Computers Without Clocks

Continuing increases in Ingil' density on VLSI chips have led to increasing problems with clock distrihution systems. This has stimulated interest in dispensing with the cloc k s that contra I virtually all ex i sti ng d igi tal sy stem s' The recent construction at Cal tech of a microprocessor chip without a clock has clearly demonstrated the feasihility of such systems. Basic principles that can be used to design computers \vithout clocks are outlined here. Handshaking and dual-rail coding constitute one important related pair of concepts. Logic circuit \evel designs of asynchronous registers. counters. shift registers. and adders are slw\'.:n. Control modules are described that can be used as building blocks for systems of varying complexity. These can he used to implement all of the hasic features of modern computers including interrupts and pipelines. The design of an asynchronous add-and-shift hinary multiplier is used to illustrate the use of these data processing and control modules. Most of the work shown uses what is generally referred to as 4-phase handshaking. but 2-phase handshaking is also discussed. The extent to which uncloc ked systems c an he t ru I y del ay i n se 1-1S i t i ve is disc ussed.