AR# 41590: 7 Series MIG DDR3 SRAM - What are the DCI VRP/VRN reference resistor values for DDR3 interfaces?
7 Series MIG DDR3 SRAM - What are the DCI VRP/VRN reference resistor values for DDR3 interfaces?
MIG uses SSTL15_T_DCI (DQ) and DIFF_SSTL15_T_DCI (DQS) for DDR3 interfaces. The 7 Series FPGAs SelectIO Resources User Guide states "Split-termination DCI creates a Thevenin equivalent circuit using two resistors of twice the resistance value (2R)." This results in VRN/VRP reference resistors of 100 ohms to achieve a 50 ohm Thevenin equivalent termination. The guidelines for Virtex-6 FPGAs is VRN = VRP = R = Z0 resulting in 50 ohm reference resistors for Z0=50 (Virtex-6 FPGAs SelectIO Resources User Guide). Why are the reference resistors different for the same I/O Standards between the two device families?
The structure of DCI changed from Virtex-6 to 7 Series FPGAs. The new structure in 7 Series FPGAs requires 2R VRP/VRN reference resistors and provides better termination. Please refer to the above noted user guides for full details.
NOTE: Xilinx recommends 40 ohm traces for DDR3 operation above 1333 Mbps. When 40 ohm traces are used, the VRN/VRP references resistors should be 80 ohm (2R) to achieve a 40 ohm Thevenin equivalent termination. Please see (Xilinx Answer 42024) for additional information on the 40 ohm trace recommendation.