tutorial6 - ERG2020 A&B Tutorial 6 Outline 1. 2. 3. 4....

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ERG2020 A&B Tutorial 6 06-03-2007 Kai Caihong chkai6@ie.cuhk.edu.hk Outline 1. Decoder 2. Three State Outputs 3. XOR gates 4. Examples
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What is Decoder Decoder is a converter which converts coded multiple inputs to coded multiple outputs. Generally, input code has fewer bits than the output code. There is a one-to-one mapping between input code words and output code words Typically n inputs, 2 n outputs 2-to-4, 3-to-8, 4-to-16, etc The general structure is shown on RHS. Enable inputs must be asserted for the decoder to perform its normal mapping function.
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3-to-8 Decoder A commercial 3-to-8 decoder is 74x138 decoder. The truth table is as follows:
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Cascading For decoding larger code words (4-to-16 decoder), multiple small-size decoders (2-to4 or 3-to-8 decoder) can be cascaded to achieve the purpose. The general principle: make use of the most significant input bit(s) as the enable inputs for the small-size decoders A design example of a 4-to- 16 decoder using 74x138s is shown in figure
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tutorial6 - ERG2020 A&B Tutorial 6 Outline 1. 2. 3. 4....

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