21.3: Adaptive Equalization and Data p qRecovery in a Dual-Mode (PAM2/4)

Serial Link Transceiver

Vladimir Stojanovic1,2, Andrew Ho1, Bruno Garlepp1, 1 1 1 2 1Fred Chen1, Jason Wei1, Elad Alon1,2, Carl Werner1,

Jared Zerbe1 and Mark Horowitz2

1 Rambus Inc1 Rambus Inc.2 Stanford University

Backplane channels0

dB]

p

• Same backplane

-20

-10

enua

tion

[d 9" FR4Same backplane• Loss is variable

50

-40

-30Atte

9" FR4, i t b

26" FR4• Custom fit• Equalization

-60

-50 via stub

26" FR4,via stub

• Modulation

• Lots of different links 0 2 4 6 8 10frequency [GHz]

• Lots of different links• Huge control overhead• Nice to have link adapt itself to the channelNice to have link adapt itself to the channel

• Adapt both coefficients and modulation/equalization type

Backplane channel – variable in timep

dB• Changes in time• Temperature

H idit

dB

• Humidity

• Slow but significant• Need to re-adjust the

equalizerCan do with slow• Can do with slow, simple hardware

GHz

G. Sheets, J. D’Ambrosia “The Impact of Environmental Conditions on Channel Performance," DesignCon 2004

Outline• Show adaptive and reconfigurable link

• Minimum hardware overhead

• Link equalization and modulation• Dual-mode (PAM2/PAM4) linkDual mode (PAM2/PAM4) link• Making the link adaptive

Results• Results

Transmit equalization – headroom constraintq

-5

0

on [d

B]

unequalizedTx Anticausal tapsPeak power constraint

20

-15

-10

Atte

nuat

io

equalized

Data

Channel

p

Ω50Ω50

0 0.5 1 1.5 2 2.5-25

-20

frequency [GHz]

Amplitude of equalized signal

Causaltaps

I

doutNoutP

d

Ω50Ω50

• Transmit DAC has limited voltage headroom

Amplitude of equalized signaldepends on the channel

0eqI

• Transmit DAC has limited voltage headroom• Tx Eq attenuates the signal

• Best to combine with feedback equalization• Best to combine with feedback equalization

Including feedback equalizationg q

1

Feedback• Feedback equalization (DFE)• Subtracts error from input 0.6

0.8

itude

Feedbackequalization

• No attenuation

• Problem with DFE0.2

0.4Am

pl

• Problem with DFE• ISI must be causal

• Problem - latency in the decision circuit

0 2 4 6 8 10 12 14 16 180

Symbol time

y• Receive latency + DAC settling < bit time

• Can increase allowable time by loop unrollingR i t bit b f th i i l d• Receive next bit before the previous is resolved

One-tap DFE with loop unrollingp p g1

αPulse response

+1

0

-1

One-tap DFE with loop unrollingp p g1

α

+1+1+α

0+α

-1-1+α

One-tap DFE with loop unrollingp p g1

α

+1+1+α

+1 α

0+α

+1-α

-α

-1-1+α

-1-α

One-tap DFE with loop unrollingp p g

+1+α

+1 α

1| 1 =−nn dd+α

+α+1-α

-α

D Q1−nd

dClknx

-1+α

-1-α dClk

0| 1 =−nn dd-α

• Instead of subtracting the error• Move the slicer level to include the noise

S f

dClk

• Slice for each possible level, since previous value unknownK.K. Parhi, "High-Speed architectures for algorithms with quantizer loops," IEEE International Symposium on Circuits and Systems, May 1990

Pulse amplitude modulationp

• PAM2 (Binary) • PAM4• PAM2 (Binary)• 1 bit / symbol• Symbol rate = bit rate

• PAM4 • 2 bits / symbol• Symbol rate = bit rate/2y Symbol rate bit rate/2

00

011

10

110

Outline• Show adaptive and reconfigurable link

• Minimum hardware overhead

• Link equalization and modulation• Dual-mode (PAM2/PAM4) linkDual mode (PAM2/PAM4) link• Making the link adaptive

Results• Results

Hardware re-use: Dual-mode receiver

D QD Qthresh (+)

lsb(+)

prDFE enable

D Q0

th h(+)

D Q D Qin0

msb

prDFE enabledClk

dClk

D Q

D Q0

1thresh(+)

0

D Qthresh (-)

dClk

prDFE enable

D Q

D Q1

0

1thresh(-)

• PAM4

lsb(-)D QdClkD Q

0

1

PAM4

Hardware re-use: Dual-mode receiver

D QD Qthresh (+)

lsb(+)

prDFE enable

D Q0

th h(+)

D Q D Qin0

msb

prDFE enabledClk

dClk

D Q

D Q0

1thresh(+)

0

D Qthresh (-)

dClk

prDFE enable

D Q

D Q1

0

1

outN

clkclk

Q

thresh(-)

lsb(-)D QdClkD Q

0

1inP

inNinP

outNoutP

outP outN

Q

• PAM4 inNclkthreshII

+2 threshII

−2 Q

PAM4

pre-amp with offset comparator

Hardware re-use: Dual-mode receiver

D QD Qthresh (+)

lsb(+)

prDFE enable

D Q0

0 D Q D Qin0

msb

prDFE enabledClk

dClk

D Q

D Q0

1

D Qthresh (-)

dClk

prDFE enable

D Q

D Q1

0

1

lsb(-)D QdClkD Q

0

1

• PAM2

Hardware re-use: Dual-mode receiver

D QD Qthresh (+)

lsb(+)

prDFE enable

D Q0

D Q D Qin0

msb

prDFE enabledClk

dClk

D Q

D Q0

1

D Qthresh (-)

dClk

prDFE enable

D Q

D Q1

0

1

lsb(-)D QdClkD Q

0

1

• PAM2 with loop-unrolled DFE tap

Hardware re-use: Dual-mode receiver

D QD Qthresh (+)

lsb(+)

prDFE enable

D Q0

thresh(+)D Q D Qin

0

msb

prDFE enabledClk

dClk

D Q

D Q0

1

thresh(-)

D Qthresh (-)

dClk

prDFE enable

D Q

D Q1

0

1

lsb(-)D QdClkD Q

0

1

• PAM2 with loop-unrolled DFE tap• Leverage multi-level properties of signals in loop-unrolling• Re-use PAM4 receiver hardware (slicers and CDR)

Partial response CDRp+1+α+1-α(0,1) (0,1)

-1-α

-1+α(1,0) (1,0)

1 α

F i l l l• Four signal levels

Partial response CDRp+1+α+1-α(0,1) (0,1)

-1-α

-1+α(1,0) (1,0)

1 α(1,1)

(0,1)+α

F i l l l(0,0)

(1,0)-α

• Four signal levels • Offset edge samplers for transitions with ISI

Otherwise timing error• Otherwise timing error• Need to filter edges – similar to PAM4

Dual-mode CDRD Q

thresh (+) edgen (+)lsbn(+), lsbn-1(+)

D Qx0 eClk

eClk

edgen (0)PAM4

msbn , msbn-1 , msbn-2

D Qthresh (-)

eClk

edgen (-)PAM2prDFE lsbn(-), lsbn-1(-)

• PAM4

( )eClk

filteredearly/late

PAM4

J. Zerbe et al, "Design, Equalization and Clock Recovery for a 2.5-10Gb/s 2-PAM/4-PAM Backplane Transceiver Cell," IEEE Journal Solid-State Circuits, Dec. 2003.

Dual-mode CDRD Q

thresh (+) edgen (+)lsbn(+), lsbn-1(+)

D Qx0 eClk

eClk

edgen (0)PAM4

msbn , msbn-1 , msbn-2

D Qthresh (-)

eClk

edgen (-)PAM2prDFE lsbn(-), lsbn-1(-)

( )eClk

filteredearly/late

• PAM2 with loop-unrolled DFE tap• Leverage multi-level properties of signals in loop-unrolling• Re-use PAM4 receiver hardware (slicers and CDR)

Outline• Show adaptive and reconfigurable link

• Minimum hardware overhead

• Link equalization and modulation• Dual-mode (PAM2/PAM4) linkDual mode (PAM2/PAM4) link• Making the link adaptive

Results• Results

Adaptation with minimum overheadTx Data

dLev

adaptivel

error

Ad i

Channel

sampler

Rx data

Adaptivemacro

thresholds

aClk

dClk

tapupdates CDRedge

aClk dClk eClkeClk

• Adaptive sampler • Generates the error signal at reference level (dLev)

tap updates

aClk dClk eClk

Generates the error signal at reference level (dLev)• Monitors the link

• Adjustable voltage and time reference• On chip sampling scope• On-chip sampling scope

• Can replace any other sampler - calibration

Dual-loop adaptive algorithmp p g• Data level reference loop

0>d)sign(eΔdLevdLevdLevinitdLevinitdLevinit

error p-p dL

0,1 >−=+ nndLevnn d)sign(eΔdLevdLev

errorinitp p

)0|( >nn designdLev

)( ndsign

Initial eyeInitial eye

nd

• Equalizer loop))sign(esign(dΔww kk += ))sign(esign(dΔww nknwnn −+ +=1

Dual-loop adaptive algorithmp p g• Data level reference loop

0>d)sign(eΔdLevdLevdLevinit dLevmid dLev

dLevinit dLevmid dLev

dLevinit dLevmid dLeverror p-p

0,1 >−=+ nndLevnn d)sign(eΔdLevdLev

… …

dLevend

… …

dLevend

… …

dLevenderrorinitp p

dLev

Initial eye Mid-way equalized EqualizedInitial eye Mid-way equalized Equalizednd

• Equalizer loop))sign(esign(dΔww kk +=

• Scale the equalizer - output Tx constraint

))sign(esign(dΔww nknwnn −+ +=1

Partial response adaptation - startp p• Extend data filter by one bit (msbn,msbn-1)

(a) Update loops only on (msbn,msbn-1)=(1,1)• Finds dLev(1,1) - “1+α”

(b) Update loops only on (msbn,msbn-1)=(0,1)• Finds dLev(0,1) – “1-α”

Partial response adaptation - endp p

2α

2α=dLev(1,1)-dLev(0,1)Iterate α finding and equalization loops

• msbn,msbn-1 filter tolerates one tap post-cursor ISI α

Outline• Show adaptive and reconfigurable link

• Minimum hardware overhead

• Link equalization and modulation• Dual-mode (PAM2/PAM4) linkDual mode (PAM2/PAM4) link• Making the link adaptive

Results• Results

Fully adaptive dual-mode linky p

• PAM2/PAM4Config Registers PAM2/PAM4• 1-10Gb/s• 0.13µm

g g

PhaseMixers

CDRLogic PLL

• 40mW/Gb/sTransmitterReflection

CancellerReceiver

Backchannel RX

Backchannel TX

• Reconfigurable dual-mode PAM2/PAM4 link• Adaptive equalization• Transmit and receive equalization• Transmit and receive equalization• DFE with loop unrolling

Dual-loop convergence – 4 tap exampleua oop co e ge ce tap e a p e

1000100PAM2, 5Gb/s, 4taps Tx Equalization

600

800

t [m

V] main tap

60

80

mV]

0

200

400

tap

wei

ght

post2 40

60

dLev

[m

0 50 100 150 200-400

-200 post1 pre1

0 50 100 150 2000

20

b f d t

• Reference loop tracks the signal• Once reference loop provides reliable error

number of updatesnumber of updates

Once reference loop provides reliable error• Equalizer taps converge quickly

Dual-loop convergence - stabilityp g y200

dLev speed 1x - eq speed 1x dLev speed 10x - eq speed 1x

0500

1000

150

[mV]

dLev speed 1x - eq speed 10x

0 20 40 60 80 100-500

0

ts [m

V]

5001000

50

100

dLev

tap

wei

ght

0 20 40 60 80 100-500

0

1000

0 20 40 60 80 1000

0 20 40 60 80 100-500

0500

• Hard to estimate analytically• Experimental results show

number of updates number of updates

Experimental results show • Both loops are stable within wide range 0.1 – 10x

of relative speeds

Improvements with loop-unrollingp p g

200 -3 tion)

0.3

0.4 [V] unequalized

100

150

200

4

-3.5

ility

dis

tribu

t

0 1

0.2

-50

0

50

[mV]

-4.5

-4

tage

pro

bab

0 1000 2000 3000 4000

0

0.1

[ps]

0 50 100 150 200

-100

[ps]

-5

log 10

(vol

t

0 15

0.2

0.25 [V] transmit equalized with one tap DFE

fully transmit equalized

• Signal as seen by the i ( hi )

0.05

0.1

0.15

receiver (on-chip scope)0 1000 2000 3000 4000

0[ps]

Conclusions• PAM2 with one-tap DFE similar to PAM4

N d b th t ll th h l• Need both to cover all the channels

D l d li k f t d h d• Dual-mode link - front-end hardware re-use• 1-10 Gb/s with 40 mW/Gb/s

• Adaptation with one extra samplerD l l l ith• Dual-loop algorithm