© H. Heck 2008Section 2.51 Module 2:Transmission Line Basics Topic 5: Modeling & Simulation OGI ECE564 Howard Heck

Modeling & Simulation EE 564 © H. Heck 2008 Section 2.52 Where Are We? 1.Introduction 2.Transmission Line Basics 1.Transmission Line Theory 2.Basic I/O Circuits 3.Reflections 4.Parasitic Discontinuities 5.Modeling, Simulation, & Spice 6.Measurement: Basic Equipment 7.Measurement: Time Domain Reflectometry 3.Analysis Tools 4.Metrics & Methodology 5.Advanced Transmission Lines 6.Multi-Gb/s Signaling 7.Special Topics

Modeling & Simulation EE 564 © H. Heck 2008 Section 2.53 Contents Overview Transistor Level Models Non-linear Models Linear Models Simulation Tools Summary

Modeling & Simulation EE 564 © H. Heck 2008 Section 2.54 Overview AccuracyDeviceInterconnectSpeed HighestTransistors Frequency Dependent Slowest   Non-linear Behavioral Lossy   LowestLinearLosslessFastest Which should we choose to use? Depends on our needs… If we need to simulate large numbers of nets (can be 1000’s), speed is critical. If we are trying to design a new feature into a buffer, such as automatic impedance control, we may wish to sacrifice speed for accuracy.

Modeling & Simulation EE 564 © H. Heck 2008 Section 2.55 Transistor Level I/O Models Example: GTL Driver V V in V out V TT R CC P1 N1 N2 N4 N3 Model Requirements:  I/O circuit schematic  Silicon process files  Device dimensions Sources:  Silicon suppliers (databooks, Spice models)  Simulation tool suppliers also supply some libraries. Drawbacks:  Many silicon suppliers are unwilling to divulge process model data.  Some use internal Spice variants with proprietary model types. Conversion to standard Spice formats, they sacrifices accuracy.  If the vendor improves the process or design, the model must be updated.  Simulations can take a lot of time to prepare and run.

Modeling & Simulation EE 564 © H. Heck 2008 Section 2.56 Model Requirements:  I out vs. V out curves for high and low states  V out vs. time curves for rising & falling transitions  Test load used for I-V & V-t curve generation Sources:  Silicon suppliers (databooks, IBIS models)  Simulation and/or experimental measurements. Many tools now have the capability to simulate the behavior of buffers using non-linear models.  HSPICE accepts IBIS (I/O Buffer Information Specification) models.  PSPICE has an analog behavioral modeling feature.  Board simulators, such as XTK by Quad Design, use non-linear models. Example: I-V curves Non-Linear I/O Models V out [V] I out [mA] nMOS pMOS

Modeling & Simulation EE 564 © H. Heck 2008 Section 2.57 Non-Linear I/O Models #2 Models can be quite complex, including many features, such as:  Multi-stage output drivers  Receiver “clamping”  Simultaneous switching effects Advantages:  Can be nearly as accurate as transistor models, but run faster.  Suitable for worst case analysis & “sensitivity” analysis.  They provide a way to develop buffer requirements by exploring the effects of changing the behavioral characteristics. Drawbacks:  Datasheets may not contain all desired information.  Some tools may not support all available features. When to use:  They can be used at any time during the design cycle, but may not be worth the effort in the early stages of the design, when several design variables are being explored & defined.  Non-linear behavioral models are the most widely used model type for designing boards.

Modeling & Simulation EE 564 © H. Heck 2008 Section 2.58 Linear I/O Models Example: our Th é venin equivalent circuit with a linear rise/fall time. Requirements:  Supply voltage(s) and effective output resistance. Note that the high and low resistances can be different.  Rise and fall times Advantages: fast Disadvantages: limited accuracy When to use:  Early in the design cycle when you are trying to bound your buffer and interconnect requirements.  For learning the basics of interconnect & transmission lines.

Modeling & Simulation EE 564 © H. Heck 2008 Section 2.59 Transmission Lines There are different approaches and different algorithms. PSpice uses discrete circuit elements (as near as I can tell).  Not very effective for high frequency designs. HSpice solves Maxwell’s equations.  Uses a quasi-static approximation. That is, it assumes that the line is purely TEM – all fields perpendicular to the direction of propagation. Not strictly true for lossy lines.  Models frequency dependent losses using the following: R o = DC resistance matrix R s = skin effect matrix G o = shunt current due to free electrons G d = loss due to rotation of dipoles under the alternating field f gd = cut-off frequency

Modeling & Simulation EE 564 © H. Heck 2008 Section Tools Requirements:  An accurate, fast transmission line engine  The ability to handle non-linear models  Features for helping the design engineer understand what is happening in the system. Types:  General circuit simulation tools (Spice): Used by nearly everyone at some point, but rarely for post-layout, full board simulations. Full versions cost several $K.  Dedicated transmission line simulators (Quad design, Interconnectix). Widely used by board designers for both pre- and post-layout simulation. They cost even more.  Complex field solvers (Speed2000, Ansoft HFSS). These typically use FDTD or TLM methods. We’ll use some variant of Spice.

Modeling & Simulation EE 564 © H. Heck 2008 Section Circuit Elements We’ll need to use the following circuit elements:  Transmission lines: lossless, lossy, coupled  Voltage & current sources: pulse and/or PWL  Resistors  Capacitors  Inductors  Ground

Modeling & Simulation EE 564 © H. Heck 2008 Section Summary I/O circuit models can be structural (transistor) or behavioral. Each has advantages and drawbacks and may be applicable in the design process. Non-linear behavioral models are widely used. IBIS is the industry standard format for behavioral modeling. We’ll use linear models and Spice in this class.

Modeling & Simulation EE 564 © H. Heck 2008 Section References S. Hall, G. Hall, and J. McCall, High Speed Digital System Design, John Wiley & Sons, Inc. (Wiley Interscience), 2000, 1 st edition. H. Johnson and M. Graham, High-Speed Signal Propagation: Advanced Black Magic, Chapter 13, Prentice Hall, 2003, 1 st edition, ISBN X. Avant! Corporation, True-HSpice™ Device Models Reference Manual, Release , Revision A, December 2001, Chapter 6, pp. 6-1 to D. Kuznetsov, “Optimal Transient Simulation of Transmission Lines,” IEEE Trans., Circs. Syst., vol. 43, pp , February 1996.

Modeling & Simulation EE 564 © H. Heck 2008 Section References J. Powell, “Spice or IBIS: How and When to Choose Between These SI Simulation Modeling Approaches,” Printed Circuit Design, pp , May K. Felton and T. Westerhoff, “High-Speed PCB Simulation: Is it Time for a Change,” Printed Circuit Design, pp :63, June L. Green, “Models for Signal Integrity Simulation: What You Need to Know About SPICE and IBIS,” Printed Circuit Design, pp , October G. Aldrich, “Designing PC Motherboards: Little Margin for Error Printed Circuit Design, March D. Duehren, et. al., “I/O-Buffer Modeling Spec Simplifies Simulation for High-Speed Systems,” September 1994, P.W. Tuinenga, A Guide To Circuit Simulation & Analysis Using Pspice, Prentice-Hall, Englewood Cliffs, NJ, 1988.