Practical Macromodels for Digital I/O Paul Franzon paulf@ncsu.edu Madhavan Swaminathan, Michael Steer madhavan.swaminathan@ece.gatech.edu mbs@ncsu.edu Acknowledgements: Ambrish Varma, Bhyrav Mutnury

Outline Background Macromodeling Techniques Proposal : The Way Forward Macromodeling Needs Requirements for Successful Macromodels Macromodeling Techniques IBIS Numerical Models Physical Proposal : The Way Forward

I/O Macromodeling Replace full Spice driver model Macromodel with Hides proprietary information Speeds up simulation

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

I/O Macromodels Desirable Goals Unique solution Orthogonality Multiple Verification Approaches Physical basis Human readability Monotonic Simulator Independent Extendable to core noise SSN

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

Basic IBIS Model 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

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, SPICE2 SPICE3,SPECTRE -Calls Spice, analyzes data Prints results IBIS Model Circuit Layout

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

IBIS Alternatives Current Work: Numerical black-box methods “Parametric” Models: Stievano, Maio, and Canavero, Politecnico di Torino Spline/Finite Time Difference Swaminathan, Georgia Tech

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

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)

Physical Model Circuit Level: Captures staged turn-on drivers M1(T) Gain loss during switching event (Vgs) Second order effects = f(Vds) w=step function only in break before make drivers CML reduces CM dependence Circuit Level: M1(T) S G Ids=k(Vgs-Vt)2 D M2(T) or etc.

Comments on Models IBIS Spline/FTD Parametric 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 Least physical Most accurate Harder to automate model production More complex numerical procedures

Proposal Collaborative Effort: NC State University Georgia Tech Politecnico di Torino

The Way Forward Find the compromise between complexity and automation, while considering other goals Spline RBF IBIS 4.0 RBF Sigmoid Power Series Reduced Order Power Series Wavelet basis fns Neural net fitting Etc. I=f(Vout) ErrorCurrentSource* I=f(Vout,Vdd/Vss) M@OP M=f(T,Vdd) Etc. *e.g. Numerically fitted error fn

The Way Forward Test on standard driver set: Evaluate 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

The Way Forward 2. Promulgate alpha standard Develop SpicetoMacromodel 3. Evaluate and propagate more broadly

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

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

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