VREF - VREF - UG861

Single-ended I/O standards with a differential input buffer require an input reference voltage (V_REF). When V_REF is required within an HP 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)). HD and XP5IO I/O banks only support INTERNAL_VREF so there are no V_REF pins in HDIO or XP5IO I/O banks. An internally generated reference voltage is enabled by using the INTERNAL_VREF constraint. HDIO only support a V_REF of 50% V_CCO for a given bank, while XP5IO can have unique V_REF levels on a nibble wide resolution. 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.

In HP I/O 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 - 1 KΩ resistor). The internal V_REF scan feature is available in HP I/O banks to account for process variations and system considerations.