Published on Apr 02, 2024
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ON
Asynchronous Chips
Introduction
==> 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
==> New problems with speeding up a clocked chip are just around the corner.
==> Clock frequencies are getting so fast that signals can barely cross the chip in one clock cycle
Discussion
==> Asynchronous, or clock less, design has advantages over the synchronous design
==> The other advantage of a clock less design is power consumption
==> Special light emission measurements of a synchronous chip (left) and an asynchronous chip (right) with the same digital functionality under the same operational conditions indicate hoe much power the chips dissipate
==> Asynhronous chips use power only during computations, while a clocked chip always consumes power because the chip is always running
Clockless Chips
==> Figure
Asynchronous Logic
==> Data signals may use either “dual rail encoding” or “data building”.
==> Each dual rail encoded Boolean is implemented as two wires.
==> 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.
==> 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
Computers Without Clocks
==> Asynchronous chips improve computer performance by letting each circuit run as fast as it can
==>Each part of an asynchronous system may extend or shorten the timing of its steps when necessary, much as a hiker takes long or short steps when walking across rough terrain
==>Asynchronous parts of otherwise synchronous systems are also beginning to appear; the Ultra SPARC IIIi processor recently introduced by SUN includes some asynchronous circuits developed by our group
Local Operation
==> A clocked system is like a bucket brigade in which each person must pass and receive buckets according to the tick tock rhythm of the clock.
==> When the clock ticks, each person pushes a bucket forward to the next person down the line.
==> When the clock tocks, each person grasps the bucket pushed forward by the preceding person.
==> Even if most of the buckets are light, everyone in the line must wait for the clock to tick before passing the next bucket
Rendezvous Circuits
==> Figure
The Need for Speed
==> Research group at Sun Microsystems concentrates on designing fast asynchronous systems.
==> They have found that speed often comes from simplicity.
==> Their initial goal was to build a counter flow pipeline with two opposing data flows – like two parallel bucket brigades moving in opposite directions.
==> They wanted the data from both flows to interact at each of these stages; the hard challenge was to ensure that every “northbound” data element would interact with every “southbound” data element. Arbitration turned out to be essential.
Conclusion
==> Clocks have served the electronics design industry very well for a long time, but there are insignificant difficulties looming for clocked design in future.
==> These difficulties are most obvious in complex SOC development, where electrical noise, power and design costs threaten to render the potential of future process technologies inaccessible to clocked design
==> Self-timed design offers an alternative paradigm that addresses these problem areas, but until now VLSI designers have largely ignored it.
==> Things are beginning to change; however, self-timed design is poised to emerge as a viable alternative to clocked design
References
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