Previous Xilinx FPGA families featuring DCI used a slightly different circuit for calibrating the controlled impedance driver and split-termination impedance from the external reference resistors placed on the VRN and VRP pins. In Xilinx 7 series FPGAs, DCI calibrated each leg of the split-termination circuit to be directly equal to the external resistor values. For example, a 7 series device with a target parallel termination of 50W to VCCO/2 requires 100W external resistors on the VRN and VRP pins.
In UltraScale devices, irrespective of the DCI termination value requirement, the external resistor on the VRP pin is required to be 240W. Instead of two resistors, only one resistor is required at an UltraScale device VRP pin. The exact value of the split-termination or single-termination resistors are determined by the user-controllable ODT attribute.
Possible ODT values for split-termination DCI standards (HSTL and SSTL) are RTT_40, RTT_48, or RTT_60.
Possible ODT values for single-termination POD standards are RTT_40, RTT_48, or RTT_60. Possible ODT values for single-termination HSUL standards are RTT_120, RTT_240, or RTT_NONE.
IMPORTANT: The ODT value represents the desired resistance to VCCO for single-termination DCI standards.
The termination value for the controlled impedance driver is determined by the DCI state machine when a DCI standard with a controlled impedance driver is chosen, using OUTPUT_IMPEDANCE attribute values. Possible values for OUTPUT_IMPEDANCE attributes are RDRV_40_40, RDRV_48_48, RDRV_60_60, and RDRV_NONE_NONE.