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Editorial Team
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Asked: 2026-05-14T01:12:49+00:00

i have been told to use ‘when’ statement to make multiplexer but not use

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i have been told to use ‘when’ statement to make multiplexer but not use ‘if’ statement as it will cause timing errors…
i don’t understand this …
so what is the difference between ‘if’ and ‘when’ ? and do they map to the same thing in hardware ?

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1 Answer

  1. Editorial Team

    OK, lets discuss some points at first on the difference between if and when statements:

    • Both are called Dataflow Design Elements.

    when statement

    • concurrent statement
    • not used in process, used only in architecture as process is sequential execution

    if statement

    • sequential statement
    • used in process as it is sequential statement, and not used outside the process

    And you know multiplexer is a component don’t need process block, as its behavior doesn’t change with changing its input, so it will be outside process, so you have to write it using when statement as it is concurrent statement.. And if you wrote it with if statement, timing errors may occur. Also all the references and also Xilinx help (if you are using Xilinx) are writing the Multiplexer block using when statement not if statement

    Reference: Digital Design Priciples & Practices, John F. Wakerly, 3rd Edition

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