Preface
What This Book Is About
We live in the high-speed digital age where embedded system developers in many cases must apply new design methodologies and perform complex signal integrity tests and measurements. This book guides the reader through the full life cycle of embedded system design from specification and simulation to test and measurement. A significant feature of the book is the explanations of the thorny issues of signal integrity engineering that are so often the cause of delay in a product's development—time to market is the signal integrity engineer's Achilles' heel. By considering the whole life cycle from simulation through to test and validation you can see the new interplay of simulation and real-time test, which drives the new design methodologies. A case in point is the design and implementation of a new high-speed serial bus where the simulation and real-time test are inextricably linked, especially where designs incorporate device driver pre-emphasis or receiver equalization.
The celebrated teacher and philosopher Archimedes said, "What we must learn to do is to learn by doing," and this book endeavors to do just that by presenting practical applications so that you can learn from the authors' experiences in embedded system design, simulation, test, and measurement. What's more, you are encouraged to consider and adopt good practices in signal integrity engineering throughout the entire life cycle of the design.
Of particular importance today is the need to meet regulatory compliance and interoperability requirements. Consequently this book treats the demands of compliance testing and interoperability design as a foremost topic. Alongside today's compliance testing is the migration to high-speed serial buses, where both topics lend themselves to some prime examples of good practice in signal integrity engineering. Therefore, the design and testing of high-speed serial buses with their associated low-voltage differential signaling are pivotal themes throughout several chapters of this book. Nevertheless, the fundamentals of signal integrity engineering underpin the understanding of compliance testing and interoperability design, and you are encouraged to refresh or learn the basics of signal acquisition, test, measurement, and device simulation, which for the most part is provided in this book.
Work as a team has become routine for engineers designing, developing, and testing digital systems. The members of the team build up companionships and regularly look to each other for advice, especially in today's complex high-speed digital world. Today, the task of developing or testing a digital system is complex, and even the design of what appears to be a simple circuit can be problematic. For example, an engineer can design a simple digital circuit that meets all the requirements, and then a year down the line a chip is changed because the new chip is cheaper and has the same functionality. What the engineer didn't know is that the new chip is now made with a smaller device size that reduces its cost and its switching time, leading to faster edge rates and signal integrity problems! This leads to the well-documented quote "There are two kinds of design engineers, those that have signal integrity problems, and those that will." As we have seen if a design is sensitive to edge rates, the component specification must make edge rate a formal product parameter since it is just not possible to anticipate the evolution of a silicon fabrication process. This book aims to be a companion to the engineer and part of the engineer's team by providing an understanding of design specification, simulation, test, and measurement along with some significant advice on maintaining signal integrity throughout the life cycle of a design.
Although we can take the big view, there are significant little problems that the engineer needs to know but, as they say, "is afraid to ask." For example, if a designer suspects ground bounce, or more accurately transients in the signal reference, and wants to measure the ground line with an oscilloscope, where is the probe ground connected? Actually, it is normally connected to a solid logic zero. Moreover, why is it that a state-of-the-art oscilloscope can give a 50% measurement error when measuring ground bounce? Well, the bandwidth of an oscilloscope is typically specified at the –3 dB point, and a voltage measured at the limit of the specified oscilloscope bandwidth will be shown as half the real voltage. Also it is important to measure the voltage but think in terms of current—since the current spike on a ground rail generates crosstalk and spurious switching. And we could go on; there are a myriad of signal integrity challenges from intermittent setup and hold violations, resulting in problems ranging from metastability to electromagnetically induced crosstalk. The ability to foresee signal integrity problems and how to avoid them is fundamental to this book.
A feature of this book is the blend of source material. Whereas a theoretical text on signal integrity is built on scientific laws and notable hypotheses, this book has sourced its applications from the authors' professional experiences, published papers, and the work of associates. This book is a blend of source material coherently assembled and expanded to provide an understanding of modern signal integrity applications. Practical issues concern us most in this book. Each chapter focuses on a day-to-day activity of the signal integrity engineer, giving advice and illustrations from the industry. Practicality forms the central theme throughout the book.
The twenty-first century is a digital era of media convergence where mobile telephony, computing, and digital broadcasting merge, and consumers expect the media to be transparent to the technology. Put simply, the sports fan expects to view an event, in real time, on his or her mobile phone, laptop, and personal music player or digital TV at a reasonable cost and with absolute reliability. We could have taken any number of other examples from industry, medicine, or the military. Today, the digital designer or maintenance engineer is expected to be accomplished at signal integrity engineering, which is at the heart of the provision of systems that will make tomorrow's innovative technologies happen. This book reflects this trend.
The Intended Audience
Signal integrity engineering is a young and evolving science where few who proclaim to know it all. Writing this book has been a journey of discovery for the authors, and we have every reason to believe it will be a rewarding journey for you. We have no doubt that some topics discussed in this book will provoke debate as there is much to be standardized in this branch of engineering. However, indisputable principles are presented in this book that underlie signal integrity engineering. These principles give the emergent engineer a basis on which to build the knowledge and understanding necessary for good signal integrity engineering. Therefore, this book is recommended reading for the student signal integrity engineer and the practicing engineer whereby the authors present a wealth of applications that illustrate good practice and show the development, test, and validation of modern digital systems.
While the book is naturally partitioned into chapters of diverse topics a common thread runs throughout the book. Each chapter provides a guide for the reader by presenting the necessary prerequisites of a topic before detailing complex design or test applications. Consequently the experienced engineer can approach a topic by stepping through the beginning of a chapter and concentrating on the detail in the applications and advanced topics. No signal integrity book claims to be all encompassing, and this book is no exception. You may need to consolidate your understanding of the theory of signal integrity engineering via in-depth theoretical texts in the Prentice Hall SI series. Nevertheless, much of this book is self-contained in terms of addressing a wide audience in signal integrity engineering.
How This Book Is Organized
To guide you through the full life cycle of embedded system design, including specification, simulation, test, and measurement, the book is structured, where possible, chronologically to follow the development cycle. However to encompass the diverse aspects of signal integrity engineering and to provide a coherent thread as you read, chapter order is a compromise of product life cycle flow and a natural grouping of signal integrity engineering topics. Therefore both compliance and serial bus simulation are found toward the latter part of the book, whereas earlier topics are prerequisites for these more advanced subjects.
Chapter 1: Introduction: An Engineer's Companion
Chapter 1 takes you into the world of device and circuit simulation, which is a major phase in the successful development of a modern digital product. A thought-provoking example is given whereby a designer is under the intense pressure of time to market where it's easy to overlook a true understanding of operating margins—will a network continue to function reliably over the range of manufacturing and operating conditions it will encounter during the useful life of the product? What are the expected primary failure mechanisms, and how do they interact with one another? Some of these complex simulation questions will be considered in the body of the introduction, but more to the point, these discussions lead the way to the in-depth chapters that examine these concerns.
Following simulation, the introduction describes in detail a number of principal innovations in signal integrity engineering test and measurement. For example, to overcome some of the traditional signal integrity engineering problems, device manufacturers currently use novel integrated signal processing functions within device drivers and receivers to apply signal pre-emphasis and equalization. However, incorrectly applied pre-emphasis generates unwanted overshoot, crosstalk, and noise. This is one illustration of the complexity in solving today's signal integrity problems. This chapter throws light on such issues and presents a pathway for the solution of such problems by describing the basics of eye diagrams, which form the basis of many of today's automated compliance tests and signal analysis measurements.
Chapter 2: Chip-to-Chip Timing and Simulation
This chapter covers the circuits used to store information in a CMOS state machine and how they fail. A set of SPICE simulations and spreadsheet budgets introduces the common-clock architecture, the first of three paradigms for transferring digital signals between chips. Even though the source-synchronous and high-speed serial paradigms are more prevalent in contemporary systems, the common-clock architecture is not dead yet. A solid approach for timing common-clock transfers is a useful thing to have in the toolbox.
IO circuits play a pivotal role in signal integrity engineering, yet we seldom get to lay our eyes on a schematic for one of them. A handful of CMOS IO circuits get used time and again, and Chapter 2 examines their pertinent electrical characteristics. The chapter also covers the assumptions we make when using behavioral models for these circuits. Studying these circuits provides a basis for understanding the more esoteric circuits. This chapter and others make repeated references to the accuracy and quality of the models we use in signal integrity simulations.
Chapter 3: Signal Path Analysis as an Aid to Signal Integrity
This chapter describes signal path analysis based on intuitive time-domain reflectometry (TDR) techniques. TDR measurement theory and its application are described in detail, given that TDR is fundamental to the understanding of signal integrity effects, such as impedance mismatches and circuit board (PCB) issues. In particular, it provides an ideal vehicle for illustrating some principal signal integrity challenges and their solutions. Another facet of this chapter is the introduction to Vector Network Analysis (VNA), which is an important frequency domain measurement methodology used, among other things, to accurately characterize high-speed signal paths. A particular feature of this chapter is the introduction to the design, development, and test of Low Voltage Differential Signaling (LVDS) signal paths. However an understanding of basic transmission line theory underpins good practice in signal path design.
Today, with high-speed digital signal transmission, even the shortest passive PCB trace can exhibit transmission line effects. Transmission line theory encompasses electromagnetic field concepts and generally attracts complex mathematical analysis. However, using TDR, leads intuitively to a basic understanding of transmission line theory, even though some of the basic concepts require a few simple calculations.
Chapter 4: DDR2 Case Study
The DDR2 case study tackles the million-dollar question for a common source-synchronous bus: Will the interface operate with positive timing margin over the lifetime of the product without incurring the high costs associated with excessive conservatism? This approach involves picking the interface apart piece by piece—understanding how many mV of crosstalk a DIMM connector generates and how many ps of eye closure go along with it. Under the pressure of a project schedule, it is often tempting to gather a set of models, construct a simulation, and be done with the exercise. This chapter challenges the reader to take a deeper look.
Chapter 5: Real-Time Measurements: Probing
Central to the book and this chapter is the challenge of data acquisition whereby the problematic issues of the ideal nonintrusive probe are examined. How is signal fidelity achieved in a modern signal integrity measurement, and how are unwanted measurement artifacts avoided? Analog signal measurement is carefully investigated to demonstrate the importance of correctly connecting to a system under test because the analog features of a high-speed digital signal determine signal integrity. The chapter provides practical advice on probing and how to avoid probe problems. Special attention is given to the probing of today's fast-edge signals and Low Voltage Differential Signaling (LVDS).
Chapter 6: Testing and Debugging: Oscilloscopes and Logic Analyzers
This chapter reviews modern signal integrity testing from both an analog and digital viewpoint, since at the high frequencies encountered in today's designs the two are inextricably linked. The emphasis is on frequencies over 1 GHz, where the measurement tools of choice are the digital oscilloscope and the logic analyzer. The chapter provides practical examples to show how detailed observation of both analog and digital waveforms, side by side, can provide the data necessary for the understanding of the most challenging signal integrity timing budget issues, such as setup and hold violations, data skew, and metastable states. Real-world illustrations show these problems and how they can be detected and debugged.
Chapter 7: Replicating Real-World Signals with Signal Sources
A core theme throughout signal integrity engineering is the behavioral analysis of a digital circuit in terms of its analog properties and notably how the behavior of a digital circuit is determined. Sections of this book are devoted to the methods used to simulate models where computer-generated outputs show signals and data in a variety of formats in response to an array of simulated inputs. However, the fundamental method used to determine the real-time characteristics and operation of an actual circuit, or prototype, is to externally control and observe the circuit or device under test. Most of this book is about signal acquisition and measurement, and this chapter provides the balance; it is about the other half of the story—the signal source—which is used to control a circuit. Put simply, this chapter is about the externally provided signal that is used as a real-time stimulus for electronic measurements. This chapter describes and demystifies the complex issues of modern signal sources and shows how they can be used to stress a digital circuit to expose signal integrity faults.
Chapter 8: Signal Analysis and Compliance
The proliferation of digital systems, such as the new high-speed buses, has created numerous interoperability and compliance standards. This chapter explains how real-time test and measurement is the cornerstone of compliance testing. It describes the various standards, with particular emphasis on high-speed serial buses, and shows why, at frequencies of more than 2.5 GHz, real-time test and measurement is the only way to achieve compliance. The practical use of logic analyzers and oscilloscopes for compliance validation is examined, whereby techniques such as automated eye diagrams and statistical analysis are discussed in detail. This chapter is essential reading for the signal integrity engineer who needs to understand the challenges of meeting regulatory compliance tests.
Chapter 9: PCI Express Case Study
High-Speed Serial (HSS) is the last of the three major paradigms for transferring data between chips, and PCI Express is an excellent example of a high-speed serial interface. The PCI Special Interest Group published a set of guidelines that will keep designs out of trouble. However, situations often arise that force a designer to depart from the well-traveled path, either by breaking one of the guidelines or by trading one off for another. In these cases, it is helpful to acquire an understanding of how much each interconnect component contributes to the jitter budget and how many picoseconds remain after accounting for all relevant effects. Starting with a set of models and design rules, Chapter 9 examines the characteristics of each component in the time and frequency domains and then combines them one at a time to arrive at a total jitter budget.
Chapter 10: The Wireless Signal
The success of cellular technology and wireless data networks has caused the cost of basic radio frequency (RF) components to plummet. This has enabled manufacturers outside the traditional military and communications markets to embed relatively complex RF devices into all sorts of commodity products. RF transmitters have become so pervasive that they can be found in any number of embedded systems. Therefore this book introduces RF test and measurement for completeness in the understanding of signal integrity engineering. Moreover, given the challenge of characterizing the behavior of today's high-speed logic devices, this chapter provides an understanding of how radio frequency parameters such as jitter are measured. Although this chapter provides a detailed decision of the real-time spectrum analyzer (RTSA), the chapter also offers a detailed introduction to the Swept Spectrum Analyzer (SA) and the Vector Signal Analyzer (VSA).
The Website That Accompanies This Book, www.informit.com/title/0131860062
Color Pictures and Illustrations
Illustrations and pictures are used throughout a number of chapters in this book to allow the reader to become involved with instrumentation applications. In particular the chapters describing test and measurement use logic analyzer, oscilloscope, and spectrum analyzer displays to quantify and simplify what would otherwise be difficult and wordy descriptions. However, in keeping with most other books, the illustrations are understandable in monochrome, but some of the detail or features of a picture can be lost. Therefore, downloading the figure files from the website, www.informit.com/title/0131860062, allows the reader to view the descriptive color images and relate them to the accompanying text contained in the following three chapters:
• Chapter 6, "Testing and Debugging: Oscilloscopes and Logic Analyzers"
• Chapter 8, "Signal Analysis and Compliance"
• Chapter 10, "The Wireless Signal"
Simulation Models
Chapters 2, 4, and 9 demonstrate the allocation of picoseconds using case studies of three interface paradigms: common clock, source synchronous, and high-speed serial.
The model kit for Chapter 2 includes SPICE transistor models for an ancient 3.3 V 0.5 um CMOS process, IO circuits, and some simple networks.
Chapter 4 features behavioral simulation of standard DDR2 IO circuits, lossy transmission lines, vias, and a DIMM connector. Although the chapter discusses DIMM connector crosstalk, the model kit only contains single-line models because the DIMM connector model is proprietary.
The PCI Express case study in Chapter 9 demands more accurate interconnect models: coupled s-parameter representation of a ball-grid array, the corresponding via field, and edge connector. The simulations utilize a simple 100 ohm de-emphasized driver model found in the Agilent ADS library. This model is not included in the kit.
All models are suitable for simulation in SPICE or a behavioral simulator.