Decoders, Encoders, and Multiplexers Four Important Circuits student version

Decoders, Encoders, and Multiplexers Four Important Circuits student version

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1 EC262 Topic 9: Four Important Circuits The Binary Decoder A binary decoder is a circuit that has n inputs and (usually) up to 2 n outputs. Based on the n inputs, one of the decoder’s outputs is selected. To motivate the idea, consider the small section of a Central Processing Unit (CPU) shown below. This CPU has a four special purpose high-speed registers to hold important data. To write data to a register, we need to place a logical 1 on its Write Activate , and then the data can be fed in on the input marked Data In . Say we want to write the data value 2014 to register 2 (and only register 2!). When we want to write data to any register, we activate the Write signal. We would then place the data 2014 on the line labeled Register Data . What happens? How do we ensure that the data 2014 This is where a decoder comes in! On the decoder’s 2-bit input, we will place the
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2 Design a two-input, four-output decoder. Truth table: Circuit: Suppose you would like to implement this decoder using an active low output. What would be your truth table? Circuit: From A. Marcovitz, Introduction to Logic Design , McGraw Hill, 2010
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3 Decoders usually have one or more enable inputs. When an enable input is active, the encoder works as described above. When the enable input is not active, all of the decoder’s outputs are set to the inactive state (i.e., 0 if active high, 1 if active low). Consider the decoder shown below, as a block diagram and as a circuit.
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Decoders, Encoders, and Multiplexers Four Important Circuits student version

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