THE DESIGNER S GUIDE TO VERILOG-AMS

THE DESIGNER S GUIDE TO VERILOG-AMS

THE DESIGNER S GUIDE BOOK SERIES Consulting Editor Kenneth S. Kundert Books in the series: The Designer s Guide to Verilog-AMS ISBN: 1-00-80-1 The Designer s Guide to SPICE AND Spectre ISBN: 0-79-9571-9

THE DESIGNER S GUIDE TO VERILOG-AMS First Edition June 00 KENNETH S. KUNDERT Cadence Design Systems OLAF ZINKE Cadence Design Systems KLUWER ACADEMIC PUBLISHERS NEW YORK, BOSTON, DORDRECHT, LONDON, MOSCOW

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Contents Chapter 1 Introduction 1 1 Hardware Description Languages The Verilog Family of Languages Mixed-Signal Simulators Applications of Verilog-AMS...5 Component Modeling Test Benches Simulation Acceleration Mixed-Signal Design Top-Down Design Chapter Top-Down Design Mixed-Signal Design Productivity 1 5 Traditional Approaches to Mixed-Signal Design.1 Bottom-Up Design. Moving to Top-Down Design Principles of Top-Down Design.1 A Shared Design Representation. Every Change is Verified. Verification Planning. Multiple Passes.5 Executable Specifications and Plans A Rigorous Top-Down Design Process Simulation and Modeling Plans System-Level Verification Mixed-Level Simulation Bottom-Up Verification Final Verification Test Further Benefits of Top-Down Design...5.6 5.1 5. 5. 5. 5.5 Improves Communications Between Engineers Improves Productivity Improves Ability to Handle Complex Designs Allows Parallel Execution of Design Tasks Supports IP Reuse 1 5 8 8 10 10 11 11 1 1 16 17 17 19 0 1 6 7 8 9 0 0 0 0 1 1 1

Contents 6 Final Words on Top-Down Design Chapter Analog Modeling 1 Resistor 5 6 7 8 9 10 11 1 1 1 1 1.1 Capacitor 1. Inductor 1. Voltage and Current Sources A Simple Circuit.1 Conservative Systems Motor.1 Natures and Disciplines Junction Diode. Junction Diode with Series Resistor Probes and Sources Series and Parallel RLC Resistive Port Relay 6.1 Non-Ideal Relay 6. Ideal Mechanical Stop 6. Ideal Diode Voltage-Controlled Oscillator Periodic Sample and Hold 8.1 Smoothing the Output Time Interval Measurement Analog to Digital Converter Digital to Analog Converter Lossy Inductor Tolerances Elements of Style Chapter Mixed-Signal Modeling Mixed Signal Models Modeling Discrete Behavior.1 Language Basics. Integers and Reals Modeling Mixed-Signal Behavior.1 Analog and Digital Contexts. From Digital to Analog. From Analog to Digital Structural Verilog-AMS.. Connecting Analog and Digital Discipline Resolution Automatic Connect Module Insertion Modeling Connect Modules 5 5 9 0 1 1 6 50 51 5 58 60 6 65 67 69 71 7 7 77 79 80 8 87 88 9 96 99 99 100 100 109 111 11 115 118 11 1 1 18 11 vi

Contents Chapter 5 Language Reference 1 Basics 0 5 System Functions and Tasks 5.1 Simulator Interface 5. Display Tasks 5. File Operation Tasks 5. Random Numbers 6 7 1.1 1. 1. 1..1....5.6 Signals.1. Expressions...5.6.7.8.9 6.1 6. 6. 6. 6.5 6.6 6.7 6.8 7.1 7. Comments Identifiers Keywords Compiler Directives Data Types Constants Variables Parameters Natures and Disciplines Ports, Nets, and Nodes Branches Continuous-Time Signal Access Contributions Operators Functions Mathematical Functions Logical Functions Environment Functions Analog Operators Thresholding Functions Limiting Functions Small-Signal Stimulus Functions User-Defined Functions Analog Behavior Analog Processes Procedural Blocks Assignments Contributions Conditionals Iterators User-Defined Analog Functions Analog Events Discrete-Event Behavior Initial and Always Processes Procedural Blocks 19 19 19 19 150 151 15 15 155 157 159 16 167 167 168 169 17 17 17 175 175 175 177 188 188 189 190 190 190 191 19 19 195 196 196 198 198 00 0 0 0 08 09 09 vii

Contents 8 9 10 7. 7. 7.5 7.6 7.7 7.8 7.9 8.1 8. 8. 8. 8.5 Appendix A Compatibility 1.1... Concurrent Blocks Assignments Nets and Registers Timing Control Conditionals Iterators User-Defined Functions and Tasks Mixed Behavior Discrete-Event Values in an Analog Process Discrete Events in an Analog Process Continuous-Time Values in an Initial or Always Process Continuous Events in an Initial or Always Process Calling Functions Hierarchy 9.1 Modules 9. Instantiation 9. Gate-Level Descriptions 9. Hierarchical Names 9.5 Mixed Signal Structure Other Features of Verilog-HDL Verilog-HDL Compatibility SPICE Compatibility.1 Scope of Compatibility. Accessing SPICE Objects from Verilog-A/MS. Preferred Primitive, Parameter and Port Names. Other Issues Spectre Compatibility Using Verilog-A with Spectre Accessing Spectre Objects from Verilog-A Spectre s Implementation of Verilog-A AMS Designer Compatibility Using Verilog-AMS with AMS Designer Referencing SPICE Referencing VHDL-AMS 11 1 15 16 0 0 0 5 5 5 6 6 6 7 9 0 5 5 6 6 7 9 1 1 1 7 8 9 57 57 viii

Preface The Verilog Hardware Description Language (Verilog-HDL) has long been the most popular language for describing complex digital hardware. It started life as a proprietary language but was donated by Cadence Design Systems to the design community to serve as the basis of an open standard. That standard was formalized in 1995 by the IEEE in standard 16-1995. About that same time a group named Analog Verilog International formed with the intent of proposing extensions to Verilog to support analog and mixed-signal simulation. The first fruits of the labor of that group became available in 1996 when the language definition of Verilog-A was released. Verilog-A was not intended to work directly with Verilog-HDL. Rather it was a language with Similar syntax and related semantics that was intended to model analog systems and be compatible with SPICE-class circuit simulation engines. The first implementation of Verilog-A soon followed: a version from Cadence that ran on their Spectre circuit simulator. As more implementations of Verilog-A became available, the group defining the analog and mixed-signal extensions to Verilog continued their work, releasing the definition of Verilog-AMS in 000. Verilog-AMS combines both Verilog-HDL and Verilog-A, and adds additional mixed-signal constructs, providing a hardware description language suitable for analog, digital, and mixed-signal systems. Again, Cadence was first to release an implementation of this new language, in a product named AMS Designer that combines their Verilog and Spectre simulation engines. At the time this preface was written, all but the oldest commercial circuit simulators support Verilog-A, and each of the major ICCAD vendors offer mixed-signal simulators that support Verilog-AMS. Verilog-A is extensively used in both device modeling for circuit simulation and for behavioral modeling of analog systems and adoption of Verilog-AMS is growing rapidly. Verilog-AMS is continuing to evolve. Version.1 of the Verilog-AMS standard is based on the IEEE Verilog 16-1995 standard. It was released in January 00. The committee charged with the development of Verilog-AMS (www.eda.org/verilogams) is currently working to improve and update the standard. Progress is currently being made to update the basis of the standard to the latest version of Verilog-HDL, IEEE 16-001. They are also working to integrate Verilog-AMS into SystemVer-

Preface ilog. Finally, extensions are being added to support compact semiconductor models and table models. The intent of Verilog-AMS is to let designers of analog and mixed-signal systems and circuits create and use models that describe their designs. Once a design is described in Verilog-AMS, simulators are used to help designers better understand and verify their designs. Verilog-AMS allows designs to be described at the same level as does SPICE, but at the same time allows designs to also be described at higher more abstract levels. This range is needed for the larger more complex mixed-signal designs that are becoming commonplace today. This book starts in Chapter 1 with a brief introduction to hardware description languages in general and Verilog-AMS in particular. Chapter presents a formal topdown design methodology. While not used extensively today, top-down design is widely believed to be the only methodology available that can efficiently handle large complex mixed-signal designs. This chapter presents a refined and proven top-down methodology that overcomes many of the problems with existing top-down methodologies. Chapter and Chapter introduce the Verilog-A and Verilog-AMS languages. The important concepts of the languages are presented using practical and easy to understand examples. These chapters are intended to be read from beginning to end and are designed to take engineers with a working knowledge of programming concepts to the point where they are comfortable writing a wide range of Verilog-A and Verilog-AMS models. However, they do not cover all the details of the languages. Chapter 5 is a reference guide to the languages. It presents all of the details, but not in a completely linear fashion. Though it can be read from beginning to end, it was written with the expectation that most would use it as a reference, looking up just the details they need when they need them. As such, it, as with the rest of the book, is extensively cross referenced and indexed. A word about the conventions used in this book. As new ideas and definitions are presented, a few keywords will be set in bold italics to make them easier to find and to call your attention to them as important points. Code is set in a sans serif font with keywords in bold and comments in italics. When in text, identifier names are set in italics. Acronyms that are spoken as words rather than letters are set in small caps; for example, SPICE. Besides the normal cross references found in the text, you will also find references that appear like this: (5.p157). These abbreviated references include the chapter number, the section number, and finally the page number. Finally, all models presented in this book have been verified with the simulators from Cadence, either Spectre or AMS Designer as appropriate. This book has two companion websites on which you can find updated information about both this book and its subject matter. www.designers-guide.com contains inforx

Preface mation about the book, including an errata sheet, the latest versions of the models given in this book, articles that contain additional information about both modeling and Verilog-AMS, and links to other sites that would be of interest. In addition, it also provides a discussion forum where you can ask questions and have conversations with other practicing design engineers. www.verilog-ams.com provides a burgeoning library of high quality user contributed Verilog-A and Verilog-AMS models. It is our intention to continually update and improve this book. As such, we would like to ask for your help in the process. Please send your comments, suggestions, experiences, feedback and reports of errors to either ken@designers-guide.com or describe them at www.designers-guide.com/forum. Ken Kundert Olaf Zinke April 1, 00 xi