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From The Free On-line Dictionary of Computing (05 January 2017) [foldoc]:
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://cs.man.ac.uk/amulet/async/.
(1995-01-18)