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**ECE 2372 Modern Digital System Design**

Section 4.2 Decoders 1

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Decoders A decoder is a circuit used to “translate” a code word to another code representation. 2

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Decoders n – 2n Decoder: n-input lines (n-bit word input) and 2n output lines. For each possible word, one (and only one) output is asserted to a logical 1, all other lines will remain at logical 0. 3

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Decoders Parallel 2-Bit Decoder (with inputs A and B, and outputs m0, m1, m2, and m3) is described by the truth table: 4

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Decoders 5

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Decoders 6

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Decoders Enable Controls provide a selection to either inhibit (disable) or allow (enable) the decoder output. 7

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Decoders An alternative decoder structure is: 8

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**74139 2-Line-to-4-Line Decoder/Demultiplexer**

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**74139 2-Line-to-4-Line Decoder/Demultiplexer**

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**74139 2-Line-to-4-Line Decoder/Demultiplexer**

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**74139 2-Line-to-4-Line Decoder/Demultiplexer**

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**74139 2-Line-to-4-Line Decoder/Demultiplexer**

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**74138 3-Line-to-8-Line Decoder/Demultiplexer**

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**74138 3-Line-to-8-Line Decoder/Demultiplexer**

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**74138 3-Line-to-8-Line Decoder/Demultiplexer**

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**74138 3-Line-to-8-Line Decoder/Demultiplexer**

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**74238 3-Line-to-8-Line Decoder/Demultiplexer**

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**74138 3-Line-to-8-Line Decoder/Demultiplexer**

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**74138 3-Line-to-8-Line Decoder/Demultiplexer**

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**74154 4-Line-to-16-Line Decoder/Demultiplexer**

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**74154 4-Line-to-16-Line Decoder/Demultiplexer**

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**74154 4-Line-to-16-Line Decoder/Demultiplexer**

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**74154 4-Line-to-16-Line Decoder/Demultiplexer**

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**74154 4-Line-to-16-Line Decoder/Demultiplexer**

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**74154 4-Line-to-16-Line Decoder/Demultiplexer**

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**Applications of Decoders**

Implementing Logic Functions Using Decoders: The Decoder selects an output line corresponding to the minterm represented by the binary value of the input word. This can be used to implement any switching function, often with fewer integrated circuits. 27

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**Applications of Decoders**

Consider the (simplified) 3-Line-to-8-Line Decoder and the switching function: 28

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**Applications of Decoders**

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**Applications of Decoders**

Consider the (simplified) 3-Line-to-8-Line Decoder and the switching function: 30

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**Applications of Decoders**

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**Applications of Decoders**

Recall, the 4-person vote-tallying machine may be implemented by: 32

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**Applications of Decoders**

Address Decoding: One of 2n – 1 devices is selected when the address of that device is broadcast over a common data bus and decoded by the decoder. 34

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**Applications of Decoders**

BCD to Decimal Decoders: The input is a BCD word (DCBA), the output is one of the output lines (Y0 – Y9). The inputs are related to the outputs by the truth table (note don’t-cares). 35

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**Applications of Decoders**

Display Decoders: LED displays require decoders to illuminate the appropriate LED for a given code word. An LED emits light when it is forward biased and current flows from anode to cathode. Typically a current limiting resistor must be included in series with the LED. 37

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**Applications of Decoders**

A number of LEDs (as in a display) may be connected common anode (LEDs illuminate with logical 0 input). 38

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**Applications of Decoders**

A number of LEDs (as in a display) may be may be connected common cathode (LEDs illuminate with logical 1 input). 39

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**Applications of Decoders**

In a seven-segment display the individual LEDs are arranged: 40

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**Applications of Decoders**

The numerals 0 – 9 are represented by: 41

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