RE: Issue with Generate

From: Francoise Martinolle (fm@cadence.com)
Date: Sat Feb 26 2005 - 05:31:14 PST

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    SystemVerilog did not change the rules for constant expressions.

    Francoise
        '

    -----Original Message-----
    From: owner-btf@boyd.com [mailto:owner-btf@boyd.com] On Behalf Of Kausik
    Datta
    Sent: Saturday, February 26, 2005 4:47 AM
    To: 'Steven Sharp'; pgraham
    Cc: btf@boyd.com
    Subject: RE: Issue with Generate

    Hi Steven,
    I agree with your point, at least V2K LRM doesn't support the hier-ref in
    constant expression, I am not sure of SV.
    But the following example is passed by MTI simulator.

    module top;
      generate
        if (I1.p == 1 )
          M2 I1();
        else
          M3 I1();
      endgenerate
      M3 I1();
    endmodule

    module M2;
      parameter p = 10;
    endmodule

    module M3;
      parameter p = 1;
    endmodule

    Thanks
    Kausik
     

    -----Original Message-----
    From: owner-btf@boyd.com [mailto:owner-btf@boyd.com] On Behalf Of Steven
    Sharp
    Sent: Saturday, February 26, 2005 2:01 AM
    To: pgraham@cadence.com; kausikd@cal.interrasystems.com
    Cc: btf@boyd.com
    Subject: RE: Issue with Generate

    >No. In 1364-2001 LRM generate doesn't define any scope.
    >I think in latest draft 3 or 4 some scope is defined like that.

    That change is intended to be regarded as an erratum, so generate constructs
    should define a scope in 1364-2001. But as you say, the issue is not with
    the defparam. The example could be modified to get around that issue, by
    naming the scope and adding it to the hierarchical name in the defparam.

    >Issue is with the line
    >
    > if ( I1.p == 1 )
    >
    >Even if you remove the defparam line none of the simulator will be able
    >to elaborate this design.

    And they are correct. The condition of a conditional generate must be a
    constant expression. Hierarchical references are not legal in constant
    expressions. Therefore, this is illegal by the existing LRM.
    I was unable to find a place in the LRM text that made clear that
    hierarchical references are not legal in constant expressions, but you can
    track it down through the BNF.

    The kind of circular dependency that appears in this example is part if why
    hierarchical references are not legal in constant expressions.

    Steven Sharp
    sharp@cadence.com

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