21.4: Common-mode Backchannel Co o ode ac c a eSignaling System for Differential

High-speed LinksHigh speed Links

Andrew Ho1, Vladimir Stojanovic1,2, Fred Chen1,Andrew Ho , Vladimir Stojanovic , Fred Chen ,Carl Werner1, Grace Tsang1, Elad Alon1,2,

Ravi Kollipara1, Jared Zerbe1, Mark Horowitz2

1 Rambus Inc.2 Stanford University

The Need for a Backchannel

TX Data

dLev

error

Channel

adaptivesampler

RX data

Adaptivemacro

aClk

dClk

?

update

thresholds

CDRedge

dClk

updatetaps

tap updatesaClk dClk eClk

eClk

Backchannel

• New techniques to mitigate effects of band-limited channel• Close loop tuning of transmit side circuits

Transmitter Receiver

• Close-loop tuning of transmit-side circuits

• Need Low bandwidth information flow from RX to TX

System Limitationsy• Reverse information flow can be a significant

constraint on systems that haveconstraint on systems that have• Limited pins• Minimal control data routing capabilityg p y• Fixed link arrangements

• Typical Solutionsyp• Control plane mechanism• Pairing with sister channels in opposite direction

• Requirements for adaptation• BER < 0.5 still converges• Lower BER faster convergence

An alternate approach – Backchannel on same wires by signaling in the common-modewires by signaling in the common mode

• Differential signaling leaves common-mode as a potential methodpotential methodA self-contained backchannel solution

Outline○ Motivation

Si l I t it M d li & A l i○ Signal Integrity Modeling & Analysis○ System Implementation○ Test System & Results○ Conclusion

Signal Integrity Modelingg g y gDiff. TX

Rx DataFC

Diff. RX

Tx DataFC

1 2

Channel

RxClk

Tx DataBC

TxClk

Rx DataBC

3 4CM TXCM RX

3 4

s21

s

21

s

+

s31

3 4

s24

+ s34

• 4-port model – s31 and s24 captures crosstalk between differential and common-mode linksdifferential and common mode links

Potential Error Sources:CM-to-Differential Crosstalk

-20

-10

R h l

C to e e t a C ossta

-40

-30

dB)

Raw channel

-60

-50|H| (

d

107 108 109-80

-70 Rise time 1.8ns

10 10 10Frequency (Hz)

• Reduced Common-mode TX rise time decreases crosstalk noise to Differential RXcrosstalk noise to Differential RX

Potential Error Sources:Common-mode Modulation NoiseCo o ode odu at o o se

Rp +Diff. RX-

+Diff. TX

-DataFC

Data

Rn

p

DataBC

CM TX

• Backchannel and forward channel data are uncorrelatedBackchannel transmitter modulates CM at input of Diff RX• Backchannel transmitter modulates CM at input of Diff. RX

• Changes in Diff. RX as function of CM = differential noise

RX Common-mode Sensitivity

25

30

0

20

y

Operating

AllocatedMargin

15

20

tivity

(mV)

-40

-20

set(m

V) OperatingRange

OperatingRange

5

10Sen

sit

-80

-60Offs

0.4 0.6 0.8 1 1.20

Common-mode Voltage (V)0.4 0.6 0.8 1 1.2

-100

Common-mode Voltage (V)

• CM signal range and swing are chosen to minimize change in receiver characteristics5mV forward channel margin CM range 0 75 to 1 V• 5mV forward channel margin CM range 0.75 to 1 V

Thru-channel Freq Responseq p0

10

-5

(dB)

-15

-10

|H| (

107 108 109-20

• Flat frequency response – little margin degradation due to inter-symbol interference

10 10 10Frequency (Hz)

to inter symbol interference

Potential Error Sources:Differential-to-CM Crosstalk

20

-10

e e t a to C C ossta

-30

-20

dB)

s31 before pre-amp

50

-40|H| (

d

107 108 109-60

-50

s31 after pre-amp

10 10 10Frequency (Hz)

• Low-pass filtering pre-amp reduces high frequency crosstalk from differential TXcrosstalk from differential TX

Potential Error Sources:Differential TX CM Glitch

-20

-10Differential transmittercommon-mode glitch

e e t a C G tc

Outputp

-40

-30

dB)Outputn

-60

-50|H| (

d

Common-mode glitch filtered by the pre-amp

106 107 108 109 1010-80

-70

CM = (Outputp + Outputn) / 210 10 10 10 10Frequency (Hz)

• Rise/fall mismatch & differential skew CM Glitch• Low-pass filtering pre-amp reduces high frequency noise• Low-pass filtering pre-amp reduces high frequency noise

from differential TX’s CM glitch

Common-mode Backchannel RequirementsCommon mode Backchannel Requirements

● Reduced generated noise to forward channelR d t it i ti● Reduce transmit rise time

● Limit swing range

● Reduced noise from forward channel● Low frequency operation● Low frequency operation● Add low-pass filter

Outline○ Motivation

Si l I t it M d li○ Signal Integrity Modeling● System Implementation○ Test System & Results○ Conclusion

RZ Signaling Schemeg g

• 3-level SignalingL i d f i i i• Long period of inactivity

RZ Signaling Schemeg g

• Oversampling Tracking ReferenceS h i i• Synchronization

• Reference Voltage

Common-mode Transmitter

• Reduces generated differential noise by• Slew rate control

Cl l l t t h d d d t t t• Closely layout-matched cascoded output stage

Common-mode Receiver

• Majority vote allows trade off between bandwidth and noise tolerancenoise tolerance

• Oversampling digital integrator provides time constant scaling with backchannel operating frequency

Common-mode Receive Preampp

V

Vref

Vin,CM

2 t• 2-stage preamp• provides 3x gain• sets low-pass filter bandwidth at 650MHz

Outline○ Motivation

Si l I t it M d li○ Signal Integrity Modeling○ System Implementation● Test System & Results○ Conclusion

Die Photo & Summaryy

T h l 0 13 Config Registers

PhCDR

Technology: 0.13 μmSupply:

Vdd 1 VF d Ch l Phase

MixersCDRLogic PLL

Forward Channel:Bandwidth: 1-10GbpsModulation: 2PAM

4PAM

TransmitterReflectionCancellerReceiver

4PAMBackchannel:

Bandwidth: 1-16MbpsCM Swing: 6 100 mV

CM RX

CM TX

CM Swing: 6-100 mV

Measured Common-mode Tracking RXeasu ed Co o ode ac g

Received BackchannelSignal

Tracking VoltageReferencee e e ce

• Backchannel RX tracks incoming common-mode signal

Measured Common-mode RX DAC DNLeasu ed Co o ode C

Margin loss due to DNL

Received BackchannelSignal

Tracking VoltageReferencee e e ce

• Differential nonlinearity (DNL) reduces margin• Test chip successful adapted transmit equalizer despiteTest chip successful adapted transmit equalizer despite

DNL issue

Measured Backchannel Performance

-1

0

10]-1

0

10]

4

-3

-2

or r

ate

[log

4

-3

-2

op r

ate

[log

-6

-5

-4

Pack

et e

rro

-6

-5

-4

Pac

ket d

ro

0 20 40 60 80 100-7

transmit swing [mV]0 20 40 60 80 100

-7

transmit swing [mV]

• Packet Error• Any erroneous received

bit in un-dropped packet

• Packet Drop• Parity Check Error• Synchronization Loss pp pSynchronization Loss

• Forward channel operating at 6.4Gbps

TX Adaptation Convergence

100(BC swing 18.75mV)

100(BC swing 25mV)

p g

25

50

75

lost updates due to packet drop e [ls

b]

25

50

75

e [ls

b]

-25

0

25

code

-25

0

25

code

500 1000 1500 2000-50 # updates sent from RX

500 1000 1500 2000-50 # updates sent from RX

Packet Error Rate = 3%P k t D R t 2%

Packet Error Rate = 8%P k t D R t 63%

• Packet Error divergenceP k t D d l d

Packet Drop Rate = 2% Packet Drop Rate = 63%

• Packet Drop delayed convergence

Minimal Effect on Forward Channel

Backchannel OFF Backchannel ON

● No detectable margin loss in forward channel operating at 5Gbps 2PAMoperating at 5Gbps 2PAM

Minimal Effect on Forward Channel

Backchannel OFF Backchannel ON

● No detectable margin loss in forward channel operating at 5Gbps 4PAMoperating at 5Gbps 4PAM

Outline○ Motivation

Si l I t it M d li○ Signal Integrity Modeling○ System Implementation○ Test System & Results● Conclusion

Conclusions• Common-mode is a promising method for low rate self-

contained backchannelcontained backchannel• System analysis necessary to understand effects on

forward channel• 16Mbps CM backchannel with 1-10Gbps forward channel

• Successfully adapt transmit equalizer coefficients for swing as low as 20mVswing as low as 20mV

• Packet Error Rate < 10-3 with CM Swing > 50mV• Minimal effect on a 1V-swing forward channel for g

backchannel swing up to 100mV