Liming Xiu

Liming Xiu

VP of Product Development, Kairos Microsystems Corporation.

Chief Clock Architect, Novatek Microelectronic Inc., 2009-2012.

Senior Member of Technical Staff (SMTS), Texas Instruments Inc., 1995-2009.

IEEE Circuit And System Society VP, 2009-2010.

Liming Xiu earned B.S.and M.S. degrees in physics from Tsinghua University, China, in 1986 and 1988, respectively. He earned his MEEE from Texas A&M University, USA, in 1995. In

1995-2009, he worked for Texas Instruments, Dallas, USA (Senior Member Technical Staff). In 2009-2012, he was chief clock architect of Novatek Microelectronics, Taiwan. From 2012–2015, he was VP of Kairos Microsystems, Dallas, USA. From 2015, he works for BOE Technology Group, Beijing, China, as chief scientist on IC technology. He served as VP of IEEE CASS in years 2009-2010. He is the inventor of Flying-Adder frequency synthesis architecture; the promoter of Time-Average-Frequency concept and theory. He has 25 US patents. He has published numerous IEEE journal and conference papers,

published three books as solo author: 《VLSI Circuit Design Methodology Demystified》, 《Nanometer Frequency Synthesis beyond Phase-Locked Loop》, and 《From Frequency to Time-Average-Frequency: A Paradigm Shift in the Design of Electronic system》.

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PREFACE

"I have no special talents. I am only passionately curious."

-- Albert Einstein

In the great Einstein's view, passion, desire -- and above all curiosity -- are the very ignition switches to spark discovery and creation. More than two decades ago, when I was studying physics in Tsinghua University (Beijing, China), this confession seemed counterfactual. After 20 years of involvement in scientific and engineering work, it is gradually starting to make sense to me. Nowadays, there are 7 billion people living on this planet. If all the people who ever lived on Earth were included, this enormous number would be exponentially larger. Among this gigantic population there are countless gifted people who are born with talent. However, history shows that only a tiny handful of people have made paramount contributions to the understanding of the world we all live in. The force that separates these all-time greats from the exceptional group of the talented is the passion to ask what and why, sincerely and unyieldingly.

FREQUENCY IS CHANGED

I am neither the great nor the gifted. But this force of curiosity does have its hold over me. In my career as a VLSI circuit design professional, I have had the fortune to work in many different areas (please see my other book: VLSI Circuit Design Methodology Demystified: A Conceptual Taxonomy, 2007). This unique experience provides me with the opportunity to observe everything from a broader viewpoint, the ability to see things in the bigger picture. In the meantime, it engages my curiosity. It often drives me to challenge the conventional way of doing things. One particular example is the clock signal used in the VLSI circuit. As both a circuit level phase-locked loop (PLL) designer and a system-on-a-chip integration level PLL user, I have seen the story from both sides. I distinctly remember one afternoon in the summer of 2003, after spending a long time explaining the flying-adder architecture (invented in the late 1990s) to one of my colleagues, a question suddenly occurred to me: What is frequency? Why must all the cycles have equal lengths in time? In common sense, this question looks foolish and dangerous for anyone to ask. Curiosity about this issue has intrigued me for several years (secretly, for fear of being treated as an illiterate). In 2008, after a long period of serious investigation from both theoretical and experimental perspectives, I had built up enough nerve to formally introduce the concept of "time-average-frequency." It removes the constraint that all clock cycles must have the same length-in-time. This seemingly ridiculous or insignificant step is a bold move philosophically. Its aim is the two long-lasting problems in this field: arbitrary frequency generation and fast response in frequency switching. It will have profound influence in VLSI circuit design since clock signal is used in every chip. Along the running history of our progressive understanding of this world, it is shown that all the great advancements originate at the concept level. The greatest example is provided by Einstein. By changing our view of the two fundamental concepts of time and space, he brought us one giant step closer to the ultimate understanding of the universe. This has forever changed the way we live. In this book, the most important message that I want to share with reader is: the concept of clock frequency is changed.

"Your time is limited, so don't waste it living someone else's life. Don't be trapped by dogma -- which is living with the results of other people's thinking. Don't let the noise of others' opinions drown out your own inner voice. And most important, have the courage to follow your heart and intuition."

-- Steve Jobs

The spirit behind this excerpt from Steve Jobs's famous speech (Stanford University, 2005) is not unfamiliar. Similar wisdom has been expressed in the past by great philosophers and pioneers. But Mr. Jobs's testimony is more touching and real to us as individuals because he lived in our time. He noticeably changed the face of technology and the modern way of life, and he preached his passion in a way that was pleasantly contagious. During the pursuit of time-average-frequency, I sometimes felt frustrated because this new thinking contradicts conventional wisdom. On several occasions, a painful price had to be paid to uphold what I believe. Today, whenever Jobs's remark is replayed, I feel a bit of warmth and encouragement. Looking at his journey, it is confirmed again that all the greats have their own obstacles. The key to success is not superior intellect or powerful financial muscle. Instead, it is the intrinsic drive to believe, to achieve, and to change. This book is my case of this testimony.

SIMPLE AND ELEGANT

Coupled with curiosity, the other important part of my mindset is the tenacious desire to pursue simplicity and elegance in almost everything. I admire beautiful things in life: beautiful music, beautiful art, beautiful literature, beautiful sportsmanship, a beautiful soul -- the list goes on and on. During the creation of the flying-adder circuit, simplicity drove me to search unrelentingly for the simplest structure that required the minimum number of transistors possible. Elegance compelled me to ensure that there is a sophisticated and yet beautiful mechanism behind the simple circuit. I am a passionate believer of the "Principle of Least Action" (Pierre - Louis Maupertuis, 1774). I apply it to my circuit design whenever I can. I hope that I can convey this attitude to readers throughout this book.

TIME, NUMBER, AND THE BEAUTY OF MATHEMATICS

The key focus of this book -- frequency -- is closely related to the thing that we called time. Time is a major subject of religion, philosophy, and science. Among great thinkers, there are two distinct standpoints on time. One view is that time is part of the fundamental structure of the universe, a dimension in which events occur in sequence. The opposing view is that time does not refer to any kind of physical container that events and objects move through. Instead, time is part of a fundamental intellectual structure (made of space, number, and time) within which humans sequence and compare events. In this second view, time is a virtual subject, neither an event nor a thing, and thus is not itself measurable.

Another mysterious product from human brain is the number. The world is virtually made of numbers. Numbers were invented to fulfill the need to organize our life quantitatively, beyond just qualitatively. It is generally believed that this is one of the major reasons why humans and all other species have followed different evolutionary paths (language is among the others). In our daily life, time and number are connected though an entity called the atomic clock: the definition of second. In VLSI circuit design, time and number are related by a special signal called clock. In this engineering practice, however, the relationship between time (frequency) and number has not reached the harmonization achieved in our daily life. In this book, one of the goals is to see if something can be done to improve the situation (digital-to-frequency converter, the counterpart of digital-to-analog Converter). In this effort, two important mathematical tools are used: Number Theory and Fourier Analysis. During this process of reasoning and learning from several "beautiful minds," I am amazed at the power and the striking beauty of mathematics. I am deeply touched by the mysterious harmony rooted in our number system. In this book, I want to share this joy with reader.

PLAY TIME AS WE PLAY LEVEL

The entire VLSI circuit design business is built on the fact that we use level (voltage or current level) to represent information. In analog processing, level is organized in multiple elevations. In the digital domain, it is in binary fashion. As process technology advances, some momentous changes emerge: the transistor is switching faster and faster, and the supply voltage is reduced lower and lower. Consequently, time (or rate-of-switching) becomes an attractive option to represent information. This will unquestionably influence the way that we design circuits. In this book, a million-dollar question is asked: "Can we play time as we play level?"

This book is organized in the following way:

Chapter 1 discusses how the clock signal is used in all electronic applications. The aim of this chapter is to understand our targeting problem in depth. Chapter 2 briefly reviews the existing clock generation techniques. This chapter focuses on the explanation of how this problem is conventionally dealt with. Chapter 3 looks at the root of the clock problem. It investigates the very concept of frequency and introduces the breakthrough viewpoint that leads us on an entirely new path. Chapter 4 presents the supporting technology, flying - adder architecture, which implements this new concept into circuitry. This is the hardware implementation of this novel approach introduced in chapter three. Based on the time-average-frequency concept and the flying-adder circuit, Chapter 5 coins a new device: the digital-to-frequency converter. Chapter 6 shows some examples of using this innovative technology to build cheaper, faster, and better systems. It illustrates the strength of this new technology. Chapter 7 is the visionary discussion of using "time" for signal processing. It brings forth new directions for future chip design. Its goal is to inspire the next generation researcher and engineer with new opportunities.

This book was inspired by Stay Hungry, Stay Foolish, which I second from the bottom of my heart. This mindset is the invisible hand that has created our magnificent civilization out of the void. It will serve as the lighthouse to guide us in the journey of seeking the ultimate paradise. It is my wish that this book can play a role in achieving the goal of designing "cheaper, faster, and better" electronic products that will ultimately make for a more enjoyable life.

I would like to thank my dear wife, Zhihong You, for supporting me in the completion of this book. Without her selfless effort, this book would never have been published. She has always stood beside me through both "thick and thin." As a fellow professional who works in similar area and was trained in the same schools, her gifted mental might is highly respected by me. Fortunately, it appears that her exceptional competence has been passed to our lovely daughters Katherine and Helen. I also want to thank Katherine Xiu for helping me in English proofreading and in creating the index.

Liming Xiu

First draft, Nov. 2011, HsinChu, Taiwan.

Final version, May, 2012, Dallas, USA.

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前言

"我沒有特殊的才能,, 我只有執著的好奇心.."

- 亞伯特 愛因斯坦

在偉大的愛因斯坦眼裡, 激情, 渴望及超越一切的好奇心, 是催生重要發現與點燃發明創造的關鍵.約二十多年前, 對於當時正在中國北京清華大學學習物理的我而言, 愛因斯坦的此番自白似乎過於自謙而與事實不符. 經過二十年在科學研究與工程實踐中的浸淫, 我開始逐漸理解了這位科學偉人這段自白背後的真實含義. 當今的世界有七十億人口, 如果曾經生活在地球上的所有人都包含在內的話, 那數目將會遠超過此數. 在這個巨大的人群中, 有無數的人生來就具有異乎尋常的能力. 但是歷史上只有極為少數的人對於認識及了解我們所居住的世界做出過重大的貢獻. 那股將此類偉人與擁有天賦異質的人群區分開的力量, 就是追求 "知其所以"與"知其然"的激情. 這種探索是真誠的, 是不帶功利色彩的, 是不屈不撓的.

"頻率變了 "

我既非偉人也無天賦異稟. 但這種對事物的好奇心在我身上有強烈的烙印. 在我的電路設計職業生涯中, 我有幸涉足許多領域(詳見我的另一本書: VLSI Circuit Design Methodology Demystified: A Conceptual Taxonomy ). 這種獨特經歷使我能從更寬, 更高的視野觀察問題. 同時這種多方面的經歷也強化了我的好奇心. 它使我經常對傳統的解決問題的方法產生疑問與挑戰. 一個具體的例子就是電路設計中最重要的時鐘信號. 在我的職業生涯中, 我既是時鐘電路的設計者, 又是晶片層次時鐘電路的使用者. 這使我對這個電路設計中的關鍵信號有十分全面深入的瞭解. 我清楚地記得在 2003 年夏季的一個下午, 在花了很長時間給一個同事解釋 Flying-Adder PLL電路(發明於上世紀九十年代末期)之後, 我的腦中突然閃過這個問題: 什麼是 "頻率" ? 為什麼時鐘信號中所有週期都必須等長 ? 就常理而言, 這是一個愚蠢的問題, 是一個有顯而易見答案的問題. 但好奇心使我對此問題不甘罷手. 基於害怕被同事嘲笑為無知的心理, 我找尋答案的努力一直是秘密地進行的. 經過幾年嚴肅的理論探索和工程實踐, 在 2008 年我終於累積了足夠的勇氣, 一個全新的概念Time-Average-Frequency 被正式提出了. 它移除了在一個時鐘信號中所有周期都必須等長的這種限制. 這個看似荒謬與無足輕重的一小步, 在認知上是勇敢而意義深遠的一次突破. 提出此概念的終極目標是解決在頻率合成這個領域中二個長期懸而未決的問題: 任意頻率的合成與頻率切換的快速反應. 它將對未來的電路設計方法產生重大的影響, 因為所有芯片都使用時鐘這個關鍵信號. 在本書中, 我希望與讀者分享的最重要的訊息就是: 頻率變了.

"你的生命是有限的, 不要模仿別人的樣板去生活. 不要生活在教條中, 你的生活不要被局限於別人設計的框架中. 不要讓外界的雜音淹沒你內心的真實感受. 要有勇氣用行動去追隨你的直覺和靈魂."

- 史提夫 賈伯斯

在史提夫賈伯斯 2005年史丹佛大學的著名演講中, 這段摘錄背後的含意並不陌生. 在賈伯斯之前的歷史歲月中, 類似的哲理已被許多的智者與先覺闡述過. 但賈伯斯的證言更加動人和具有真實感, 因為他生活在我們的時代. 他深刻地詮釋了 "科技" 這個詞對我們的含義. 他以一己之力影響了我們現代生活方式. 他科技宣道的方式充滿激情, 極具感染力. 在專注於Time-Average-Frequency 的研究期間, 我時常因為這個與傳統方法相抵觸的新觀念而感覺到壓力, 進而有時變得沮喪. 在若干場合, 為堅持信念, 我曾經付出過沉痛的代價. 今天, 當重溫賈伯斯的這段話時, 我感受到一絲溫暖與鼓舞. 回顧他的奮鬥歷程, 歷史再一次證明了所有成功者都曾面臨困難. 成功的關鍵不是超群的智力, 也不是龐大的錢袋, 而是不屈不撓執著的信念. 這本書是我對此哲理的證言.

"簡潔與優美 "

除了好奇心之外, 我的性格中另外一個重要的特癥是不倦地追求簡潔與優美. 我欣賞生活中所有優美的東西: 優美的音樂, 優美的藝術, 優美的文學, 優美的運動精神, 優美的智慧結晶. 在 Flying-Adder 電路的研發過程中, 追求簡潔的信念驅使我無情地尋找使用最少電晶體的電路. 對優美的鍾愛激發我盡全力保證在簡潔電路設計背後有精密而巧妙的機制. 通過本書, 我希望我能把這種理念和風格傳遞給讀者.

"時間, 數字與美妙的數學 "

本書的主軸 -頻率- 是與我們稱之為 "時間" 的這個東西緊密相連的. 在宗教, 哲學與科學的範疇內,時間是一個重大的議題. 在嚴肅的思想家中, 對時間有兩種截然不同的認識. 一種觀點認為時間是宇宙基礎架構中的一個實在的維度. 在此維度中, 事件依序發生. 另一種相反的觀點認為時間與任何實在的現實架構無關. 時間是我們認知結構中的一部份. 在不依附於任何實體的人的認知結構中, 有三個基本組成部份: 空間, 數字及時間. 在此精神框架中, 人對事物進行排序和比較, 進而安排和組織我們的生活. 在這第二種看法中, 時間既不是事件也不是實體, 它只存在於我們的精神世界裡. 它是不可感知的, 是不可測量的.

人腦的另一個神秘作品是數字. 這個世界實際上是由數字所組成的. 數字的發明使我們能從定性地安排生活, 進步到定量地組織生活. 人類之所以能和其它動物走上不同的進化路程, 數字的產生和運用是其中一個重要的原因 (語言是另一個重要的原因之一). 在我們日常生活中, 時間與數字是透過一個稱為原子鐘的物體聯繫起來的. 這個關聯就是對 "秒" 的定義. 而在電路設計中, 時間和數字是通過一個特殊信號 - 時鍾信號 - 相關聯的. 但在此工程實踐中, 時間和數字的關聯操作遠未達到它們在日常生活中所達到的和諧程度. 本書目的之一就是試圖對此狀況提供一些改進(Digital-to-Frequency Converter, Digital-to-Analog Converter 的配對物). 在此努力中, 數論及傅立葉分析這兩個重要的數學工具起了重大作用. 在此艱難的探索過程中, 我有幸受教於若干位數學奇才. 在他們的指引下, 我有機會體驗到了數學的力量和其中的和諧之美. 植根於數字體系中的神秘和諧讓我嘆為觀止. 在本書中, 我希望能與讀者分享這種喜悅.

"像操縱強度一樣操縱時間 "

全部的集成電路設計理論和實踐都是建立於一個基礎之上. 這個基礎就是: 信息是用強度(或稱大小,高低)來表示的. 在模擬電路設計中, 強度是通過多層次結構來體現的. 在數字電路設計中, 強度只有兩個選擇. 隨著半導體製造技術的迅猛發展, 有一些重大的變化出現了: 電晶體翻轉速度越來越快,電源電壓越來越低. 在此背景下, 用時間(或更準確地說是用變化速率, rate-of-switching)來表達信息變成一個可能和有利的選擇. 這種趨勢將毫無疑問地影響我們未來設計電路的方法. 在本書中, 有一個極具價值的問題被提出: 我們是否可以像操縱強度一樣自如地操縱時間 ?

這本書是以下列方式組織寫作的

第一章探討時鐘信號是如何被應用於各種電子系統中的. 本章的目的是對我們的研究對象進行深入的瞭解. 第二章簡短地回顧現行的技術. 通過此討論, 我們可以看到在目前通用的手段中, 時鐘信號是如何被產生的. 第三章聚焦於時鐘信號的根本. 它對頻率這個重要的基礎概念進行了重新檢驗. 它對此問題引入了一個全新的觀察角度, 進而開闢了研究時鐘問題的一個新的方向. 第四章介紹此新概念的支持電路Flying-Adder 電路. 此章是第三章所闡述的全新概念的硬體實現. 基於第三章的新觀念和第四章的硬體電路, 第五章引入了一個新的電路元件Digital-to-Frequency Converter. 第六章包含了若干實例來展示此一新技術的威力. 這些實例體現了 "更便宜, 更快, 更好" 的精神. 第七章是關於用 "時間" 來作信息處理的前瞻性討論. 它提出了幾個可用於將來電路設計的新方向. 本章的目的是激勵下一代的研究者與工程師向前看.

我極為贊同 "求知若渴, 虛心若愚" 這句名言. 這本書的形成就受益於這句至理名言. 這種精神是那隻從虛無中創造出我們熠爛文明的無形之手. 它將是我們人類在尋找極樂天堂征途中的燈塔. 在追求生產 "更便宜, 更快, 更好" 的電子產品的永久目標中, 這種精神是最具價值的原動力. 在此波瀾壯闊, 精采紛呈, 永無止境的追求中, 我希望本書能起到一些作用.

我願藉此機會對我妻子, 由志宏, 表示由衷的感謝. 如果沒有她無私的幫助, 本書不可能出版. 不論在順利的時候和在困難的處境, 她始終與我站在一起. 做為一個與她學習和畢業予同幾所大學的校友, 做為一個與她工作在類似領域的從業者, 我對她的天賦聰明才智極為敬慕. 幸運的是, 有跡象表明,她的天賦才華已經遺傳給了我們可愛的女兒: 凱瑟琳和海倫.

修黎明, 新竹, 台灣, 2011年11月