The System Designer's Guide to VHDL-AMS: Analog, Mixed-Signal, and Mixed-Technology Modeling

872 Pages·2002·38.11 MB·English

by  Peter J. Ashenden

Preface
VHDL-AMS is a language for describing digital, analog and mixed-signal systems. It
is an extension of the digital hardware description language VHDL, which arose out
of the United States government's Very High Speed Integrated Circuits (VHSIC) pro-
gram. In the course of this program, it became clear that there was a need for a stan-
dard language for describing the structure and function of integrated circuits (ICs).
Hence the VHSIC Hardware Description Language (VHDL) was developed. It was
subsequently developed further under the auspices of the Institute of Electrical and
Electronic Engineers (IEEE) and adopted in the form of the IEEE Standard 1076, Stan-
dard VHDL Language Reference Manual, in 1987. This first standard version of the
language is often referred to as VHDL-87.
Like all IEEE standards, the VHDL standard is subject to review every five years.
Comments and suggestions from users of the 1987 standard were analyzed by the
IEEE working group responsible for VHDL, and in 1992 a revised version of the stan-
dard was proposed. This was eventually adopted in 1993, giving us VHDL-93. A sec-
ond round of revision of the standard was started in 1998. That process was
completed in 2001, giving us the current version of the digital language, VHDL-2001.
In the early 1990s, the need for a hardware description language supporting an-
alog and mixed-signal modeling became apparent, and an IEEE Working Group was
established. The Working Group developed a set of design objectives for an exten-
sion to VHDL and completed a draft Language Reference Manual in 1997. The draft
was refined and subsequently approved in 1999, becoming IEEE Standard 1076.1 Def-
inition of Analog and Mixed Signal Extensions to IEEE Standard VHDL.
VHDL-AMS is designed to fill a number of needs in the design process. First, it
allows description of the structure of a system, that is, how it is decomposed into sub-
systems and how those subsystems are interconnected. Second, it allows the specifi-
cation of the function of a system using familiar programming language and equation
forms. Third, as a result, it allows the design of a system to be simulated before being
manufactured, so that designers can quickly compare alternatives and test for correct-
ness without the delay and expense of hardware prototyping. Fourth, it allows the
detailed structure of a design to be synthesized from a more abstract specification, al-
lowing designers to concentrate on more strategic design decisions and reducing time
to market.
xix
xx Preface
This book presents a structured guide to the modeling facilities offered by the
VHDL-AMS language, showing how they can be used for the design of digital, analog,
mixed-signal and mixed-technology systems. The book does not purport to teach en-
gineering design, since that topic is large enough by itself to warrant several textbooks
covering its various aspects. Instead, the book assumes that the reader has at least a
basic grasp of electronics design concepts, such as might be gained from a first course
in electronics design in an engineering degree program. Some exposure to computer
programming and to analog and radio-frequency system design will also be beneficial.
An understanding of multidisciplinary engineering will be useful for those interested
in using VHDL-AMS for "mechatronics" design. Unlike some other books on VHDL-
AMS, this book does not assume the reader is already familiar with VHDL for digital
system modeling. Instead, it presents the whole of VHDL-AMS as a unified language
for digital, analog and mixed-signal modeling. This book is suitable for use in an in-
termediate or advanced-level course in analog and mixed-signal circuit design or
"mechatronics" design. It will also serve practicing engineers who need to acquire
VHDL-AMS fluency as part of their changing job requirements.
One pervasive theme running through the presentation in this book is that mod-
eling a system using a hardware description language is essentially a software design
exercise. This implies that good software engineering practice should be applied.
Hence the treatment in this book draws directly from experience in software engineer-
ing. There are numerous hints and techniques from small-scale and large-scale soft-
ware engineering presented throughout the book, with the sincere intention that they
might be of use to readers.
We are particularly pleased to be able to include this book in the Morgan Kauf-
mann Series in Systems on Silicon. Modeling for simulation and synthesis is a vital part
of a design methodology for large-scale systems. VHDL-AMS allows models to be ex-
pressed at a range of levels of abstraction, from device level up to algorithmic and
architectural levels. It will play an important role in future design processes for ana-
log, mixed-signal and mixed-technology systems, many of which must function cor-
rectly in environments that are neither digital nor electronic.
Structure of the Book
The Designer's Guide to VHDL-AMS is organized so that it can be read linearly from
front to back. This path offers a graduated development, with each chapter building
on ideas introduced in the preceding chapters. Each chapter introduces a number of
related concepts or language facilities and illustrates each one with examples. Scat-
tered throughout the book are five case studies, which bring together preceding ma-
terial in the form of extended worked examples.
Chapter 1 introduces the idea of a hardware description language and outlines the
reasons for its use and the benefits that ensue. It then proceeds to introduce the basic
concepts underlying VHDL-AMS, so that they can serve as a basis for examples in sub-
sequent chapters. The next three chapters cover the aspects of VHDL-AMS that are
most like conventional programming languages. These may be used to describe the
behavior of a system in algorithmic terms. Chapter 2 explains the basic type system
of the language and introduces the scalar data types. Chapter 3 describes the sequen-
Structure of the Book xxi
tial control structures, and Chapter 4 covers composite data structures used to repre-
sent collections of data elements.
In Chapter ,5 the main facilities of VHDL-AMS used for modeling digital hardware
are covered in detail. These include facilities for modeling the basic behavioral ele-
ments in a design, the signals that interconnect them and the hierarchical structure of
the design. In Chapter 6, the facilities of VHDL-AMS used for modeling analog and
mixed-signal hardware are covered. These include facilities for modeling the equa-
tions that represent analog behavior, the terminals and quantities that interconnect
parts of a design and the facilities for expressing the interaction between the analog
and digital portions of a design. Chapter 7 describes the way in which VHDL-AMS
models are processed for simulation or other forms of analysis and discusses execu-
tion of models and solution of equations. The combination of facilities described in
these early chapters is sufficient for many modeling tasks, so Chapter 8 brings them
together in the first case study.
The next group of chapters extends this basic set of facilities with language fea-
tures that make modeling of large systems more tractable. Chapter 9 introduces pro-
cedures and functions, which can be used to encapsulate behavioral aspects of a
design. Chapter 10 introduces the package as a means of collecting together related
parts of a design or of creating modules that can be reused in a number of designs.
It also describes a number of packages standardized by the IEEE for use in VHDL-AMS
designs. Chapter 11 then covers aliases as a way of managing the large number of
names that arise in a large model. Chapter 21 describes generic constants as a means
of parameterizing the behavior and structure of a design. Chapter 13 deals with
Laplace and discrete transforms for frequency domain modeling and with small-signal
noise modeling. The material in this group of chapters is brought together in the next
case study in Chapter .41
The third group of chapters covers advanced modeling features in VHDL-AMS.
Chapter 51 deals with the important topic of resolved signals. While this language
facility forms the basis of many real-world models, its treatment in this book is left to
this late chapter. Experience has shown that the ideas can be difficult to understand
without a solid foundation in the more basic language aspects. Chapter 16 deals with
the topics of component instantiation and configuration. These features are also im-
portant in large real-world models, but they can be difficult to understand. Hence this
book introduces structural modeling through the mechanism of direct instantiation in
earlier chapters and leaves the more general case of component instantiation and con-
figuration until this later chapter. In Chapter ,71 generated regular structures are cov-
ered. Chapter 18 brings the material in this group of chapters together in the third
case study.
The fourth group of chapters covers language facilities generally used for system-
level modeling. Chapter 19 is a detailed treatment of the related topics of guarded
signals and blocks. Chapter 20 introduces the notion of access types (or pointers) and
uses them to develop linked data structures. This leads to a discussion of abstract data
types as a means of managing the complexity associated with linked data structures.
Chapter 12 covers the language facilities for input and output using files, including
binary files and text files. Chapter 22 describes the attribute mechanism as a means
of annotating a design with additional information. Chapter 23 then presents the
fourth case study.
xxii Preface
The last group of chapters draws the tour of VHDL-AMS to a close by covering
the remaining language facilities and addressing system design issues. Chapter 24 is
a miscellany of advanced topics not covered in the previous chapters. Chapter 25 dis-
cusses the use of VHDL-AMS features for modeling the integrated system of analog
and digital subsystems and their interactions. Chapter 26 concludes with the final case
study.
The Case Studies
The case studies explore the design of a radio-controlled (RC), electric-powered air-
plane system. They are intended for educational purposes and include analog,
mixed-signal and mixed-technology models. They demonstrate VHDL-AMS language
features and embody clear guidelines for system design and analysis.
The system from which all of the case studies are derived is shown in Figure P-1.
The overall system works as follows. The ground-based operator gives analog throttle
(propeller) and rudder commands using control joysticks as shown in the upper left-
hand corner of the diagram. The command signals are digitized, encoded into a serial
format and transmitted over an FM radio link. The signals are picked up by receiver
electronics on the airplane, decoded and converted to pulse-width modulated signals
that are proportional to the original analog control settings. These varying pulse-
width signals are converted back to analog signals and sent to the amplifiers and ser-
vos controlling the propeller speed and rudder position. The system architecture pro-
vides an expandable design to which channels for control surfaces or other functions
can easily be added.
FIGURE Pi1
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A
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, (42 )V ,/i)cl Assembly
metsyS diagram for the CR airplane .metsys
Notes to Instructors xxiii
At first glance this may appear to be a rather simple system. However, under clos-
er inspection, it contains a wide variety of modeling challenges: digital, analog, mixed-
signal, mixed-technology, as well as behavioral and structural VHDL-AMS models,
represented with multiple levels of hierarchy. Various aspects of the CR airplane sys-
tem are explored in this book in five case studies. Each case study is designed to
illustrate specific uses of VHDL-AMS.
Case Study :1 Mixed-Signal Focus
The first case study focuses on the encoding, transmission and decoding of the analog
throttle and rudder command signals as they make their way through the system. An-
alog-to-digital and digital-to-analog conversions are highlighted to show mixed-signal
modeling with VHDL-AMS.
Case Study 2: Mixed-Technology Focus
This case study focuses on the mixed-technology (mixed-domain) capabilities of
VHDL-AMS. In particular, the s-domain servo control blocks and the electrical and
mechanical system components are discussed.
Case Study 3: Power Systems Focus
This case study shows how power systems can be modeled with VHDL-AMS. A buck
converter is modeled with two useful levels of abstraction, and the effects of power
fluctuations on the system are simulated.
Case Study 4: Communications System Focus
This case study compares phase-locked loop and discrimination methods for signal
recovery in the RF receiver.
Case Study 5: Full System Analysis
This case study looks at the CR airplane as an entire system. It also highlights how
different modeling abstraction levels may be used to make large system simulations
as efficient as possible. The value of early requirements exploration, frequent sub-
system integration and validation, and bottom-up design verification are explored.
Notes to Instructors
Whereas a complete reading of this book provides a complete coverage of the lan-
guage, there are several shorter paths through the material. Some suggested minimal
paths for readers with different requirements are as follows:
xxiv Preface
For an introductory course in modeling using VHDL-AMS: Chapters 1 to 7, 9,
,01 21 and ,51 plus the case study in Chapter 8. Chapter 11 may be included
if time permits.
For a more advanced course: add Chapters 13, 16 and ;71 the case studies in
Chapters 14 and ;81 and a selection of the remaining chapters as time permits.
Chapter 25 will be helpful in formulating a modeling flow around VHDL-AMS.
For readers who wish to focus primarily on analog modeling: Chapters ,1 ,2 6,
7, 13 and 25 are the most relevant.
For readers proficient in using conventional programming languages: treat
Chapters 2 to 4 as review.
(cid:12)9 For readers with some previous introductory-level background in VHDL-AMS:
treat Chapters 1 to 4 as review.
Each chapter in the book is followed by a set of exercises designed to help the
reader develop understanding of the material. Where an exercise relates to a partic-
ular topic described in the chapter, the section number is included in square brackets.
An approximate "difficulty" rating is also provided, expressed using the following
symbols:
@ quiz-style exercise, testing basic understanding
O basic modeling exercisewl0 minutes to half an hour effort
@ advanced modeling exercisewone half to two hours effort
O modeling project~half a day or more effort
Answers for the first category of exercises are provided in Appendix .F The re-
maining categories involve developing VHDL-AMS models. Readers are encouraged
to test correctness of their models by running them on a VHDL-AMS simulator. This
is a much more effective learning exercise than comparing paper models with paper
solutions.
CD.ROM
This book includes a CD-ROM containing source code of all of the case study and
example models in the book, supplemental resources related to the text and examples
and a library of models that are useful building blocks for building larger systems or
for learning the power of VHDL-AMS "by example." The case studies and model
library have been coordinated to work with the SystemVision simulation and model-
ing environment offered as a commercial product by Mentor Graphics Corporation.
The SystemVision environment includes a schematic entry tool, a VHDL-AMS simu-
lator and a waveform viewing facility. Full hierarchical SystemVision schematics are
available for the case studies in this book. To learn more, you can visit the Web site
.noisiVmetsyS/moc.rotnem A downloadable version of the environment, for educa-
tional purposes, is available from this Web site. The limited-capacity educational-ver-
Resources for Help and Information xxv
sion tools allow you to experiment with the example models and to verify designs
suggested in the exercises. They can also be used with selected portions of the case
studies.
Resources for Help and Information
While this book attempts to be comprehensive in its coverage of VHDL-AMS, there
will no doubt be questions that it does not answer. For these, the reader will need to
seek other resources. A valuable source of experience and advice, often overlooked,
is one's colleagues, either at the workplace or in user groups. User groups generally
hold regular meetings that either formally or informally include a time for questions
and answers. Many also run e-mail lists and on-line discussion groups for problem
solving.
Accellera, an EDA industry association, sponsors the EDA Industry Working
Groups Web server (www.eda.org). The server has links to Web pages and reposito-
ries of several VHDL standards groups and user groups. Another Web site of interest
is www.vhdl-ams.com, hosted by Avant! Systems. It provides tutorial material and ex-
ample models provided by VHDL-AMS tool vendors.
Readers who have access to the Usenet electronic news network will find the
news group comp.lang.vhdl a valuable resource. This discussion group is a source of
announcements, sample models, questions and answers and useful software. Partic-
ipants include VHDL and VHDL-AMS users and people actively involved in the lan-
guage standard working group and in VHDL and VHDL-AMS tool development. The
"frequently asked questions" (FAQ) file for this group is a mine of useful pointers to
books, products and other information. It is archived at www.eda.org.
One resource that must be mentioned is IEEE Standard 1076.1, Definition of An-
alog and Mixed Signal Extensions IEEE to Standard .LDgtV It extends the standard for
digital VHDL, IEEE Standard 1076, IEEE Standard VHDL Language Reference Manual.
Together, the two standards are the authoritative sources of information about VHDL
and VHDL-AMS. However, since they are definitional documents, not a tutorial, they
are written in a complex legalistic style. This makes them very difficult to use to an-
swer the usual questions that arise when writing VHDL-AMS models. They should
only be used once you are somewhat familiar with VHDL-AMS. They can be ordered
from the IEEE at standards.ieee.org.
This book contains numerous examples of VHDL-AMS models that may also serve
as a resource for resolving questions. The VHDL-AMS source code for these examples
and the case studies, as well as other related information, is available for on-line ac-
cess at
www. m kp. o c m/vh dl-a ms
Although we have been careful to avoid errors in the example code, there are no
doubt some that we have missed. We would be pleased to hear about them, so that
we can correct them in the on-line code and in future printings of this book. Errata
and general comments can be e-mailed to us at
xxvi Preface
vhdl-book@ashenden, com.au
Acknowledgments
The development of this book has been very much a team effort, and the authors
are extremely grateful to all who have contributed. Most important, we would like to
acknowledge the generous support of Mentor Graphics Corporation, for contributing
staff time and other resources to the development of the case studies. After we de-
cided on the CR airplane as the system to be designed, Mike Donnelly developed the
system specification and detailed architecture. He created the original "back of the
napkin" sketch and worked out how to partition the system into the individual case
studies. From this starting point, Scott Cooper and Tom Egel worked through the de-
tailed design and drafted the case study chapters. Scott also took on the Herculean
effort of testing the code examples, not only in the case study chapters, but in other
chapters also. In addition, Rehan Shaukat from Portland State University assisted with
preparation of the CD-ROM during his internship at Mentor. Without the efforts of
these people and the support provided by Mentor, the case studies, which are central
to the book, would not have been possible.
We are also grateful to Ken Bakalar, one of the developers of VHDL-AMS, for
helping us to resolve questions of interpretation of the language definition and to for-
mulate modeling guidelines. Gary Pratt also helped us with modeling guidelines and
suggestions for examples to illustrate them. Thanks are due to Alan Mantooth of the
University of Arkansas, who used a draft of this book in his course.
This book has also benefited greatly from the input of the reviewers: Gary Pratt
and Maged Fikry of Mentor Graphics, Steve Drager of Air Force Research Labs, Peter
Wilson of the University of Southampton, Peter Frey of Cadence, Paul Menchini, Jim
Barby of the University of Waterloo and David Barton of AverStar. Also, Hal Carter
of the University of Cincinnati helped to shape the structure of the book. Any remain-
ing errors or omissions are our fault.
Finally, and by no means least, we would like to thank the staff at Morgan Kauf-
mann Publishers, led by Senior Editor Denise Penrose, for their support for this
project. Denise has kept us focused, and her commitment to quality has been an in-
spiration to us.
chapter one
Fundamental Concepts
In this introductory chapter, we describe what we
mean by digital, analog and mixed-signal system
modeling and see why modeling and simulation
are an importantpart of the design process. We see
how the hardware description language VHDL-
AMS can be used to model digital, analog and
mixed-signal systems and introduce some of the
basic concepts underlying the language. We com-
plete this chapter with a description of the basic
lexical and syntactic elements of the language, to
form a basis for the detailed descriptions of lan-
guage features that follow in later chapters.
2 chapter one Fundamental Concepts
1.1 Modeling Systems
If we are to discuss the topic of modeling digital, analog and mixed-signal systems,
we first need to agree on what we mean by each of these terms. Different engineers
would come up with different definitions, depending on their background and the
field in which they were working. The primary difference between digital and analog
systems is their representation of values and behavior with respect to time.
First, we focus on what we mean by a digital system. Some designers may con-
sider a single VLSI circuit to be a self-contained digital system. Others might take a
larger view and think of a complete computer, packaged in a cabinet with peripheral
controllers and other interfaces, as a system. In both of these cases, it is not the size
or complexity of the system that makes it digital; rather, it is how data values are rep-
resented. For the purposes of this book, we consider any digital circuit that processes
or stores information quantized over time into discrete values to be a digital system.
We thus consider both the system as a whole and the various parts from which it is
constructed.
With this definition of digital systems in mind, we now turn to analog systems. In
contrast to digital systems, information in analog systems has values that vary contin-
uously with time. We often use continuous ordinary differential and algebraic equa-
tions to describe the varying values within analog systems. Like a digital system, we
can consider an analog system as a whole or as an assembly of constituent compo-
nents. An analog system may be a single device, such as a resistor or capacitor, or it
may be a complete system including components such as mixers, oscillators, filters
and amplifiers.
Given these definitions of digital and analog systems, we now consider how these
systems interact. We call combined analog and digital systems langis-dexim systems.
In such systems, the analog and digital portions interact via analog-to-digital and dig-
ital-to-analog converters. An example of a mixed-signal system is a chemical-process
control system with analog sensors, filters and actuators combined with digital com-
puters to determine flow rates for different fluids. The interactions between the digital
and analog portions of the system are accomplished by analog-to-digital converters
that sample sensor values for processing by the computer and by digital-to-analog
converters that enable the computers to control actuators and thus fluid flow.
In addition to electronic systems, we can also consider mechanical, optical, fluid-
ic, thermal and electromagnetic systems, since they also represent information in con-
tinuously varying form. Note that we may view a particular system from more than
one of these perspectives. For example, a microelectromechanical accelerometer
used to control deployment of an automobile's airbag may include digital and analog
electrical, mechanical and thermal functionality. Similarly, the thermal effects on the
operation of a transistor involve multiple domains in a single component. Thus our
discussion covers a range of systems modeled in different energy domains and span-
ning levels of abstraction from the low-level devices that make up the components to
the top-level functional units.
If we are to encompass this range of views of digital, analog and mixed-signal
systems, we must recognize the complexity with which we are dealing. It is not hu-
manly possible to comprehend such complex systems in their entirety. We need to