Singer, Theodore Smith, Dexter Smith, Leonard Strong, Peter F. Kalin, Theodore Kincaid, Marshall Lucchini, E. Cali, Lloyd Canepa, Michele Ciampa, Carmela M. Blaauw, Gerrit Burkhart, William Burns, Robert J.
#BCD OUTPUT LOGIX PRO SERIES#
The Bell Telephone Laboratories Series (1 ed.).
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The machine remains stuck in the power-up state or can be moved only between other can't-happen states in a walled garden of states. In some cases, there is no combination of inputs that can exit the state machine into a normal operational state. It is sometimes possible that some states that are nominally can't-happen conditions can accidentally be generated during power-up of the circuit or else by random interference (like cosmic radiation, electrical noise or heat). Such circuits can be represented by a state machine. That is, those circuits that depend on the previous output(s) of the circuit as well as its current external inputs. Power-up states įurther considerations are needed for logic circuits that involve some feedback. In synthesized hardware, however, the actual value of such a signal will be either 0 or 1, but will not be determinable from the circuit's inputs. In simulation, an X value can result from two or more sources driving a signal simultaneously, or the stable output of a flip-flop not having been reached. In the VHDL hardware description language such values are denoted (in the standard logic package) by the letter "X" (forced unknown) or the letter "W" (weak unknown). In the Verilog hardware description language such values are denoted by the letter "X". "Don't care" may also refer to an unknown value in a multi-valued logic system, in which case it may also be called an X value or don't know. ĭon't-care states can also occur in encoding schemes and communication protocols. Write-only registers, as frequently found in older hardware, are often a consequence of don't-care optimizations in the trade-off between functionality and the number of necessary logic gates.
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Įxamples of don't-care terms are the binary values 1010 through 1111 (10 through 15 in decimal) for a function that takes a binary-coded decimal (BCD) value, because a BCD value never takes on such values (so called pseudo-tetrades) in the pictures, the circuit computing the lower left bar of a 7-segment display can be minimized to a b + a c by an appropriate choice of circuit outputs for dcba = 1010…1111. In 1958, Seymour Ginsburg proved that don't-care circuit minimization does not necessarily yield a minimal result. The designer of a logic circuit to implement the function need not care about such inputs, but can choose the circuit's output arbitrarily, usually such that the simplest circuit results ( minimization).ĭon't-care terms are important to consider in minimizing logic circuit design, including graphical methods like Karnaugh–Veitch maps and algebraic methods such as the Quine–McCluskey algorithm. Both these types of conditions are treated the same way in logic design and may be referred to collectively as don't-care conditions for brevity. An input that is known never to occur is a can't-happen term. In digital logic, a don't-care term (abbreviated DC, historically also known as redundancies, irrelevancies, optional entries, invalid combinations, vacuous combinations, forbidden combinations, unused states or logical remainders ) for a function is an input-sequence (a series of bits) for which the function output does not matter.