Computer chips of today are synchronous. They contain a main clock, which controls the timing of the entire chips. There are problems, however, involved with these clocked designs that are common today.

One problem is speed. A chip can only work as fast as its slowest component. Therefore, if one part of the chip is especially slow, the other parts of the chip are forced to sit idle. This wasted computed time is obviously detrimental to the speed of the chip.

Asynchronous, or clock less, design has advantages over the synchronous design.

The first of these advantages is speed. Chips can run at the average speed of all its components instead of the speed of its slowest component, as was the case with a clocked design. Also the need to have a clock running at a speed such that the signal can reach all parts of the chip is eliminated. Therefore, the speed of an asynchronous design is not limited by the size of the chip.

An example of how much an asynchronous design can improve speed is the asynchronous Pentium designed by Intel in 1997 that runs three times as fast as the synchronous equivalent. This speedup is certainly significant and proves the usefulness of a clock less design.

ASYNCHRONOUS LOGIC

Data-driven circuits design technique where, instead of the components sharing a common clock and exchanging data on clock edges, data is passed on as soon as it is available. This removes the need to distribute a common clock signal throughout the circuit with acceptable clock skew. It also helps to reduce power dissipation in CMOS circuits because gates only switch when they are doing useful work rather than on every clock edge.

There are many kinds of asynchronous logic. Data signals may use either “dual rail encoding” or “data building”. Each dual rail encoded Boolean is implemented as two wires. This allows the value and the timing information to be communicated for each data bit. Bundled data has one wire for each data bit and another for timing. Level sensitive circuits typically represent a logic one by a high voltage and a logic zero by a low voltage whereas transition signaling uses a change in the signal level to convey information. A speed independent design is tolerant to variations in gate speeds but not to propagation delays in wires; a delay insensitive circuit is tolerant to variations in wire delays as well.

The purest form of circuit is delay-insensitive and uses dual-rail encoding with transition signaling. A transition on one wire indicates the arrival of a zero, a transition on the other the arrival of a one. The levels on the wires are of no significance. Such an approach enables the design of fully delay-insensitive circuits and automatic layout, as the delays introduced by the layout compiler can’t affect the functionality (only the performance). Level insensitive designs can use simpler, stateless logic gates but require a “return to zero” phase in each transition.