Book Review

Ray Perez

High Speed Digital Design:A Handbook of Black Magic

Authors: Drs. Howard W.Johnson and
Martin Graham
Publisher: Prentice Hall, 1993
Reviewer: R.Perez

The main objective of book reviews in the EMCS Newsletter is to let our readers know of the main features or salient points of EMC related books that have recently been published. Rarely, do we go back and review a book that has been published several years ago, because the normal thinking is that the book is already familiar to many of our readers. However, when I personally believe that a book is unique enough (and EMC related) to provide good material to those beginning in the EMC field, I “dust-off” such books from my personal library and will submit one to our editor as a review candidate.

This book was published in 1993 by an electronic digital designer in the industry for electronic designers. It is mostly a practical book, but theory, practical approaches and solutions are mixed in a unique way so as to make this book an excellent one in the art of teaching electronic design for built-in immunity to noise and interference. The material itself is not new, but the format when this book was published in 1993 is indeed new.

This book is highly recommended for those involved in analog and digital design (specially digital). If you call yourself an EMC engineer this book also is of great value to you and it would be an excellent companion to those more traditional EMC books that you may already have in your library.

Finally, this book is reviewed for the sake of those thousands of electronic engineers entering the job market who are at least aware of the importance of noise control in their future designs. This book should be of great benefit.

From a broad point of view, the book is divided into 12 chapters and three appendices. The first three chapters introduce analog circuit terminology, the high speed properties of logic gates, and standard high speed measurements techniques, respectively. The first three chapters form the fundamentals of this book and are very important in every serious consideration of high speed logic design. The remaining chapters (four through 12), each treat a specialized subject in high speed logic design. Appendix A collects highlights from each section and each chapter of the book, listing the most important ideas and concepts presented. The author thinks that it can be used as a check list for system design or as an index to the text when facing a difficult problem. Appendix B details the mathematical assumptions behind various forms of rise time measurements. This section helps relate results given in this book to other sources and standards of nomenclatures.

Appendix C lists a series of standard formulas used in the book and their MathCad implementation.

Chapter one’s title is called “Fundamentals” and it addresses how passive circuit elements affect signal propagation interactions (ringing and reflections) between signals (or crosstalk) and interactions with the natural world (electromagnetic interference). The author begins the chapter with a study of high speed digital design and relationships among frequency, time, and distance. The empirical formulation of knee frequency is derived and the role of such frequency in the flat frequency response and the processing of short time events in digital circuits is discussed. The chapter also discusses the difference between lumped and distributed systems and the fact that the response of any system of conductors to an incoming signal depends on whether the system is smaller than the effective length of the fastest electrical feature in the signal. Four kinds of parasitic reactances are reviewed which are typical of high speed digital circuits: capacitance, inductance, mutual capacitance, and mutual inductance. The author provides diagnostic rules to instantly characterize which kind of reactance is present in the device under question. Two good sections in chapter one are the discussion of mutual capacitance among two circuits and the relation of mutual capacitance to crosstalk as well as mutual inductance and its role in crosstalk. These discussions are done in the time domain with examples and measurements. Chapter two addresses the high speed properties of logic gates.

All logic families exhibit trade off among power, speed, and packaging and this chapter looks at these issues in great detail. The chapter covers the study of power consumption of high speed logic circuits in the four main categories: input power, internal dissipation, drive circuit dissipation, and outer power delivered to the load (quiescent power is also introduced briefly). Many examples are discussed and numerous handy simple formulas are derived which I personally found very useful when deriving the need for “power budgets” in my PCB designs. The important subject of logic speed is also covered in chapter two. Theoretical digital logic designs focus on the propagation delay of logic gates. Practical problems in high frequency, however, often depend solely upon a more subtle specification: the minimum output switching time. Faster switching times cause proportional increases in problems with return currents, crosstalk, and ringing that are independent of propagation delay. Logic families having minimum switching times much faster than the propagation delays suffer unnecessary penalty in system design because the device packaging, board layout and connectors must accommodate fast switching times while the logic timing benefits only from propagation delay. The chapter effectively discusses the two distinct mechanisms which can cause problems in fast switching times IC: effects created by sudden changes in voltage and effects created by sudden changes in current. The very important and related subject of voltage margin is generously discussed also. One of the most important sections in the book is the last section of chapter two where a thorough review of the subject of packaging is given. This is even more important in these days where the number of packaging schemes is growing greatly to accommodate even more faster ICs and the fact that packages at high logic speeds suffer from problems with lead inductance, lead capacitance, and heat dissipation.

Chapter three deals with the limitations of scientific instruments in making digital measurements. This chapter describes not only real world problems in making accurate measurements with the oscilloscope, but provides a great deal of examples, tips, cautions, and formulas that make this chapter extremely useful (it has been for this reviewer). If you are an EMC engineer that works frequently with compliance testing, this chapter should be a good companion for you and should be put together in a binder with those chapters, that you probably already have, dealing with measurements in the frequency domain using the spectrum analyzer. Chapter four addresses the analysis of transmission lines in the time domain. The chapter compares the use of transmission lines in logic circuits versus the old method of point to point wiring. Transmission lines provide less distortion, less radiation, and less crosstalk; the price paid, of course, is that more driving power is needed. The chapter compares and gives examples of signal distortions, radiation and crosstalk problems arising from point to point wiring, and the benefits when compared to the use of transmission lines. The material covered concerning transmission lines is typical of that covered in other electromagnetic books such as the infinite uniform transmission line, the low-loss transmission line, skin effect, frequency response in the skin effect region, and dielectric effects (including losses). Any combination of practical source and local impedances connected to a real transmission line will degrade its performance. This degradation may be slight or it may be devastating, depending upon the particular source and local impedances used with the transmission line. Therefore, in chapter four considerable attention is paid to the effects of source and local impedances in such topics as reflections, end and source terminations, settling time in poorly terminated lines, high and low source impedance with unterminated lines, capacitive loading, uniform loaded lines, and right angle bendings. The chapter ends with a good discussion on the different relationships between impedances and propagation delays.

In chapter five, it is noted that ground and power planes in high speed digital systems perform three critical functions: provide stable reference voltages for exchanging digital signals, distribute power to all logic devices, and control crosstalk between signals.

The first approach is to assume short traces for which lumped analysis and mutual inductance is appropriate, later the traces are treated as long lines where separate coupling into its forward and reverse parts are applied. The chapter ends with summary rules for designing good printed circuit board layer stacks for the control of crosstalk. The author outlines a handy group of empirical formulas that are reasonably accurate and several measurement exercises and examples are outlined. Chapter six addresses the very important subject of terminations. This chapter has also been very useful to this reviewer. When a line is such that cable length exceeds about one sixth of the electrical length of a rising edge, the cable needs terminations. Without terminations, reflections at either end of a long cable renders signal transmission impossible. When a line is short it may still need terminations if it is driving capacitive loads. This chapter addresses three main topics: comparison of ends versus series terminations, selection of appropriate terminating resistors, and crosstalk among terminating components. Chapter seven covers vias. The term via commonly refers to a hole in a printed circuit board. A via can be used for mounting a through-hole component or for routing traces between layers. The only difference, from our point of view, is that during assembly a through hole has one leg of a component soldered into it, while a trace routing via remains empty. The chapter covers mechanical properties of vias, capacitance and inductance of vias and current return and its relation to vias.

The very important subject of power distribution in digital systems is discussed in chapter eight. This chapter describes how power systems provide stable voltage references and distribute power. The need and pitfalls encountered in providing a stable voltage reference is discussed with examples in the chapter. Power “rules” are outlined for the designer. Power distribution problems encountered in the process of supplying uniform voltage are also discussed in this chapter. Topics such as resistance, inductance of distribution wiring and board level filtering using bypass capacitances are discussed in good detail. Procedures for using bypass capacitance arrays are discussed as well as procedures for using bypass capacitance in different design scenarios. A very novel chapter is chapter nine, which discusses the subject of connectors. Chapter nine examines connector properties which are important in high speed digital design. After reading this chapter, you will know what properties are important in your application and how to test a connector system. The primary electrical factors discussed concerning high performance in connectors are: how connectors create crosstalk (a mutual inductance effect), how series inductance slows down signal propagation and creates electromagnetic interference, and how parasitic capacitance slows down signal propagation. The chapter provides many illustrations and examples. The chapter ends with the resolution of EMI problems in connectors using filtering and shielding. Chapter 10 is an extension of chapter nine, but specifically addresses ribbon cables. The term ribbon cable refers to any cable having multiple conductors bound together in a flat wide strip. Ribbon cable wiring always runs parallel to each other at precisely controlled separations. The chapter covers ribbon cable signal propagation and ribbon cable crosstalk. Formulas and measurement techniques are provided. The chapter ends with proper design of ribbon cable connectors.

Chapters 11 and 12 cover clock distribution and clock generation respectively. Not only are clocks the fastest signals, but they are also the most heavily loaded. Clocks connect to every flip flop in a system, while individual data wires fan out to only a few devices each. Chapter 11 provides special attention to clocks. The chapter examines clock drivers, special clock routing rules, and peculiar circuits used to improve the distribution of clock signals. The very basic but extremely important subjects of time margins and clock skew are discussed first. This material is followed by a discussion on the usage of low impedance drivers and low impedance distribution lines. Brief introductions are provided concerning source termination of multiple clocked lines and how to control crosstalk in clock lines. A fairly good discussion on delay adjustments for eliminating clock skew is also provided in the chapter. Chapter 11 ends with methods for canceling parasitic capacitances of clock repeaters. Chapter 12 covers clock oscillators. Standard industry practice has shifted from designing oscillators to specifying oscillators. Chapter 12 focuses on how to properly specify and use oscillators and crystals; this includes frequency specifications, allowed operating conditions, electrical and mechanical manufacturing issues and reliability. Finally, the subject of clock jitters (deviations of clock output transitions from their ideal position, mostly caused by amplified noise) is discussed in good detail.

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