DataTypes - Please vote no

From: Clifford E. Cummings (cliffc@sunburst-design.com)
Date: Sun Nov 21 2004 - 20:37:21 PST

  • Next message: Francoise Martinolle: "RE: DataTypes - Please vote no"

    Hi, All -

    My apologies in advance to Kathy and the entire datatypes team for all the
    hard work they have done, only to have me come late to the game and express
    severe doubt about the datatypes proposal. Unfortunately, due to the volume
    of SV email, volume of datatypes email and the Thursday schedule for
    datatypes meetings, I have been unable to keep up in a timely manner. Due
    to unfortunate circumstances, I had a few hours to catch up on what I think
    is all of the datatype email that has circulated in the past month+

    I found the email by Steve Sharp dated November 11, where Steve is
    apparently explaining to Stu why the net datatypes are useful. I think I
    have the same question(?) Why are net datatypes so important? Nobody that I
    teach has asked for these capabilities, but almost everyone I teach is
    disgusted at the declaration distinction between nets and regs.

    In short. Most engineers (myself included) would like the tool to figure
    out the distinction between a variable assignment and a net driver without
    the requirement to change a declaration.

    IMO - every common net/variable usage could be declared as a wire. All
    existing net/variable rules would apply depending on whether a declared
    wire was assigned within a procedural block (variable-rules) or whether it
    was driven by a module, UDP, gate-primitive or continuous assignment
    (net-rules). An error would occur if a user tries to make both procedural
    and driver-assignments to the same identifier. First usage would determine
    local usage-rules (whether the identifier will be treated like a net or a
    variable in the local module).

    This seems to satisfy one of the main reasons for the datatypes proposal,
    allowing variable datatypes on nets. Seems like most identifiers could be
    declared as a wire and datatypes applied. Since there are some limitations
    for datatypes on nets (?), errors would be flagged if an assignment were
    made to an illegally declared datatype.

    The rest of the datatypes proposal could be reasonably adapted to common
    wire/variable declarations.

    Steve pointed out that designers do not think about making a procedural
    assignment to a variable and then driving it onto a net. Or taking the
    value from a net and procedurally assigning it to a variable. Steve is
    right. As design engineers, we could care less. We like to make behavioral
    descriptions for logic and then connect it to other logic. Until I joined
    the Verilog Standards Groups, I did not even know that simulators
    frequently run a procedurally assigned variable through a continuous
    assignment to get it out of a module onto a resolved net.

    VHDL does not force net/variable distinctions. Verilog should not force
    them either.

    Considering the amount of activity, explanation and confusion surrounding
    the datatypes proposal, and since is does not address my fundamental
    concern surrounding datakinds and datatypes, I will be voting no and will
    encourage other members of the P1800 committee to vote no to this proposal.
    I believe it is being rushed to meet a deadline and does not address far
    more basic concerns that designers have with datakinds/types.

    Sorry.

    Regards - Cliff Cummings

    ----------------------------------------------------
    Cliff Cummings - Sunburst Design, Inc.
    14314 SW Allen Blvd., PMB 501, Beaverton, OR 97005
    Phone: 503-641-8446 / FAX: 503-641-8486
    cliffc@sunburst-design.com / www.sunburst-design.com
    Expert Verilog, SystemVerilog, Synthesis and Verification Training

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