18.104.22.168. Fly-By Network Design for Clock, Command, and Address Signals
If you design with discrete components, you can choose to use one or more fly-by networks for the clock, command, and address signals.
The following figure shows an example of a single fly-by network topology.
Every SDRAM component connected to the signal is a small load that causes discontinuity and degrades the signal. When using a single fly-by network topology, to minimize signal distortion, follow these guidelines:
- Use ×16 device instead ×4 or ×8 to minimize the number of devices connected to the trace.
- Keep the stubs as short as possible.
- Even with added loads from additional components, keep the total trace length short; keep the distance between the FPGA and the first SDRAM component less than 5 inches.
- Simulate clock signals to ensure a decent waveform.
The following figure shows an example of a double fly-by network topology. This topology is not rigid but you can use it as an alternative option. The advantage of using this topology is that you can have more SDRAM components in a system without violating the 0.69 tCK rule. However, as the signals branch out, the components still create discontinuity.
You must perform simulations to find the location of the split, and the best impedance for the traces before and after the split.
The following figure shows a way to minimize the discontinuity effect. In this example, keep TL2 and TL3 matches in length. Keep TL1 longer than TL2 and TL3, so that it is easier to route all the signals during layout.
You can also consider using a DIMM on each branch to replace the components. Because the trace impedance on the DIMM card is 40-ohm to 60-ohm, perform a board trace simulation to control the reflection to within the level your system can tolerate.
Using the fly‑by daisy chain topology increases the complexity of the datapath and controller design to achieve leveling, but also greatly improves performance and eases board layout for SDRAM implementations.
You can also use the SDRAM components without leveling in a design if it may result in a more optimal solution, or use with devices that support the required electrical interface standard, but do not support the required read and write leveling functionality.
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