Time (ns)

Vol

tage

(V)

PRBS7

E2C =485 mV

2×157 mV = 1.54

Repeat calculation for all candidate pulse shapes

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Maximum E2C for Various Filter Types at 2.7 Gb/s

1 2 3 4 5 61

2

3

4

5

6

4.5

55.5

6

6.5

increasing E2C

PSfrag replacements

Total Taps

Taps per UI

E2C

1

1/2

1/3

1/4

1/5

1/6

Tap Delay(in UI)

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Increasing Total Taps

1 2 3 4 5 61

2

3

4

5

6

4.5

55.5

6

6.5

PSfrag replacements

Total Taps

Taps

per

UI

E2C

1

1/2

1/3

1/4

1/5

1/6

Tap Delay(in UI)

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Increasing Granularity

1 2 3 4 5 61

2

3

4

5

6

4.5

55.5

6

6.5

PSfrag replacements

Total Taps

Taps

per

UI

E2C

1

1/2

1/3

1/4

1/5

1/6

Tap Delay(in UI)

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Increasing Taps per UI

1 2 3 4 5 61

2

3

4

5

6

4.5

55.5

6

6.5

PSfrag replacements

Total Taps

Taps

per

UI

E2C

1

1/2

1/3

1/4

1/5

1/6

Tap Delay(in UI)

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Hardware Proof-of-Concept

CombinerPower

CombinerPower

CombinerPower

OscilloscopeParBERT Chip-to-ChipChannel

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Channel Introduces ISI and Crosstalk

2.7 Gb/s

PRBS: 231–1

input to channel

square pulse

output fromchannel

no aggressors

output fromchannel

two aggressors

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Filter Opens the Eye

output from channel

two aggressors

square pulse input

jitterRMS = 53 ps

pulse shape chosen tomaximize E2C

3 total taps

2 taps per UI

jitterRMS = 33 ps

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Filter Improves Bit Error Rate

−1 −0.5 0 0.5 1

10−15

10−10

10−5

100

Crosstalk Aware Pulse ShapeSquare Pulse

PSfrag replacements

Bit

Err

orR

ate

Sampling Phase (Unit Intervals)

−1 −0.5 0 0.5 110

−10

10−8

10−6

10−4

10−2

100

Crosstalk−Aware Pulse ShapePre−emphasis Pulse Shape

PSfrag replacements

Bit

Err

orR

ate

Sampling Phase (Unit Intervals)

square pulse:BER = 10−5

crosstalk-aware pulse:BER < 10−12

pre-emphasis pulse:BER = 10−7

crosstalk-aware pulse:BER = 10−5

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Conclusion

Crosstalk is significant inboard-to-board channels

Received eye opening canbe increased by takingcrosstalk into accountwhen equalizing

Crosstalk-aware pulseshape decreased BER by102 at 2.7 Gb/s

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