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5.5 Encoders

input code word. map. output code word. enable inputs. Encoder. Return. Next. 5.5 Encoders.

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5.5 Encoders

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  1. input code word map output code word enable inputs Encoder Return Next 5.5 Encoders • A encoder is a multiple-input, multiple-output logic circuit that converts coded inputs into coded outputs, where the input and output codes are different. The output code generally has fewer bits than the input code, and there is one-to-one mapping from input code words into output code words.

  2. Binary encoder I0 Y0 I1 Y1 I2 : : : : Yn-1 I2n-1 Return Back Next 5.5 Encoders • Binary encoder • The most common decoder circuit is an 2n-to-nencoder or binary encoder. Such a encoder has an 2n-bit binary input code and n-bitoutput binary code. Write the truth table for 8-to-3 binary encoder and the output logic function expressions.

  3. Return Back Next 5.5 Encoders Simulation Suppose the inputs I2 and I4 are both 1, what is the output ? If multiple requests can be made simultaneously, the encoder gives undesirable results.

  4. 74X148 EI I7 A2 I6 A1 I5 A0 I4 I3 GS I2 EO I1 I0 5 6 4 7 3 9 2 1 13 14 12 15 11 10 Return Back Next 5.5 Encoders • Priority Encoders The solution is to assign priority to the input lines, so that when multiple inputs are asserted, the encoding device produces the number of the highest-priority input. Such a device is called a priority encoder. • The 74x148 Priority Encoders

  5. Return Back Next 5.5 Encoders • Input data I0-I7 • Enable input EI • Output GS is asserted when the device is enabled and one or more of the request are asserted. • Output EO is asserted if EI is assert but no request input is asserted, it designed to be connected to the EI input of another ‘148 that handles lower-priority requests.

  6. Return Back Next 5.5 Encoders • Truth table for a 74x148 8-input priority encoder

  7. Return Back 5.5 Encoders • Logic diagram for the 74x148 priority encode (p379 Figure 5-50) • Cascading Binary Encoders (p380 Figure 5-51)

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