Tracing UNKNOWN drivers

Discussion in 'VHDL' started by Yaseen Zaidi, Dec 20, 2006.

  1. Yaseen Zaidi

    Yaseen Zaidi Guest

    On Post Map and Post Route model simulations intermittently I get few X
    bytes. I set up a trace check for source in ModelSim and found out that
    it leads to X_BUF with 658 ps pathpulse which connects to vitalbehavior
    (gsr, prld, dly of clk, set, rst etc). It's only bit 0 of the bus
    that goes to X at times.

    What do I do to get consistent timing?

    Best wishes.
     
    Yaseen Zaidi, Dec 20, 2006
    #1
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  2. Yaseen Zaidi

    KJ Guest

    Yaseen Zaidi wrote:
    > On Post Map and Post Route model simulations intermittently I get few X
    > bytes. I set up a trace check for source in ModelSim and found out that
    > it leads to X_BUF with 658 ps pathpulse which connects to vitalbehavior
    > (gsr, prld, dly of clk, set, rst etc). It's only bit 0 of the bus
    > that goes to X at times.
    >
    > What do I do to get consistent timing?
    >


    1. Perform static timing analysis. The output of this will be the
    following bits of information:
    - Maximum clock frequency for each clock in the design.
    - Setup time of input pins relative to whatever clocks sample those
    input pins.
    - Clock to output time of all synchronous output pins.
    - Propogation delays to all output pins that are combinatorial
    functions of input pins.

    2. Make sure your testbench does not violate any of the above timing.

    3. If output signals are blipping to 'X' for a little bit then those
    signals are outputs of combinatorial logic and not directly out of a
    flip flop. If this momentary blip is an issue for whatever reason then
    you need to get rid of it. To get rid of the blip, change the output
    to be a clocked output. Obviously this changes the function somewhat,
    delaying the output to occur some Tco after the clock edge but if the
    signal is not allowed to blip even momentarily then a flip flop is the
    only output type that help you meet that goal.

    KJ
     
    KJ, Dec 20, 2006
    #2
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