A Paradigm Shift
The word “paradigm” is defined in the dictionary as “a framework containing the basic assumptions, ways of thinking, and methodology that are commonly accepted by members of a scientific community”. In his influential book “The Structure of Scientific Revolutions” published in 1962, Thomas Kuhn uses the term “a paradigm shift” to indicate a change in the basic assumptions (the paradigms) within the ruling theory of science. Today, this term “paradigm shift” is used widely, both in scientific and non-scientific communities, to describe a profound change in a fundamental model or perception of events.
Ever since the clock concept was introduced into microelectronic system design, it was assumed that all the cycles in a clock pulse train have to be equal in length (a rigorous clock signal). One reason that this form of clock signal has dominated microelectronic system design for a long time is that, in the past, the requirement for IC clocking was mostly straightforward. A clock signal with a fixed rate was sufficient for most systems. However, the complexity of future systems changes the game. Low power operation, low electromagnetic radiation, synchronization among networked devices (e.g. Internet of Things), complex data communication schemes, etc., all require a clock signal that is flexible.
Another reason behind the dominance of this style of rigorous clock is that time, which shows its existence and its flow indirectly through the use of a clock pulse train, is not a physical entity that can be controlled and observed directly. Thus, creating flexible clock is an inherently difficult task. It demands effort beyond simply playing with various techniques at circuit level. Philosophically it requires an adjustment, at fundamental level, in our thinking about the way of clocking microelectronic system. The “anomaly” in this case is a new perspective on the concept of clock frequency. In this line of argument, the materials presented in this book induce a paradigm shift in the field of microelectronic system design.
Clock is an enabler for system level innovation
Viewing from high level, there are four fundamental technologies supporting the entire IC design business: processor technology, memory technology, analog/RF technology and clock technology. In the past several decades, a tremendous amount of effort has been spent on the development of the first three technologies. Clock technology falls behind in this race. One of the key reasons for this is that clock technology deals with a special entity: time. Neither is it directly observable nor is it directly controllable. The circuit designer can only play with it indirectly, through voltage and/or current. This lag, however, provides us an opportunity to make significant progress. It is a battleground for new ideas. It is a potential birthplace for great inventions. It is one of the enablers for system level innovation.
What is new on clock? flexibility versus spectrum purity
When the term “flexible clock” is used, it refers to a clock signal: 1) whose frequency can be arbitrarily set; and 2) whose frequency can be changed quickly. Preferably, these two features shall be achieved simultaneously and be available to clock user at a reasonable cost. A rigorous clock has the characteristic of high spectrum purity, which is beneficial to certain applications. There are, however, many more applications where spectrum purity is not of high concern. Instead, a clock signal possessing the capability of small frequency granularity and fast frequency switching is more useful. Therefore, there is a crucial trade-off to be made when an IC design problem is investigated. In the past, clock of high spectrum purity was the undeniable winner. However, for future microelectronic system design, this is not necessarily always the case.
“Jittery” clock is not necessarily a bad thing
The essence of a clock pulse train is to create a series of “moments in flow-of-time” by utilizing the mechanism of “voltage-level-crossing-a-threshold”. The resulting moments are used as the reference points for other events happening inside the microelectronic system. Therefore, the requirement on those moments is that their location-in-time must be predictable and precise. Jitter is a parameter measuring this quality. Thus, jittery clock is undesirable since it reduces the effectiveness of the clock in coordinating other events. However, jitter is not without any use. An obvious example of its applicability is that jitter in a clock signal can help reduce its electromagnetic radiation since it spreads the clock energy. Another not-so-obvious, and more valuable, use of “jittery” clock is to trade the irregularity-in-moment with the flexibility. The flexibility associated with a clock signal refers to its capability of fine frequency resolution and fast frequency switching. When use with care of this irregularity-in-moment, a clock signal can be made flexible by intentionally introducing “controlled jitter” into it. This capability is important for certain applications. Indeed, it outweighs the requirement on clock’s spectrum purity in such applications. Hence, jittery clock is not necessarily a bad thing.
The power of idea
Many times in human history, the power of an idea has changed the landscape of our civilization. Such ideas include liberty, romanticism, Marxism, Zionism, among others. Each of these great ideas leaded to a profound movement that changed the way we live. In science and technology, the latest example of such an idea would be Einstein’s theory of relativity. It links the space and time together, resulting in a thing called space-time. This breakthrough idea, which was regarded as a ridiculous one by most people when it made its debut, is proven to be one of the greatest in human history. This idea is an “anomaly” that later leads to a great paradigm shift in science.
In book “Nanometer Frequency Synthesis beyond Phase Locked Loop”, a new perspective on clock frequency was introduced. While the materials presented in that book focuses on building the circuit at component level, this book will answer the question of how to use it in upper level to create better systems. This book is the continuation in this route of new microelectronic system design methodology. Quoted from Steve Job: think different.