External Memory Interface Handbook Volume 2: Design Guidelines: For UniPHY-based Device Families

ID 683385
Date 3/06/2023
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2.3.3. Terminations for DDR3 SDRAM Registered DIMM

The difference between a registered DIMM (RDIMM) and a UDIMM is that the clock, address, and command pins of the RDIMM are registered or buffered on the DIMM before they are distributed to the memory devices. For a controller, each clock, address, or command signal has only one load, which is the register or buffer. In a UDIMM, each controller pin must drive a fly-by wire with multiple loads.

You do not need to terminate the clock, address, and command signals on your board because these signals are terminated at the register. However, because of the register, these signals become point-to-point signals and have improved signal integrity making the drive strength requirements of the FPGA driver pins more relaxed. Similar to the signals in a UDIMM, the DQS, DQ, and DM signals on a RDIMM are not registered. To terminate these signals, refer to “DQS, DQ, and DM for DDR3 SDRAM UDIMM”.