Why do some 3 to 5 decoders only have certain inputs decoded?

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SUMMARY

The discussion focuses on the functionality of 3-to-5 decoders, specifically addressing why certain inputs are decoded while others are not. It clarifies that the outputs are designed to respond only to specific 3-bit inputs, with other states categorized as "don't care." The design process aims to find unique combinations of bits to generate required outputs efficiently. Additionally, the ENABLE (EN) signal is identified as a crucial component in the decoding process.

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  • Understanding of digital logic design principles
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  • Knowledge of binary number representation
  • Basic concepts of combinational logic and enable signals
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  • Study the design and implementation of 3-to-5 decoders in digital circuits
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Digital circuit designers, electrical engineering students, and professionals working with combinational logic systems will benefit from this discussion.

alvinliu
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I don't know why they circle the 001 .011. 100.101. 110 .because I saw the textbook showed for 3to 4 decoder. Y0 =1 since 00. Y1 = 1 since 01. Y2 =1 since 10,Y3 = 1 since 11.

But for this problem . I am so confused.
 

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My guess is that the need was for an output only for the listed 3-bit inputs. Other states come under the "don't care" category. All that is required in the 'design' process is to find a unique combination of bits on the input (the fewer the better) to generate each of those required outputs. There is more than one valid design. e.g., an alternative which won't conflict with any of the other decoding appears to be Y1=I1.and.I0

There would be plenty of applications where they'd want every input decoded to an output, so none of these shortcuts would be possible.

The EN signal looks like an ENABLE line.
 
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