What is "c-element"

thumb|upright=1.5|Delays in the naïve (Earle latch based) implementation and environment thumb|upright=1.5|Timing diagram of a C-element and inclusive OR gate thumb|upright=1.8|Majority gate realization of C-element and inclusive OR gate (a); Realizations proposed by Maevsky (b), Starodoubtsev (c) and Murphy (d) thumb|upright=2.2|Static implementations of two- and three input C-element thumb|upright=1.7|Semi-static implementations of two- and multiple input C-element based on the ideas from thumb|upright=1.7|David cell (a) and its fast implementations: gate-level (b) and transistor-level (c) The Muller C-element (C-gate, Hysteresis flip-flop or sometimes, coincident flip-flop, two-hand safety circuit) is a small digital block widely used in design of asynchronous circuits and systems. It has been specified formally in 1955 by David E. Muller and first used in ILLIAC II computer. In terms of the theory of lattices, the C-element is a semimodular distributive circuit, whose operation in time is described by Hasse diagram. The C-element is closely related to the Rendezvous and Join elements, where an input is not allowed to change twice in succession. For asynchronous circuits, where relations betweens some delays are known in advance, the requirements for C-element can be less stiff. Earlier techniques for implementing the C-element include Schmidt trigger, Eccles-Jordan flip-flop and last moving point flip-flop.