15.10.2 Support for VREF I/Os

VREF I/Os are fully supported in your design. Here are the rules for assigning VREF I/Os in the designer software:

• You cannot unplace I/Os from a block.
• You can change VREF pins in the top of a block only if this will not result in I/Os becoming unplaced from the block.
• If a macro needs a VREF pin, you cannot unset the VREF pin until you first assign a different VREF pin to it.
• If you have I/O Banks with only VCCI set on a block, you can modify those I/O Banks to include VREF technology.
• Setting and unsetting VREF pins on a block must not result in unplacing any I/O from the block.
• You can change the I/O technology to "do not need a VREF" (for example, from SSTL to LVTTL).
• The default VREF pin setting will fail if any I/O from the block becomes unplaced.
• The automatic I/O Bank setting tool will not unplace any I/O from a block.