verilog 'pullup' and VHDL

Discussion in 'VHDL' started by Mark McDougall, Oct 26, 2006.

  1. Hi,

    I've got a problem which I'm sure has been solved before.

    I'm doing mixed-language simulation in ModelSim, and have two VHDL
    entities connected by an 'inout' port. The top-level of my testbench is
    Verilog, so I use a 'wire' to connect the two ports with a 'pullup'.

    Trouble is, the entities (3rd party code) specifically compare against
    logic level '1' on the wire in certain circumstances. So when the line
    isn't driven, I see 'H' on ModelSim wave output but the VHDL code fails.

    Is there any way in Verilog of forcing a pullup to '1', so that VHDL
    comparisons against '1' succeed?

    Or is there another/better way of achieving this end?

    I'd prefer not to change either entity as it's 3rd party code.

    Mark McDougall, Oct 26, 2006
    1. Advertisements

  2. I don't think this has anything to do with it being a mixed language
    setup. When I was coding VHDL a few years ago I ran into the exact
    same thing with a completely VHDL setup. The top-level drove an 'H' on
    the tristate bus, similar to the Verilog pullup.

    I found two solutions. One, change the compare against '1' to compare
    equal '1' or 'H'. I also tried using one of the millions of VHDL
    functions (can't remember the name or the library - I've been happily
    back doing Verilog for the last four years) that converted 4-value
    logic to 2-value.

    I think both of these would require you to change the entities, which
    you didn't want to do. If anyone does have a way of fixing this just
    by changing the testbench, I'd like to see it.

    unfrostedpoptart, Oct 26, 2006
    1. Advertisements

  3. I've found that there's a *single* assignment in the main entity that
    reads the inout port and maps it to a signal. For now I've changed that
    to an if-else construct so I can at least simulate the design. I may
    even decide to leave it as-is for synthesis too.

    I'd still like to know if there's a solution...

    Mark McDougall, Oct 26, 2006
  4. Mark McDougall

    KJ Guest

    The 'To_X01" function is what you want. Assuming that 'Sig1' is the signal
    that has the pullup and 'Sig2' is a newly created signal that will be used
    in place of 'Sig1' inside the entity where it is currently being read

    signal Sig1, Sig2: std_logic;
    Sig2 <= To_X01(Sig1);

    KJ, Oct 26, 2006
  5. Yes, I am now using that in place of my earlier 'fix' using if-else.
    However, this is done in the entity itself.

    I want to know if it can be done in the testbench - where I don't know
    which end of the wire is driving or when!?!

    Mark McDougall, Oct 26, 2006
  6. Mark McDougall

    KJ Guest

    Unless the testbench has access to the 'output enable' for all of the
    drivers of the signal then the answer is 'no'. I'm assuming that you
    don't have access to such output enable signals in the testbench since
    if you did you would probably have figured out when to have the pullup
    resistor drive a '1' or a 'Z'.

    The problem though is not in your testbench but in the entity that is
    incorrectly using this signal when using it as an input, so this is
    where the proper fix needs to be made. Since (from the first post)
    this is in 3rd party code then you should open a bug report to that 3rd
    party and have them fix it (if you're paid up on support). If the 3rd
    party doesn't provide support (or you're not paid up) then you need to
    fix it yourself anyway.

    Adding the To_X01() function is probably benign enough that it can be
    done correctly while waiting for the 3rd party support or if there are
    questions from your bosses about why the 3rd party code needs to be
    modified (explain to them, the code has a bug, it's not perfect).

    The only work around to fixing it properly is to have the resistor
    drive a '1' when it 'knows' that nobody else is driving it (or at least
    when the offending receiver is not using it as input) and a 'Z'
    otherwise. The only way the pullup would 'know' is through some
    additional information (i.e. the output enables). If you don't have
    those output enables on the entity of all the drivers of that signal
    then you would have to modify that 3rd party code also to get it...and
    doing it that way would be a definite kludge and a flag that you're not
    applying the proper fix.

    KJ, Oct 26, 2006
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.