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Altera_Forum
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14 years ago

CLKIN Pin as General Purpose Input

I'm doing a Cyclone III design (simulation only, no hardware yet) and am using one of the dedicated clock input pins as a general purpose input. It compiles OK & meets timing constraints so I'm assuming everything is OK. I tried assigning it as a fast input register but the fitter spit it back at me, this probably makes sense. Can I proceed with confidence that everything will work as expected?

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  • Altera_Forum's avatar
    Altera_Forum
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    14 years ago

    Yes, clock input pins can be used as normal inputs (but not outputs!) with no issues.

  • Altera_Forum's avatar
    Altera_Forum
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    11 years ago

    --- Quote Start ---

    Why can't I really use the CLKIN pin as a output?

    --- Quote End ---

    Because the pin on the FPGA is a dedicated clock input pin which does not support output. Search for "dedicated clock input pin" in the Cyclone II handbook.

    If you just want to use the given pin(s) on the connector so you can drive your old board, find another pin/net you don't care about and solder a wire to the connector.
  • Altera_Forum's avatar
    Altera_Forum
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    10 years ago

    Just to add on, you can use Quartus II Fitter compilation to verify if a specific pin usage can be implemented on the device. Fitter will help to check against the placement rule and any violation. You should also do a functional simulation with your design to check on the behavior to your expectation.