External Memory Interface Handbook Volume 2: Design Guidelines

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

3.3.3. FPGA OCT Features

Many FPGA devices offer OCT. Depending on the chosen device family, series (output), parallel (input) or dynamic (bidirectional) OCT may be supported.

For more information specific to your device family, refer to the respective I/O features chapter in the relevant device handbook.

Use series OCT in place of the near-end series terminator typically used in both Class I or Class II termination schemes that both DDR2 and DDR3 type interfaces use.

Use parallel OCT in place of the far-end parallel termination typically used in Class I termination schemes on unidirectional input only interfaces. For example, QDR‑II type interfaces, when the FPGA is at the far end.

Use dynamic OCT in place of both the series and parallel termination at the FPGA end of the line. Typically use dynamic OCT for DQ and DQS signals in both DDR2 and DDR3 type interfaces. As the parallel termination is dynamically disabled during writes, the FPGA driver only ever drives into a Class I transmission line. When combined with dynamic ODT at the memory, a truly dynamic Class I termination scheme exists where both reads and writes are always fully Class I terminated in each direction. Hence, you can use a fully dynamic bidirectional Class I termination scheme instead of a static discretely terminated Class II topology, which saves power, printed circuit board (PCB) real estate, and component cost.

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