UPGRADE YOUR BROWSER

We have detected your current browser version is not the latest one. Xilinx.com uses the latest web technologies to bring you the best online experience possible. Please upgrade to a Xilinx.com supported browser:Chrome, Firefox, Internet Explorer 11, Safari. Thank you!

AR# 41590

7 Series MIG DDR3 SRAM - What are the DCI VRP/VRN reference resistor values for DDR3 interfaces?

Description

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?

Solution

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.
AR# 41590
Date Created 05/13/2011
Last Updated 02/06/2013
Status Active
Type General Article
Devices
  • Kintex-7
  • Virtex-7
IP
  • MIG