A special Dynamic ODT use case is available (when supported by the target memory vendor) for users that wish to reduce the address/control pin count by one.
When this Dynamic ODT configuration is used, an ODT pin at the FPGA is not needed and the ODT ball at the memory is wired High.
In this configuration, RTT nom is disabled and ODT is programmed with RTT(WR) during writes.
Note: This usage of Dynamic ODT is supported by Micron as stated in their data sheet; see the Dynamic ODT Special Use Case section.
If you are not using a Micron DDR3 SDRAM, please work with your vendor on support for this special use case.
Dynamic ODT offers a limited set of termination values (i.e., RZQ/2 and RZQ/4).
IBIS simulations should be run to ensure one of these options is viable.
The generated 7 Series MIG DDR3 rtl can be modified to support this configuration.
This answer record details the changes required.
Note: This Answer Record is a part of the Xilinx MIG Solution Center (Xilinx Answer 34243).
The Xilinx MIG Solution Center is available to address all questions related to MIG.
Whether you are starting a new design with MIG or troubleshooting a problem, use the MIG Solution Center to guide you to the right information.
1. Open the "user_design/rtl/phy/ddr_phy_init.v" module and uncomment the following lines:
NOTE: Starting in MIG v1.5, available with ISE 14.1, this step can be skipped. Go directly to Step 2.
2. Modify the following top level parameters in the "example_design/rtl/example_design.v" and "user_design/rtl/core_name.v" modules:
As per the Jedec standard, Dynamic ODT cannot be used during Write Leveling.
For this reason, the 7 Series MIG PHY turns Dynamic ODT off and sets RTT_NOM to 40 ohm before Write Leveling.
After Write Leveling completes, the MR1 and MR2 mode registers are reprogrammed with the Dynamic ODT settings (RTT_NOM=Disabled and RTT_WR=60).
When running a simulation, you will see Dynamic ODT disabled settings during initialization, but then a second set of MR1/MR2 commands sent after Write Leveling completes showing the appropriate Dynamic ODT settings as follows:
NOTE: This special ODT use case is available for to users that have placed ODT on a site that violates the rules in (Xilinx Answer 45633)