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Praxis_Kernfeld's avatar
Praxis_Kernfeld
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2 hours ago
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Why does Fitter show "Dedicated Pin" as "Reference Clock Source by" for downstream PLL in cascade?

 

Hi everyone,

I created a design that connects two PLLs in a simple cascaded configuration.

 

However, I noticed something that seems inconsistent in the Fitter report:

  •  In the Resource SectionPLL Usage Summary, the "Reference Clock Sourced by" field for the downstream side PLL shows "Dedicated Pin". This seems unusual because it does not reflect the actual connection (which uses the upstream side PLL’s cascade output).
  • On the other hand, the "PLL Refclk Select" section in the report looks correct and matches the intended cascading setup.

Could anyone explain why this discrepancy occurs?

Is this normal behavior or a reporting issue in Quartus Prime?

 

Environment:

  • Tool: Quartus Prime Standard Edition version 23.11
  • Target Device: Cyclone V GX (5CGXFC5F6M11C6)
  • Target IP: "Altera PLL" or "PLL Intel FPGA"

 

Sample Design:

  • Simple_Two_plls_shortest_cascading_refclk_from_clkpin.qar

 

Settings Summary (Only the key parameters are listed.):

Upstream side PLL:

In [General] tab

  • PLL Mode: Integer-N PLL
  • Operation Mode: Normal
  • Feedback Clock: Global Clock

In [Cascading] tab

  • "Create a 'cascade_out' signal to connect with a downstream PLL" : Enabled

           ( PLL Use As Upstream PLL. → Create cascade_out signal )

In [Settings] tab

  • Bandwidth Preset: Low

 

Downstream side PLL:

In [General] tab

  • PLL Mode: Integer-N PLL
  • Operation Mode: Normal
  • Feedback Clock: Global Clock

In [Cascading] tab

  • "Create an adjpllin or cclk signal to connect with an upstream PLL" : Enabled

         ( PLL Use As Downstream PLL → Create adjpll_in or cclk signal )

In [Settings] tab

  • Bandwidth Preset: High

 

Thank you for any insights on this!

 

  • Hi,

    Yes, this is just reporting limitation and will not cause functional impact.

    Please treat "Reference Clock Sourced by" as "Don't Care" for the PLL2 ( downstream side PLL) and rely on the "PLL Refclk Select" section.

2 Replies

  • Hi ShengN,

     

    Thank you for the clarification. That makes sense, and I understand now.

    I appreciate your support and clear explanation.

     

    Best regards,

    Praxis_Kernfeld

  • ShengN_altera's avatar
    ShengN_altera
    Icon for Super Contributor rankSuper Contributor

    Hi,

    Yes, this is just reporting limitation and will not cause functional impact.

    Please treat "Reference Clock Sourced by" as "Don't Care" for the PLL2 ( downstream side PLL) and rely on the "PLL Refclk Select" section.