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Encoder and decoder

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Encoder & Decoder
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Encoder and decoder

subject : digital logic and design DLD

subject : digital logic and design DLD

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Encoder and decoder

1. 1. BUITEMS 1
2. 2. ENCODERS Encoder is a combinational circuit which is designed to perform the inverse operation of the decoder. An encoder has n number of input lines and m number of output lines. An encoder produces an m bit binary code corresponding to the digital input number. The encoder accepts an n input digital word and converts it into an m bit another digital word. BUITEMS 2
3. 3. BLOCK DIAGRAM BUITEMS 3
4. 4. TYPES OF ENCODERS Examples of Encoders are following. 1. Priority encoders 2. Decimal to BCD encoder 3. Octal to binary encoder 4. Hexadecimal to binary encoder BUITEMS 4
5. 5. PRIORITY ENCODER This is a special type of encoder. Priority is given to the input lines. If two or more input line are 1 at the same time, then the input line with highest priority will be considered. There are four input D0, D1, D2, D3 and two output Y0, Y1. Out of the four input D3 has the highest priority and D0 has the lowest priority. That means if D3 = 1 then Y1 Y0 = 11 irrespective of the other inputs. Similarly if D3 = 0 and D2 = 1 then Y1 Y0 = 10 irrespective of the other inputs. BUITEMS 5
6. 6. BLOCK DIAGRAM BUITEMS 6
7. 7. TRUTH TABLE BUITEMS 7
8. 8. LOGIC CIRCUIT BUITEMS 8
9. 9. DECIMAL TO BCD ENCODER A decimal to BCD encoder has 10 input lines D0 to D9 and 4 output lines Y0 to Y3. Below is the truth table for a decimal to BCD encoder. Below boolean functions are formed by ORing all the input lines for which output is 1. For instance Y0 is 1 for D1, D3, D5, D7 & D9 input lines. Y0 = D1 + D3 + D5 + D7 + D9 Y1 = D2 + D3 + D6 + D7 Y2 = D4 + D5 + D6 + D7 Y3 = D8 + D9 The decimal to BCD encoder can therefore be implemented with OR gates whose inputs are determined directly from truth table as shown in the image below. BUITEMS 9
10. 10. TRUTH TABLE BUITEMS 10
11. 11. BLOCK DIAGRAM BUITEMS 11
12. 12. CIRCUIT DIAGRAM BUITEMS 12
13. 13. LIMITATIONS WITH DECIMAL TO BCD ENCODER A decimal to BCD encoder has limitations. 1. For 10 bits input, there can be 210 possible combinations, out of which only 10 are used using 4 output lines. 2. The discrepancy of 0 output due to all inputs being 0 or D0 being 0 is resolved by using additional input known as Enable 3. Only one input can be active at any given time. BUITEMS 13
14. 14. 74HC147 (DECIMAL TO BCD ENCODER) BUITEMS 14
15. 15. 74LS148 (8 LINE TO 3 LINE ENCODER) Priority encoders are available in standard IC form and the TTL 74LS148 is an 8-to-3 bit priority encoder which has eight active LOW (logic “0”) inputs and provides a 3-bit code of the highest ranked input at its output. Priority encoders output the highest order input first for example, if input lines “D2“, “D3” and “D5” are applied simultaneously the output code would be for input “D5” (“101”) as this has the highest order out of the 3 inputs. Once input “D5” had been removed the next highest output code would be for input “D3” (“011”), and so on. BUITEMS 15
16. 16. BLOCK DIAGRAM AND TRUTH TABLE BUITEMS 16
17. 17. THE END BUITEMS 17