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HAlex's avatar
HAlex
Icon for Occasional Contributor rankOccasional Contributor
2 years ago
Solved

Unconnected inputs of Cyclone III

Hello

I am using EP3C40F484C8N FPGA in my board.

As now I want to remove some components assembled on PCB and connected to the FPGA inputs I want to know what modifications must be done in my Quartus based FPGA design - like defining weak pull-ups on input pins, or simply removing these pins from the design or do nothing?

1. For inputs just defined as inputs but without connection to any logic inside FPGA.

2. For inputs defined and connected to the logic inside FPGA but this logic is not used.

Thank you

Alex

  • HAlex's avatar
    HAlex
    2 years ago

    I will leave these pins defined as inputs (for any case) and will assign an internal weak pull up on them.

8 Replies

  • FvM's avatar
    FvM
    Icon for Super Contributor rankSuper Contributor
    2 years ago
    Unconnected pin behaviour is set in device and pin options dialog, tri-stated with weak pull-up is the default option and fits most cases. For pins connected to FPGA fabric, the option can be set in assignment editor or pin planner. Consider that Cyclone 3 clock inputs have no weak pull-up feature and should be externally connected if ever possible.
    • HAlex's avatar
      HAlex
      Icon for Occasional Contributor rankOccasional Contributor
      2 years ago

      Hello

      The option in device and pin dialog is for unused pins. In my case I have a defined inputs, some of them are connected to the FPGA logic (but actually not used by Nios or not effect on any essential output). FPGA doesn't know that the inputs are going to be disconnected. My question is if I need to recompile my FPGA (and remove unconnected pins/define them as weak pull-ups) or simply do nothing and leave design as is without recompilation?

      Thank you.

      Alex

      • FvM's avatar
        FvM
        Icon for Super Contributor rankSuper Contributor
        2 years ago
        Floating inputs can cause additional current consumption and switching noise. Therefore you want to avoid floating inputs by any available means.
  • _AK6DN_'s avatar
    _AK6DN_
    Icon for Frequent Contributor rankFrequent Contributor
    2 years ago

    Do you truly mean 'unused inputs' or are you really just referring to 'unused pins'?

    Typically I include this setting in my .qsf files:

    set_global_assignment -name RESERVE_ALL_UNUSED_PINS "AS OUTPUT DRIVING GROUND"

    So that for any pins I don't refer to they just get set as a low output. So no floating inputs or pullups needed.

    • FvM's avatar
      FvM
      Icon for Super Contributor rankSuper Contributor
      2 years ago

      I'm talking about floating inputs. These are FPGA pins with input buffer (e.g. all GPIO pins) that or not either externally or internally driven, at least by a weak pull-up. Read it as a comment on your previous idea to leave defined inputs without external driver or internal pull-up.

      Regarding driving unused pins to ground, I see no adavantage but a risk of inadvertently shorting board signals. I always prefer the default weak pull-up setting. It has been suggested in old application notes to utilize unused pins as additional ground return, this may be useful for old PQFP packages with few ground pins. It's not necessary with BGA or exposed ground pin packages.

      • HAlex's avatar
        HAlex
        Icon for Occasional Contributor rankOccasional Contributor
        2 years ago

        I will leave these pins defined as inputs (for any case) and will assign an internal weak pull up on them.

  • WZ2's avatar
    WZ2
    Icon for Frequent Contributor rankFrequent Contributor
    2 years ago

    Hi there,

    I wanted to check if you have any further questions or concerns. If not, I will go ahead and mark this issue as resolved.

    Additionally, we would greatly appreciate it if you could take a moment to fill out our survey. Your feedback is valuable to us and helps us improve our support quality.

    Thank you for your time and cooperation.

    Best regards,

    WZ


    • HAlex's avatar
      HAlex
      Icon for Occasional Contributor rankOccasional Contributor
      2 years ago

      Hello

      I don't have any further questions - and already marked this issue as accepted solution (to assign weak pull up on input pins for any case).

      Thank you.