Forum Discussion

HAlex's avatar
HAlex
Icon for Occasional Contributor rankOccasional Contributor
3 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 Quart...
  • HAlex's avatar
    HAlex
    3 years ago

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

_AK6DN_'s avatar
_AK6DN_
Icon for Frequent Contributor rankFrequent Contributor
3 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
    3 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
      3 years ago

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