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Altera_Forum's avatar
Altera_Forum
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17 years ago

Output Pins stuck at VCC or GND

Hi all. I've just finished writing and simulating a program which functions as a calculator.. However upon compiling the program, there is this warning which says:

"Output Pins stuck at VCC or GND".

Just so you all know I'm still relatively new to VHDL and am not particularly well versed yet. I wish to know how can I solve this problem. The pins are actually 2 pins which I intend to interface with a Seven Segment Display. And both pins are stuck at VCC, therefore permanently lighting those two LEDs.

I have two questions:

a) Why are those pins stuck at VCC when I did not assign any pins yet.

b) How do I avoid this problem.

Thanks for those who can help.

28 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    17 years ago

    Hi,

    since you haven't yet assigned then don't worry(the fitter felt free and chose to tie them up to vcc). Once you drive your outputs then the fitter will obbey your assignment.

    The fitter actually follows default (unused pins assignment)
  • Altera_Forum's avatar
    Altera_Forum
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    17 years ago

    --- Quote Start ---

    Why are those pins stuck at VCC when I did not assign any pins yet.

    --- Quote End ---

    Very unlikely. You get this message for pins, that have been assigned but are not driven by logic. Either you connected no logic to the pin or the connected logic has a constant output. The purpose of the warning is making you check for logic that has unintentional constant output.
  • Altera_Forum's avatar
    Altera_Forum
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    17 years ago

    If you mean by "assign" the pin assignment rather than logic drive then what might have happened is that the fitter was left free to assign pins as it liked. In that case possibly you are driving some outputs high which the fitter chose to assign them to those pins. user-unassigned project pins can sometimes be damaging in real systems, so beware...

  • Altera_Forum's avatar
    Altera_Forum
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    17 years ago

    My previous statement wasn't fully correct. Pins not assigned to a design top port are treated according to the designs unused pin rule, but not reported with the said stuck-to message. The message appears with port pins driven intentionally or unintentionally by a constant driver. If the output is expected not to have constant output, the logic should be corrected.

  • Altera_Forum's avatar
    Altera_Forum
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    17 years ago

    Hi all. Thanks for all your help. I looked through my code and it seems that I did assign logic 1 to those pins. Thanks again :)

  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    Hi,what about "the input pins do not drive the logic" ?In reality,I do have used them in my project.

  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    Quartus will optimize your design and remove unnecessary logic.

    If you have a input pin but, somehow, your design's behavior does not depend on it's value, Quartus will optimize it away.

    If this is not the intended design, then you have an error in your logic.
  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    --- Quote Start ---

    In reality,I do have used them in my project.

    --- Quote End ---

    May be. But after fitting the design, there was no logic. Any superfluous logic, that doesn't connect to output pins, is automaticly removed.
  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    In my project I use a "component" sentence.But ,when I compile the component only ,that pin do work(the warning disappear,I can see its route),it'll have no routes if it connects to upper structure.So why?

  • Altera_Forum's avatar
    Altera_Forum
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    16 years ago

    Hard to say.

    I suggest that you simulate your design. You should see that it does not react to those input pins that do not drive logic.

    Then try to find out why.