# Decoder

Decoder
For the drum and bass musician, see Decoder (artist). For the band, see Decoder (band)
A Digitrax DH163AT DCC decoder in an Athearn locomotive before the shell goes on.

A decoder is a device which does the reverse operation of an encoder, undoing the encoding so that the original information can be retrieved. The same method used to encode is usually just reversed in order to decode. It is a combinational circuit that converts binary information from n input lines to a maximum of 2n unique output lines.

In digital electronics, a decoder can take the form of a multiple-input, multiple-output logic circuit that converts coded inputs into coded outputs, where the input and output codes are different. e.g. n-to-2n, binary-coded decimal decoders. Enable inputs must be on for the decoder to function, otherwise its outputs assume a single "disabled" output code word. Decoding is necessary in applications such as data multiplexing, 7 segment display and memory address decoding.

The example decoder circuit would be an AND gate because the output of an AND gate is "High" (1) only when all its inputs are "High." Such output is called as "active High output". If instead of AND gate, the NAND gate is connected the output will be "Low" (0) only when all its inputs are "High". Such output is called as "active low output".

Example: A 2-to-4 Line Single Bit Decoder

A slightly more complex decoder would be the n-to-2n type binary decoders. These type of decoders are combinational circuits that convert binary information from 'n' coded inputs to a maximum of 2n unique outputs. We say a maximum of 2n outputs because in case the 'n' bit coded information has unused bit combinations, the decoder may have less than 2n outputs. We can have 2-to-4 decoder, 3-to-8 decoder or 4-to-16 decoder. We can form a 3-to-8 decoder from two 2-to-4 decoders (with enable signals).

Similarly, we can also form a 4-to-16 decoder by combining two 3-to-8 decoders. In this type of circuit design, the enable inputs of both 3-to-8 decoders originate from a 4th input, which acts as a selector between the two 3-to-8 decoders. This allows the 4th input to enable either the top or bottom decoder, which produces outputs of D(0) through D(7) for the first decoder, and D(8) through D(15) for the second decoder.

A decoder that contains enable inputs is also known as a decoder-demultiplexer. Thus, we have a 4-to-16 decoder produced by adding a 4th input shared among both decoders, producing 16 outputs.

## Row select

Most kinds of random-access memory use a n-to-2n decoder to convert the selected address on the address bus to one of the row address select lines. A decoder is a device which does the reverse of an encoder, undoing the encoding so that the original information can be retrieved. The same method used to encode is usually just reversed in order to decode.

## Instruction decoder

In CPU design, the instruction decoder is the part of the CPU that converts the bits stored in the instruction register -- or, in CPUs that have microcode, the microinstruction -- into the control signals that control the other parts of the CPU.

A simple CPU with 8 registers may use 3-to-8 logic decoders inside the instruction decoder to select two source registers of the register file to feed into the ALU as well as the destination register to accept the output of the ALU. A typical CPU instruction decoder also includes several other things.

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### Look at other dictionaries:

• décoder — [ dekɔde ] v. tr. <conjug. : 1> • 1959; de dé et code, d apr. l angl. to decode ♦ Didact. Traduire dans un autre code, en langage clair (un message formulé en code). ⇒ déchiffrer, décrypter. ♢ Analyser le contenu de (un message) selon le… …   Encyclopédie Universelle

• decoder — decodér s. n., pl. decodére Trimis de siveco, 10.08.2004. Sursa: Dicţionar ortografic ﻿ DECODÉR s.n. Dispozitiv al unui calculator electronic, care permite decodarea (decoda) informaţiilor. [< fr. décodeur]. Trimis de LauraGellner, 17.02.2005 …   Dicționar Român

• decoder — de‧cod‧er [diːˈkəʊdə ǁ ˈkoʊdər] noun [countable] a piece of equipment that you need in order to receive certain types of television service * * * decoder UK US /diːˈkəʊdər/ noun [C] ► a piece of equipment that changes electronic signals into a… …   Financial and business terms

• decoder — /deˈkoder, ingl. dɪˈkəudə(r)/ [vc. ingl., comp. di de «de » e coder «codificatore»] s. m. inv. decodificatore …   Sinonimi e Contrari. Terza edizione

• decoder — [dē kōd′ər, dē′ kōd′ər] n. 1. a person who decodes messages 2. a device that decodes scrambled messages sent by telephone 3. in an electronic digital computer, a circuit device that determines the content of a given instruction or performs… …   English World dictionary

• Decoder — Даррен Бил Darren Beale 2006 год …   Википедия

• Decoder — Unter einem Dekodierer oder Decoder (engl.) versteht man in der Regel einen Umsetzer, Konverter oder Wandler für digitale oder analoge Signale. Er kann mit einem Umsetzer bzw. einer Kodiereinheit, auch Kodierer oder Encoder genannt, eine logische …   Deutsch Wikipedia

• Decoder — Decodierer; Entschlüsseler * * * De|co|der 〈m. 3〉 Gerät zum Dekodieren * * * De|co|der [auch: dɪ koʊdɐ ], der; s, [engl. decoder, zu: to decode, ↑ dekodieren] (Elektronik): Vorrichtung zur Decodierung codierter Signale, ↑ Befehle …   Universal-Lexikon

• decoder — UK [diːˈkəʊdə(r)] / US [dɪˈkoʊdər] noun [countable] Word forms decoder : singular decoder plural decoders a piece of equipment that changes digital electronic signals into a picture and sound on your television …   English dictionary

• Decoder — De|co|der [di koudə] der; s, <aus gleichbed. engl. decoder zu to decode »entschlüsseln«; vgl. ↑Code> Datenentschlüsseler in einem ↑Computer, Stereorundfunkgerät, Nachrichtenübertragungssystem; Ggs. ↑Encoder …   Das große Fremdwörterbuch