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Practical Macromodels for Digital I/O

Paul Franzonpaulf@ncsu.edu

Madhavan Swaminathan, Michael Steermadhavan.swaminathan@ece.gatech.edu

mbs@ncsu.edu

Acknowledgements: Ambrish Varma, Bhyrav Mutnury

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Outline

• Background– Macromodeling Needs– Requirements for Successful Macromodels

• Macromodeling Techniques– IBIS– Numerical Models– Physical

• Proposal : The Way Forward

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I/O Macromodeling

• Replace full Spice driver model

with

• Macromodel– Hides proprietary

information– Speeds up simulation

pullup

pulldown

Power_clamp

Gnd

Power

C_comp

L_pkg R_pkg

C_pkg

Output Pin

Gnd_clamp

pullup

pulldown

Power_clamp

Gnd

Power

C_comp

L_pkg R_pkg

C_pkg

Output Pin

Gnd_clamp

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I/O Macromodeling

• Required Goals:– Vendor-independent format– Have to capture TX, RX, and package

parasitic essentials– Sufficiently accurate to be useful– Easy to automatically generate from Spice

and Measurements– Easy to verify– Easy and fast to simulate

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I/O Macromodels

• Desirable Goals– Unique solution– Orthogonality– Multiple Verification Approaches– Physical basis– Human readability– Monotonic– Simulator Independent– Extendable to core noise SSN

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IBIS

• IBIS – Input Output Buffer Information Specification– EIA Standard 656 -A– Behavioral model of I/O buffers

• Model is represented as set of VI and VT curves.

– Fast simulations– Protects proprietary information contained in

the IC

http://www.eda.org/pub/ibis/

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Basic IBIS Model

pullup

pulldown

Power_clamp

Gnd

Power

C_comp

L_pkg R_pkg

C_pkg

Output Pin

Gnd_clamp

pullup

pulldown

Power_clamp

Gnd

Power

C_comp

L_pkg R_pkg

C_pkg

Output Pin

Gnd_clamp

A Basic model consist of:•Four I-V curves: - pullup and pulldown

- PWR and GND Clamp•Two Ramps: -dv/dt_rise, dv/dt_fall•Die Capacitance and•Packaging information

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Spice2Ibis• Automatically converts a Spice deck to an

IBIS file

• Over 1,000 active users• Over 3,000 lines of code• Was critical to success of IBIS language

S2IBIS Command File

-Header information-Component Description

Parser-Extracts all relevant

information from Command file

SPICE-HSPICE,PSPICE,

SPICE2SPICE3,SPECTRE

S2IBIS-Calls Spice, analyzes dataPrints results

IBIS Model

Circuit Layout

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IBIS Limitations

• IBIS hard to scale for use with high speed I/O– Up to 100 ps delay error– Subtle inflections missed– SSN over-predicted

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IBIS Alternatives

• Current Work:

• Numerical black-box methods– “Parametric” Models:

• Stievano, Maio, and Canavero, Politecnico di Torino

– Spline/Finite Time Difference• Swaminathan, Georgia Tech

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Numerical Methods

• “Black Box” Modeling– Static Term

istatic(k) = w1(k)f1(k) + w2(k)f2(k)

• Captures output impedance of driver• IBIS:

– f1 and f2 are fitted VI tables. i.e. I(Vout)

• Spline : – f1 and f2 are numerically fitted power series, i.e. I(Vout,

….)

• Parametric:– Basis functions: Gaussian or Sigmoid

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Numerical Methods

• Black Box Modeling– Dynamic Term

• IBIS : i(k) = w1(k)V1(k)f1(k) + w2(k)V2(k)f2(k)

– + output physical Ccomp

• Spline : – f1, f2 dynamic by numerically fitted load

capacitance (captures di/dt)

• Parametric– Calculated as a function of past values,

using Basis functions– e.g. RBF using Gaussians

Vn(T)

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Physical Model

• Circuit Level:

or

M1(T)

M2(T)

Ids=k(Vgs-Vt)2GS

D

etc.

Captures staged turn-ondrivers

Gain loss during switching event (Vgs)

Second order effects = f(Vds)

w=step functiononly in break beforemake drivers

CML reduces CM dependence

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Comments on Models• IBIS

– No numerical flexibility to capture subtle physical effects– Most physical (but very first order)– Easy to automate

• Spline/FTD– Input waveform dependence– Less physical – More accurate– Relatively hard to automate

• Numerical fitting of power series

• Parametric– Input waveform dependence– Least physical– Most accurate– Harder to automate model production

• More complex numerical procedures

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Proposal

• Collaborative Effort:– NC State University– Georgia Tech– Politecnico di Torino

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The Way Forward

1. Find the compromise between complexity and automation, while considering other goals

IBIS 4.0 Spline RBF

I=f(Vout) ErrorCurrentSource*

I=f(Vout,Vdd/Vss)M@OP

M=f(T,Vdd)Etc.*e.g. Numerically fitted error fn

RBF SigmoidPower Series

Reduced Order Power SeriesWavelet basis fns

Neural net fittingEtc.

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The Way Forward

• Test on standard driver set:– Conventional, LVDS, DDR, Other CML, Emerging– Test Spice model formats in Freeda

• Evaluate– Metrics:

• Accuracy in predicting delay, peak SSN, xmitted SSN, Xtalk, refn noise

• Accuracy outside range where fitted• Macromodel utility factors listed earlier

– Esp. Complexity of model fit procedure

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The Way Forward

2. Promulgate alpha standard– Develop SpicetoMacromodel

3. Evaluate and propagate more broadly

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Proposed Consortium

• Set up industry consortium to fund this work

• Benefits to consortium members– IC vendor companies

• Macromodel tuned to their drivers• Early access to successful macromodel formats

– First to market

– CAD companies• Early access to successful formats• Influence macromodel for simulator compatibility

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Discussion (on the day)

• Acknowledgement and contribution of the highly accurate “black box” techniques contributed by Prof. Flavio Canavero and his group.

• George Katopis:1. Study and development of a tool that helps

the user with the selection of black box option based on "expected" accuracy and time.

2. Automatic generation of the black box models

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Conclusions

• Ease of Use is just as important as model accuracy

• All macromodels are numerical black box format– Key question is complexity and type of

functions used