218L11F08 - ESE218 Lecture 11: Decoders and encoders...

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10/16/08 ESE218 Fall 2008 Lecture 11 1 ESE218 Lecture 11: Decoders and encoders Outline ± Digital system and µ P ± Binary decoders simplest decoder 2x4 binary decoders active-high outputs, minterms active-low outputs, Maxterms Enable inputs and extension Implementation of binary functions using decoders ± Encoders with priority ± Summary
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10/16/08 ESE218 Fall 2008 Lecture 11 2 von Neumann architecture John Louis von Neumann (1903-1957) "The Principles of Large-Scale Computing Machines“, 1946 Promotion of the stored program concept Both data and Instructions stored in the same memory
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10/16/08 ESE218 Fall 2008 Lecture 11 3 Address decoder N inputs 2 N outputs: 0 1 1 0 0 0 Binary Decoder Only one output is active at a time! A row code: MSB = LSB = Decoded row Addressing memory cells in RAM with binary decoders The task can be solved with an array of AND gates. Each of the AND gates decodes a particular input combination.
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10/16/08 ESE218 Fall 2008 Lecture 11 4 Decoding with one level circuit: multi-input gates needed 3 2 1 0 m# 1 0 0 0 Y 3 0 1 0 0 Y 2 0 0 1 0 Y 1 1 1 0 0 X 1 0 0 0 1 Y 0 1 0 1 0 X 0 X 0 X 1 Y 1 = X 0 X 1 Y 0 = X 0 ’X 1 Y 2 = X 0 ’X 1 Y 3 = X 0 X 1 Binary inputs In this decoder always one output is hot. The decoder is always ENABLED Decoders with active-high outputs are minterm generators Y 3 1 2 3 4 5 6 9 10 8 12 13 11 Y 2 Y 1 Y 0 = m 3 = m 2 = m 1 = m 0 Number of the gate inputs number of the input variables (at least two as in this case)
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10/16/08 ESE218 Fall 2008 Lecture 11 5 Decoders with active-low outputs… 3 2 1 0 M# 0 1 1 1 Y 3 1 0 1 1 Y 2 1 1 0 1 Y 1 1 1 0 0 X 1 1 1 1 0 Y 0 1 0 1 0 X 0 X 0 Y 3 1 2 3 4 5 6 9 10 8 12 13 11 X 1 Y 2 Y 1 Y 0 Y 1 = X 0 ’+ X 1 Y 0 = X 0 + X 1 Y 2 = X 0 + X 1 Y 3 = X 0 ’+ X 1 Binary inputs One output is 0 at a time = M 3 = M 2 = M 1 = M 0 …generate Maxterms A single output is at a low logic level, all the rest are at a high logic level
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10/16/08 ESE218 Fall 2008 Lecture 11 6 Enabling capability would require just an additional input for each gate 1 1 1 0 1 Y 1 1 1 1 1 0 Y 0 0 1 1 1 1 E 1 1 0 1
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218L11F08 - ESE218 Lecture 11: Decoders and encoders...

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