Single-ended I/O standards with a differential input buffer require an input reference voltage (V REF ). When V REF is required within an I/O bank, you can use either the dedicated V REF pin as a V REF supply input (external) or an internally generated V REF (INTERNAL_VREF or V REF scan (HP I/O banks only)). An internally generated reference voltage is enabled by using the INTERNAL_VREF constraint. Output-only IOBs do not require a V REF or INTERNAL_VREF because only the receive portion of an IOB leverages the input reference voltage. For more information on this constraint, see SelectIO Interface Attributes and Constraints .
IMPORTANT: In banks where the input I/O standard has an input reference voltage requirement and uses an internally generated V REF (INTERNAL_VREF or V REF scan), connect the dedicated V REF pin to GND with a 500 Ω or 1 K Ω resistor.
In banks where the I/O standard does not have an input reference voltage requirement, connect the dedicated V REF pin to GND (with a 500 Ω or 1K Ω resistor), or leave it floating.
The internal V REF scan feature is available in HP I/O banks to account for process variations and system considerations.