What happens if the VRN or VRP pins are left unconnected?
When the VRN/VRP pins are left unconnected, the DCI attempts to match the "infinite impedance".
For LVDCI and LVDCI_DV2 standards, the output is effectively 3-stated.
For most HSTL_DCI, SSTL_DCI, and GTL_DCI standards, the on-chip terminations are turned Off completely. (The exceptions are HSTL_IV_DCI, GTL_DCI, and GTLP_DCI, for which a small part of the termination is turned On as part of the bitstream; consequently, that portion remains On even when the VRN and VRP pins are disconnected.)
Leaving VRN/VRP disconnected does not damage the device; it dissipates more power than, for example, a case in which the DCI is not turned on, since the DCI control logic is still active.
Disconnecting the VRN/VRP pins after configuration increases the impedance of on-chip terminations, but it does not turn the DCI completely Off.
NOTE: Some DCI outputs do not require a reference resistor, so they are not affected if the VRN/VRP pins are left unconnected. See (Xilinx Answer 14128) for more information.