The receiver terminating resistor calibration is performed automatically one time during the configuration process. All analog supply voltages must be present and within the proper tolerance as specified in the Virtex-5 FPGA Data Sheet. MGTAVTTRXC powers the RX terminating resistors calibration circuit. RREF is the precision resistor that must be positioned close to the FPGA and minimizing parasitics.
If due to a design mistake, the power or the resistor or both are not present, the terminating resistor value can be overwritten independently for each transceiver by using the TERMINATION_CTRL and TERMINATION_OVRD attributes. This feature is intended for experimental purposes only and is not supported. In this way, the expected termination precision is +/- 25%.