Don't-care term
In digital logic, a don't-care term[1][2] (abbreviated DC, historically also known as redundancies,[2] irrelevancies,[2] optional entries,[3][4] invalid combinations,[5][4][6] vacuous combinations,[7][4] forbidden combinations,[8][2] unused states or logical remainders[9]) for a function is an input-sequence (a series of bits) for which the function output does not matter.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.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 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.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).
Three-state logicdigital logicminimizationKarnaugh–Veitch mapsQuine–McCluskey algorithmSeymour Ginsburgfinite-state machineKarnaugh mapbinary-coded decimalpseudo-tetrades7-segment displayWrite-only registersencoding schemescommunication protocolsmulti-valued logicVeriloghardware description languageflip-flopfeedbackstate machinecosmic radiationelectrical noisesoft errorerror detectionDecision tableSide effectShort-circuit evaluationIncomplete address decodingIncomplete opcode decodingLogic redundancyUndefined behaviourUndefined variableUninitialized variableFour-valued logicNine-valued logicHertz encodingChen–Ho encodingDensely packed decimalKarnaugh, MauriceJohn Wiley & Sons Inc.Caldwell, Samuel HawksMoore, Edward ForrestThe Journal of Symbolic LogicD. Van Nostrand Company, Inc.Aiken, Howard H.Blaauw, GerritWang, AnHarvard University PressKautz, William H.I.R.E.Wireless WorldIliffe Technical Publications Ltd.McGraw-HillNewnes BooksElsevier ScienceChapman & HallSpringer Science & Business Media, B.V.Pedram, MassoudKluwer Academic PublishersSpringer Science+Business Media, LLCIEEE International Symposium on Circuits and SystemsInstitute of Electrical and Electronics EngineersGinsburg, SeymourJournal of the ACMToshiba CorporationKatz, Randy HowardThe Benjamin/Cummings Publishing Company, Inc.Cambridge University PressSpringer Science & Business MediaThe Macmillan Press Ltd.Springer-Verlag Berlin HeidelbergIntel CorporationBeizer, BorisAddison-Wesley Professional