External Memory Interface Handbook Volume 2: Design Guidelines

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ID 683385
Date 5/08/2017
Public
Document Table of Contents

2.1.2. Dynamic ODT

Dynamic ODT is a feature in DDR3 and DDR4 SDRAM that is not available in DDR2 SDRAM. Dynamic ODT can change the ODT setting without issuing a mode register set (MRS) command.
Note: This topic highlights the dynamic ODT feature in DDR3. To learn about dynamic ODT in DDR4, refer to the JEDEC DDR4 specifications.

When you enable dynamic ODT, and there is no write operation, the DDR3 SDRAM terminates to a termination setting of RTT_NOM; when there is a write operation, the DDR3 SDRAM terminates to a setting of RTT_WR. You can preset the values of RTT_NOM and RTT_WR by programming the mode registers, MR1 and MR2.

The following figure shows the behavior of ODT when you enable dynamic ODT.

Figure 14. Dynamic ODT: Behavior with ODT Asserted Before and After the Write

Dynamic ODT: Behavior with ODT Asserted Before and After the Write

In the multi-load DDR3 SDRAM configuration, dynamic ODT helps reduce the jitter at the module being accessed, and minimizes reflections from any secondary modules.

For more information about using the dynamic ODT on DDR3 SDRAM, refer to the application note by Micron, TN-41-04 DDR3 Dynamic On-Die Termination.

In addition to RTT_NOM and RTT_WR, DDR4 has RTT_PARK which applies a specified termination value when the ODT signal is low.

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