Download presentation
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/Demultiplexer
10 10
11
74139 2-Line-to-4-Line Decoder/Demultiplexer
11 11
12
74139 2-Line-to-4-Line Decoder/Demultiplexer
12 12
13
74139 2-Line-to-4-Line Decoder/Demultiplexer
14
74138 3-Line-to-8-Line Decoder/Demultiplexer
14
15
74138 3-Line-to-8-Line Decoder/Demultiplexer
15 15
16
74138 3-Line-to-8-Line Decoder/Demultiplexer
16 16
17
74138 3-Line-to-8-Line Decoder/Demultiplexer
17 17
18
74238 3-Line-to-8-Line Decoder/Demultiplexer
18 18
19
74138 3-Line-to-8-Line Decoder/Demultiplexer
19
20
74138 3-Line-to-8-Line Decoder/Demultiplexer
20
21
74154 4-Line-to-16-Line Decoder/Demultiplexer
21
22
74154 4-Line-to-16-Line Decoder/Demultiplexer
22 22
23
74154 4-Line-to-16-Line Decoder/Demultiplexer
23 23
24
74154 4-Line-to-16-Line Decoder/Demultiplexer
24 24
25
74154 4-Line-to-16-Line Decoder/Demultiplexer
25
26
74154 4-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 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
Similar presentations
© 2025 SlidePlayer.com. Inc.
All rights reserved.