Don't-care

Don't-care

In logic synthesis and logic simulation a don't care or X value is one value in a multi-valued logic system that denotes an unknown value, or a value that the designer (for whatever reason) does not care about. In the Verilog hardware description language such values are denoted by the letter "X". 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).cite book|title=Vhdl: A Logic Synthesis Approach|author=David Naylor and Simon Jones|date=1997|publisher=Springer|id=ISBN 0412616505|pages=14–15,219,221]

An X value does not exist in hardware. In simulation, an X value can result from two or more sources driving a signal simultaneously, or the stable output of a flip-flop (electronics) 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.

Synthesis tools can use don't care values to determine where and how to perform area optimization. A synthesis tool can use don't care values to reduce the circuit sizes of finite state machines, for example.

ee also

* Karnaugh map

References


Wikimedia Foundation. 2010.

Игры ⚽ Поможем решить контрольную работу

Look at other dictionaries:

  • Don’t-Care — In der Digitaltechnik bedeutet Don’t Care, X oder , dass dieser Wert keinen Einfluss auf die Logikschaltung hat. In der Wahrheitstabelle werden Don’t Care Einträge typischerweise eingesetzt um nicht alle Eingangszustände abhandeln zu müssen, oder …   Deutsch Wikipedia

  • Don't Care — This article is about the music single. For the meaning relating to digital logic, see Don t care (logic). Don t Care Single by Obituary …   Wikipedia

  • Don't-Care — In der Digitaltechnik bedeutet Don’t Care, X oder , dass dieser Wert keinen Einfluss auf die Logikschaltung hat. In der Wahrheitstabelle werden Don’t Care Einträge typischerweise eingesetzt um nicht alle Eingangszustände abhandeln zu müssen, oder …   Deutsch Wikipedia

  • Don't Care High — Don’t Care High   Author(s) Gordon Korman Country …   Wikipedia

  • don’t care was made to care — Quot. 1959 gives the version of the traditional children’s rhyme found at Hackney, London. 1959 I. & P. OPIE Lore and Language of Schoolchildren iii. 50 To ‘don’t cares’ the traditional saying is: Don’t care was made to care, Don’t care was hung …   Proverbs new dictionary

  • Don't-care term — In digital logic, a don t care term is an input sequence (a series of bits) to a function that the designer does not care about, usually because that input would never happen, or because differences in that input would not result in any changes… …   Wikipedia

  • Don't-care (logic) — In digital logic, a don t care term is an input sequence (a series of bits) to a function that the designer does not care about, usually because that input would never happen, or because differences in that input would not result in any changes… …   Wikipedia

  • Don't Care 'bout Nothin' — Single infobox Name = Don t Care bout Nothin Type = Single Artist = Shotgun Messiah Released = 1989 | Don t Care bout Nothin is a single released by Shotgun Messiah from their Shotgun Messiah album in 1990.Track listing …   Wikipedia

  • Don't Care — • macht nichts, es gibt schlimmeres : ) • Wildcard bei Bitoperationen, dessen Wert irrelevant für das Ergebnis ist (normalerweise durch einen Asterix dargestellt) …   Acronyms

  • Don't Care — [1] macht nichts, es gibt schlimmeres : ) [2] Wildcard bei Bitoperationen, dessen Wert irrelevant für das Ergebnis ist (normalerweise durch einen Asterix dargestellt) …   Acronyms von A bis Z

Share the article and excerpts

Direct link
… Do a right-click on the link above
and select “Copy Link”