Re: Xilinx Synthesis Warning

Discussion in 'VHDL' started by Moikel, Mar 12, 2008.

  1. Moikel

    Moikel Guest

    Hello,

    Xilinx is giving me the following warning during synthesis:

    =========================================================================
    * Low Level
    Synthesis *
    =========================================================================
    WARNING:Xst:1988 - Unit <control>: instances <Mcompar__cmp_lt0001>,
    <Mcompar__cmp_ge0001> of unit <LPM_COMPARE_1> and unit <LPM_COMPARE_4>
    are dual, second instance is removed
    WARNING:Xst:1988 - Unit <control>: instances <Mcompar__cmp_lt0000>,
    <Mcompar__cmp_ge0000> of unit <LPM_COMPARE_1> and unit <LPM_COMPARE_4>
    are dual, second instance is removed

    Could someone please explain to me what this warning means? Can I
    safely ignore it? I have followed the link for the warning code to the
    Xilinx page, but it returns "page not found".

    Thanks,

    Mike
     
    Moikel, Mar 12, 2008
    #1
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  2. Moikel

    Guest

    On Mar 11, 8:01 pm, Moikel <> wrote:
    > Hello,
    >
    > Xilinx is giving me the following warning during synthesis:
    >
    > =========================================================================
    > * Low Level
    > Synthesis *
    > =========================================================================
    > WARNING:Xst:1988 - Unit <control>: instances <Mcompar__cmp_lt0001>,
    > <Mcompar__cmp_ge0001> of unit <LPM_COMPARE_1> and unit <LPM_COMPARE_4>
    > are dual, second instance is removed
    > WARNING:Xst:1988 - Unit <control>: instances <Mcompar__cmp_lt0000>,
    > <Mcompar__cmp_ge0000> of unit <LPM_COMPARE_1> and unit <LPM_COMPARE_4>
    > are dual, second instance is removed
    >
    > Could someone please explain to me what this warning means? Can I
    > safely ignore it? I have followed the link for the warning code to the
    > Xilinx page, but it returns "page not found".
    >
    > Thanks,
    >
    > Mike


    Can't say for sure, but it looks like some intermediate stage in the
    synthesizer decided to build a "less than" unit as well as a "greater
    than or equal" unit, and the warning reflects its realization that, in
    hardware, the two will always produce the opposite (i.e. dual) output,
    hence it is replacing one of them with an inverter. My guess: this is
    probably OK to ignore.

    - Kenn
     
    , Mar 12, 2008
    #2
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  3. Moikel

    Guest

    On Mar 11, 5:01 pm, Moikel <> wrote:
    > Hello,
    >
    > Xilinx is giving me the following warning during synthesis:
    >
    > =========================================================================
    > * Low Level
    > Synthesis *
    > =========================================================================
    > WARNING:Xst:1988 - Unit <control>: instances <Mcompar__cmp_lt0001>,
    > <Mcompar__cmp_ge0001> of unit <LPM_COMPARE_1> and unit <LPM_COMPARE_4>
    > are dual, second instance is removed
    > WARNING:Xst:1988 - Unit <control>: instances <Mcompar__cmp_lt0000>,
    > <Mcompar__cmp_ge0000> of unit <LPM_COMPARE_1> and unit <LPM_COMPARE_4>
    > are dual, second instance is removed
    >
    > Could someone please explain to me what this warning means? Can I
    > safely ignore it? I have followed the link for the warning code to the
    > Xilinx page, but it returns "page not found".
    >
    > Thanks,
    >
    > Mike


    To expand on what Kenn said:

    XST has found something like this:

    A = '1' when (B > C) else '0';
    D = '0' when (C >= B) else '1';

    Well, it turns out that 'A' and 'D' are the same, so it throws the 2nd
    equation away (2nd instance is removed), and substitutes 'A' for every
    'D'.

    You can ignore these warnings. Unless you're expecting the two
    signals to be different.

    G.
     
    , Mar 13, 2008
    #3
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