Forum Discussion

Steve-Mowbray-ENL's avatar
Steve-Mowbray-ENL
Icon for Occasional Contributor rankOccasional Contributor
3 months ago
Solved

Using PCIe HIP core clock out to drive IOPLL reference clock -- advise on set_false_path syntax

Hi

Need advise on suitable set_false_path design constraint construction for the following scenario where PCIE HIP core clock out is used as the reference for an IOPLL instance:

Impact of doing this is fitter takes much longer and result fails timing by a big margin so I am guessing a design constraint false path is required... My initial attempt at setting a false path 

set_false_path -from [get_clocks {u0|pcie_hip|coreclkout_hip}] -to [get_clocks {u0|iopll_1|refclk}]

reports <from> and <to> arguments are empty collections

I would be grateful for any guidance -- apologies for outright asking for someone to show me how to do this but I'm finding SDC file content and usage quite impenetrable...

Thanks

Steve

  • Steve-Mowbray-ENL's avatar
    Steve-Mowbray-ENL
    3 months ago

    Hi FvM

    So it seems when the base/parent clock for the 100MHz domain was the external crystal Quartus was treating it as an unrelated clock hence no timing failures and status/control bits between the clock domains have synchroniser depth of 2 -- when PCIe core clock out was substituted for the external crystal -- Quartus started treating both clocks as related even though decoupled using an IOPLL instance -- the following design constraints appear to correct the situation 

    set_false_path -from [get_clocks {u0|pcie_hip|coreclkout}] -to [get_clocks {u0|iopll_1|*}]
set_false_path -from [get_clocks {u0|iopll_1|*}] -to [get_clocks {u0|pcie_hip|coreclkout}]

    Thanks

    Steve

5 Replies

  • FvM's avatar
    FvM
    Icon for Super Contributor rankSuper Contributor
    3 months ago
    Hi,
    false path for clock makes no sense. You want to find out why you get timing failure in your design, which data pathes are causing it and how it can be handled correctly. False path is only appropriate if respective timing is actually don't care. Unsubstantiated false path causes most likely data corruption.

    I guess you have domain crossing data path between 125 and 100 MHz clock domain causing timing violation. If so, you need to use synchronizer to transfer data consistently.
  • Steve-Mowbray-ENL's avatar
    Steve-Mowbray-ENL
    Icon for Occasional Contributor rankOccasional Contributor
    3 months ago

    Hi FvM

    Yes agreed which is why I am out of my depth here -- there are no data paths between the 125MHz and 100MHz domains -- previously the 100MHz domain had its own dedicated crystal as shown below:

    In the external crystal configuration the project makes timing comfortably however the extra crystal has been removed from the design and using the PCIe core clock out as its substitute causes build timing to collapse...

    I note that PCIe core clock out appears to be spread spectrum for Arria 10 devices when measured on external pin which is great for EMC but perhaps is causing the fitter/timing-analyser difficulties?

    If solution is not to set a false path how do I tell Quartus to be relaxed about using core clock out as reference clock to the IOPLL instance?

    Regards

    Steve

    • FvM's avatar
      FvM
      Icon for Super Contributor rankSuper Contributor
      3 months ago
      Hi,
      spread spectrum period variation should be low enough not to affect timing closure. How does it appear in .sdc description?

      Can you show where you see timing failures, what kind of violation, which clocks involved?
      • Steve-Mowbray-ENL's avatar
        Steve-Mowbray-ENL
        Icon for Occasional Contributor rankOccasional Contributor
        3 months ago

        Hi FvM

        Ok so I found a data path between the two domains -- there is an Avalon-MM register with control bits that are passed to a component in the 100MHz domain -- confusingly the end point for these control bits has synchroniser depth of 2 and I am still unsure as to why timing would be good when the 100MHz domain was referenced to the external clock device and bad when referenced to an internal IOPLL driven by PCIe core clock out...

        Anyways I am testing the following design constraint:

        set_false_path -to {system_top:u0|psu_drv:u1|duty_cycle_ctrl_in}

        Will report back when build has completed

        Regards

        Steve