1. ECE 2372 Modern Digital System Design
Section 4.2
Decoders 1

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

3. 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

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

5. Decoders 5

6. Decoders 6

7. Decoders
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. 74139 2-Line-to-4-Line Decoder/Demultiplexer

10. 74139 2-Line-to-4-Line Decoder/Demultiplexer10 10

11. 74139 2-Line-to-4-Line Decoder/Demultiplexer11 11

12. 74139 2-Line-to-4-Line Decoder/Demultiplexer12 12

13. 74139 2-Line-to-4-Line Decoder/Demultiplexer

14. 74138 3-Line-to-8-Line Decoder/Demultiplexer14

15. 74138 3-Line-to-8-Line Decoder/Demultiplexer15 15

16. 74138 3-Line-to-8-Line Decoder/Demultiplexer16 16

17. 74138 3-Line-to-8-Line Decoder/Demultiplexer17 17

18. 74238 3-Line-to-8-Line Decoder/Demultiplexer18 18

19. 74138 3-Line-to-8-Line Decoder/Demultiplexer19

20. 74138 3-Line-to-8-Line Decoder/Demultiplexer20

21. 74154 4-Line-to-16-Line Decoder/Demultiplexer21

22. 74154 4-Line-to-16-Line Decoder/Demultiplexer22 22

23. 74154 4-Line-to-16-Line Decoder/Demultiplexer23 23

24. 74154 4-Line-to-16-Line Decoder/Demultiplexer24 24

25. 74154 4-Line-to-16-Line Decoder/Demultiplexer25

26. 74154 4-Line-to-16-Line Decoder/Demultiplexer26

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 Decoders29

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

31. Applications of Decoders31

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 2n – 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 (Y0 – Y9). 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