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Altera_Forum's avatar
Altera_Forum
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10 years ago

For different PLL settings, timings are different by TimeQuest

I didn't realize that PLL didn't change when I upgraded my design from Cyclone II to Cyclone IV. As a result, with Cyclone IV, using the MegaWizard for ALTPLL, the "currently selected device family" is kept as Cyclone -II when the box "Match project/default" is ticked. After I changed it to Cyclone IV E, the design which met the timing requirements before can't meet timing requirements any more. Can anybody tell me why Quartus-II didn't give any warning when a Cyclone-II PLL was used for the design for Cyclone IV? What is the difference between a Cyclone-II PLL and a Cyclone IV PLL? Many thanks.

2 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    10 years ago

    During Analysis, following Quartus' assessment of the PLL's parameters, you will have been given an Info message along the lines of:

    "info (15535): implemented pll "{pll_instance_name}" as cyclone iv e pll type"

    Cyclone IV's PLL is fundamentally the same as Cyclone II's - with 5 outputs instead of 3 and a few other features. So, Quartus will quite happily take the Cyclone II PLL parameters and implement then in Cyclone IV. You need not do any more - unless you're looking to use additional features available in the Cyclone IV PLL.

    As for why your design now fails timing - I'll assume you've implemented your Cyclone IV PLL exactly as your Cyclone II PLL. Cyclone IV should be able to do every bit as well as Cyclone II as you'd expect. However, in changing the PLL Quartus may now create/derive different entity and signal names such that those you've referenced in your constraints file are no longer valid. So, you need to revisit these having had Quartus 'Analyse' your new design. Ensure you 'Derive PLL Clocks' (from the Constraints menu in TimeQuest) and you should get a better result.

    Cheers,

    Alex
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    10 years ago

    This is a very good point. The PLL names should be different and the names in the .sdc should be changed. Thank you very much indeed, Alex.