UltraScale Architecture SelectIO Resources User Guide (UG571)

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1.14 English

Single-ended I/O standards with a differential input buffer require an input reference voltage (VREF). When VREF is required within an I/O bank, you can use either the dedicated VREF pin as a VREF supply input (external) or an internally generated VREF (INTERNAL_VREF or VREF 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 VREF 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 VREF (INTERNAL_VREF or VREF scan), connect the dedicated VREF pin to GND with a 500W or 1 KW resistor.

In banks where the I/O standard does not have an input reference voltage requirement, connect the dedicated VREF pin to GND (with a 500W or 1KW resistor), or leave it floating.

The internal VREF scan feature is available in HP I/O banks to account for process variations and system considerations.