ECE 2110: Introduction to Digital Systems Chapter 6 Combinational Logic Design Practices

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# ECE 2110: Introduction to Digital Systems Chapter 6 Combinational Logic Design Practices - PowerPoint PPT Presentation

ECE 2110: Introduction to Digital Systems Chapter 6 Combinational Logic Design Practices. Decoders. 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 2110: Introduction to Digital Systems Chapter 6 Combinational Logic Design Practices

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1. ECE 2110: Introduction to Digital SystemsChapter 6Combinational Logic Design Practices Decoders

2. 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

3. 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

4. Binary 2-to-4 decoder

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

6. 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

7. Complete 74x139 Decoder

8. 3-to-8 decoder

9. 74x138 3-to-8-decoder symbol