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

ID 683385
Date 3/06/2023
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2.2.1.1. External Parallel Termination

If you use external termination, you must study the locations of the termination resistors to determine which topology works best for your design.

The following two figures illustrate the most common termination topologies: fly-by topology and non‑fly-by topology, respectively.

Figure 16. Fly-By Placement of a Parallel Resistor


With fly‑by topology, you place the parallel termination resistor after the receiver. This termination placement resolves the undesirable unterminated stub found in the non‑fly-by topology. However, using this topology can be costly and complicate routing.

Figure 17. Non-Fly-By Placement of a Parallel Resistor


With non‑fly‑by topology, the parallel termination resistor is placed between the driver and receiver (closest to the receiver). This termination placement is easier for board layout, but results in a short stub, which causes an unterminated transmission line between the terminating resistor and the receiver. The unterminated transmission line results in ringing and reflection at the receiver.

If you do not use external termination, DDR2 offers ODT and Intel® FPGAs have varying levels of OCT support. You should explore using ODT and OCT to decrease the board power consumption and reduce the required board space.