Why so few clockless CPUs?

Discussion in 'Highly Technical' started by bryanl, Dec 9, 2012.

  1. bryanl

    bryanl Golden Member

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    AKA asynchronous CPUs, especially for low power applications?
     
  2. irishScott

    irishScott Lifer

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    There are numerous examples, but in a nutshell it goes back to the reliability of the system. Asynchronous processors may work great on paper (emphasis on "may", because in many cases the added hardware logic needed to compensate for a lack of clock adds cost), but in reality are highly unstable unless employed on small-scale, specialized applications. High or lower power requirements have nothing to do with it, it's all a question of complexity and reliability.

    Wikipedia sums it up nicely.
    http://en.wikipedia.org/wiki/Asynchronous_circuit

     
  3. PandaBear

    PandaBear Golden Member

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    I was taking a class back in school that the professor hijacked into teaching asynchronous circuits. The biggest problem is the result of calculation would not only be unstable, but the uncertainty increase exponentially as the logic complexity increase. It also is significantly larger in size as more gates are needed to keep result in stable zone between initial and final value, and the only advantage we see is the reduction of power usage due to cutting out unnecessary switching.

    Not only that, a seemingly harmless small change can ripple through the entire design and force you to redo a lot of the work, and make real work unpredictable in terms of schedule and very hard to validate the result.

    Since power can be reduced with other means, like low power mode or power down mode, stopping the clock, power gating, etc. Clockless circuit (let along CPU) is just not going to be a good choice.