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ECE 2372 Modern Digital System Design Section 4.2 Decoders 1.

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Presentation on theme: "ECE 2372 Modern Digital System Design Section 4.2 Decoders 1."— Presentation transcript:

1 ECE 2372 Modern Digital System Design Section 4.2 Decoders 1

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

3 Decoders n – 2 n Decoder: n-input lines (n-bit word input) and 2 n 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

4 Decoders Parallel 2-Bit Decoder (with inputs A and B, and outputs m 0, m 1, m 2, and m 3 ) is described by the truth table: 4

5 Decoders 5

6 6

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

8 Decoders An alternative decoder structure is: 8

9 Line-to-4-Line Decoder/Demultiplexer 9

10 10

11 Line-to-4-Line Decoder/Demultiplexer 11

12 Line-to-4-Line Decoder/Demultiplexer 12

13 Line-to-4-Line Decoder/Demultiplexer 13

14 Line-to-8-Line Decoder/Demultiplexer 14

15 Line-to-8-Line Decoder/Demultiplexer 15

16 Line-to-8-Line Decoder/Demultiplexer 16

17 Line-to-8-Line Decoder/Demultiplexer 17

18 Line-to-8-Line Decoder/Demultiplexer 18

19 Line-to-8-Line Decoder/Demultiplexer 19

20 Line-to-8-Line Decoder/Demultiplexer 20

21 Line-to-16-Line Decoder/Demultiplexer 21

22 Line-to-16-Line Decoder/Demultiplexer 22

23 Line-to-16-Line Decoder/Demultiplexer 23

24 Line-to-16-Line Decoder/Demultiplexer 24

25 Line-to-16-Line Decoder/Demultiplexer 25

26 Line-to-16-Line Decoder/Demultiplexer 26

27 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

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

29 Applications of Decoders 29

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

31 Applications of Decoders 31

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

33 33

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

35 Applications of Decoders BCD to Decimal Decoders: The input is a BCD word (DCBA), the output is one of the output lines (Y 0 – Y 9 ). The inputs are related to the outputs by the truth table (note don’t-cares). 35

36 36

37 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

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

39 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

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

41 Applications of Decoders The numerals 0 – 9 are represented by: 41

42 42


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