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ECE 3110: Introduction to Digital Systems Chapter 6 Combinational Logic Design Practices

ECE 3110: Introduction to Digital Systems Chapter 6 Combinational Logic Design Practices. Decoders. Previous…. Programmable Logic Devices PLAs PALs. Decoder. Multiple-input/multiple-output device. Inputs ( n ) are less than outputs ( m ). Converts input code words into output code words.

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ECE 3110: Introduction to Digital Systems Chapter 6 Combinational Logic Design Practices

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  1. ECE 3110: Introduction to Digital SystemsChapter 6Combinational Logic Design Practices Decoders

  2. Previous… • Programmable Logic Devices • PLAs • PALs

  3. Decoder • Multiple-input/multiple-output device. • Inputs ( n ) are less than outputs ( m ). • Converts input code words into output code words. • One-to-One mapping :- Each input code produces only one output code. • Input codes :- Binary Code- Your Code ! • Output Codes 1-out-of-m code Gray Code BCD Code enable inputs

  4. Note “x” (don’t care) notation. Binary Decoder • n-to-2^n decoder : n inputs and 2^n outputs. • Input code : n bit Binary Code. • Output code : 1-out-of-2^n , One output is asserted for each input code. • Example : n=2, 2-to-4 decoder

  5. Binary 2-to-4 decoder

  6. MSI 2-to-4 decoder • Input buffering (less load) • NAND gates (faster)

  7. 74x139 : Logic Symbol -Truth Table • Active Low Enable, Active Low outputs • Truth Table Logic Symbol Inputs OutputsG_L B A Y3 Y2 Y1 Y0 1 x x 1 1 1 1 0 0 0 1 1 1 0 0 0 1 1 1 0 1 0 1 0 1 0 1 1 0 1 1 0 1 1 1 1/2 74x139 Y0 Y0_L G_L G Y1 Y1_L A A Y2 Y2_L B B Y3 Y3_L

  8. Complete 74x139 Decoder

  9. 3-to-8 decoder

  10. 74x138 3-to-8-decoder symbol

  11. Decoder cascading 4-to-16 decoder

  12. More cascading 5-to-32 decoder

  13. Implementing the Canonical Sum • The binary decoder generates all minterms of n-variable logic function. • The canonical sum ( sum of minterms ) of a logic functions is obtained by adding all minterms of that function: -Match the order of input bits -Activate Enable inputs • Example : +5V 74x138 Y0 G1 Y1 G2A Y2 G2B F Y3 Y4 Z A Y5 Y B Y6 X C Y7

  14. Logic design using Decoders • Advantages : - Flexibility- Multiple-output Logic functions • Disadvantages :- Complexity : for large number of inputs ( 5-variable Function with 3 minterms ! F= AB’CD’E + A’BC’DE+A’BCDE’ ) • A practical alternative : PLD’s

  15. Decoder applications • Microprocessor memory systems • selecting different banks of memory • Microprocessor input/output systems • selecting different devices • Microprocessor instruction decoding • enabling different functional units • Memory chips • enabling different rows of memory depending on address • Lots of other applications

  16. Next… • Encoders • Reading Wakerly CH-6.5

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