lecture11 - Composing Decoders using Enable b high-order...

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CMPT 150 Combinational Circuits Page 1 Rudimentary Logic Functions b Consider the possible single-bit functions: b The first and last columns are examples of value- fixing b The second column is called transferring b The third column is called inverting CMPT 150 Combinational Circuits Page 2 Enabling Circuits b It is sometimes useful to be able to control when the input signal is allowed to pass through to an output - this is called enabling the circuit b One easy way to do this is to AND the output of your ircuit with an EN enable signal circuit with an EN enable signal E.g. CMPT 150 Combinational Circuits Page 3 b A Decoder is a combinational circuit that converts n inputs into at most 2 n outputs b 2x4 Dec A 0 A 1 E D 0 D 1 D 2 D 3 b An Encoder does the opposite: b Converts 2 n inputs into n outputs b b Uses: b b 4x2 Enc D 0 D 1 D 2 D 3 E A 0 A 1 CMPT 150 Combinational Circuits Page 4 Implementing a Decoder b For example, a 2x4 Decoder has a 2-bit input and 4-bit output Truth table: Logic Diagram: E A 1 A 0 D 0 D 1 D 2 D 3 0 0 1 1 0 1 1 1 0 1 1 1 0 X X
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Unformatted text preview: Composing Decoders using Enable b high-order inputs to one decoder, low-order inputs to all others b connect outputs of higher-order decoder to enables of each other A A 1 A 2 D D 1 D 2 D 3 A 3 D 4 D 5 D 6 D 7 D 8 D 9 D 10 D 11 D 12 D 13 D 14 D 15 E 4x16 Dec CMPT 150 Combinational Circuits Page 6 NAND Decoders b NAND gates require fewer transistors to implement E A 1 A D D 1 D 2 D 3 0 0 0 0 0 1 0 1 0 0 1 1 1 X X CMPT 150 Combinational Circuits Page 7 Example of Decoder Usage Computer instruction 4x16 Dec CMPT 150 Combinational Circuits Page 8 Fun With Decoders! b We can use a Dec to implement an arbitrary truth table fast b b How?: b Connect input variables to decoder select b b CMPT 150 Combinational Circuits Page 9 Fun with Decoders! x y z f g 0 0 0 0 0 1 0 1 0 1 1 1 1 1 1 D D 1 D 2 A A 1 A 2 0 1 1 1 0 0 1 0 1 1 1 0 1 1 1 1 1 3x8 Dec D 3 D 4 D 5 D 6 D 7 E...
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This note was uploaded on 04/27/2011 for the course CMPT 150 taught by Professor Dr.anthonydixon during the Spring '08 term at Simon Fraser.

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lecture11 - Composing Decoders using Enable b high-order...

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