An Introduction to Asynchronous Circuit Design

The purpose of this monograph is to provide both an introduction to eld of asynchronous digital circuit design and an overview of the practical state of the art in 1997. In the early days of digital circuit design, little distinction was made between synchronous and asynchronous circuits. However, since the 1960's, the mainstream of the digital circuit design enterprise has been primarily concerned with synchronous circuits. Synchronous circuits may be simply de ned as circuits which are sequenced by one or more globally distributed periodic timing signals called clocks. Asynchronous circuits are an inherently larger class of circuits, since there are may sequencing options other than global periodic clock signals. Asynchronous circuits have been studied in one form or another since the early 1950's [92] when the focus was primarily on mechanical relay circuits. A number of theoretical issues were studied in detail by Muller and Bartky as early as 1956 [138]. Since then, the eld of asynchronous circuits has gone through a number of high-interest cycles. In recent years there has been an unprecedented level of interest in both academic and industrial settings [81]. Much of this recent research e ort has focused more on theory than practice. Nonetheless, the advance of practical asynchronous circuit design techniques also has an unusual level of interest. The focus of this document is on the aspects of the asynchronous circuit design discipline which are either likely, or have been an in uence on the practical design of asynchronous circuits. The attempt is to provide an introduction to the basic concepts which provide the foundation for today's design techniques and to summarize the current practice. The text contains an extensive set of bibliographical pointers to guide the more serious student of the eld.