1992 Reports

Computers Without Clocks

Unger, Stephen

Continuing increases in logic' density on VLSI chips have led to increasing problems with clock distribution systems. This has stimulated interest in dispensing with the clock s that contra I virtually all existing digital systems. The recent construction at Cal tech of a microprocessor chip without a clock has clearly demonstrated the feasibility of such systems. Basic principles that can be used to design computers without clocks are outlined here. Handshaking and dual-rail coding constitute one important related pair of concepts. Logic circuit level designs of asynchronous registers. counters. shift registers. and adders are shown. 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 basic features of modern computers including interrupts and pipelines. The design of an asynchronous add-and-shift binary 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 unclocked systems can be truIy delay - insensitive is discussed.