Asynchronous circuits dispense with the clock signals used in conventional computers. Instead, they perform actions as soon as all necessary inputs are in place. Asynchronous computers do not require the power and "wiring" to distribute a synchronizing clock signal throughout the computing system. Clock related circuitry can occupy as much as 30% of a conventional CPU. Asynchronous circuits generate fewer strong electronic noise pulses as they do not toggle thousands of semiconductor state transitions (nearly) simultaneously at regular intervals.

Asynchronous circuits may be faster than synchronous circuits in some cases since operation time is based on the actual slowest computing elements in use rather than the slowest computing elements that could potentially be present.

Asynchronous devices should be useful for bridging subsystems running at different speeds

In addition to conventional logic gates, asynchronous logic requires arbitration and rendezvous circuit elements that are not required for synchronous computing. Arbitration circuits select amongst multiple inputs based on which input indicates stable values first. Rendezvous circuits hold output changes until all inputs are present. All current designs for arbitration circuits exhibit variable and potentially fairly lengthy delays when inputs arrive at nearly the same time. This may be a fundamental limitation.

The earliest asynchronous designs date to the 1950s, however, early synchronous designs required fewer active elements and were easier to design. Five decades later, the balances have changed, and designers are taking a new look at asynchronous circuitry, Some asynchronous components are already used in devices such as pagers. It is likely that asynchronous circuitry will largely be used in conjunction with synchronous circuitry not in place of conventional synchronous circuits.

Return To Index Copyright 1994-2002 by Donald Kenney.