General Description: How can I activate the pull-downs on the unused I/O pins, as they are pulled up, by default, with an internal pull-up resistor (20k-100k ohms)?
The Xilinx implementation tools do not yet offer the user the option of globally activating the internal pull-downs on unused I/O. However, this task can be accomplished with the help of FPGA Editor.
Follow these steps:
1. Open your placed and routed design in FPGA Editor (<design_name>.ncd)
2. Set the Edit Mode. (Go to File -> Main Properties, and set Edit Mode to "Read Write".)
3. Highlight an unused IOB and select "Add" (on the right side). This adds the IOB to your design -- you can give it any name you like.
4. Once you've added the IOB, it should appear in blue. Double-click on it to see the internals of the IOB. To make changes, you'll have to click the second-from-left button (looks like a pencil) to allow editing.
5. Click the check box next to "Pull-down".
6. To apply the edits, click the second-from-right button (looks like an arrow and small window).