Source : Free On-Line Dictionary of Computing

asynchronous logic
     
         A {data-driven} circuit 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 bundling".  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 signalling 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 signalling.  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 sensitive designs
        can use simpler, stateless logic gates but require a "return
        to zero" phase in each transition.
     
        {(http://www.cs.man.ac.uk/amulet/async/)}.
     
        (1995-01-18)