External Memory Interfaces Agilex™ 7 M-Series FPGA IP User Guide

ID 772538
Date 11/18/2024
Public
Document Table of Contents

7.6.2. General Design Considerations

Altera recommends that you route all data signals within a specific group on the same layer.

The following figure illustrates a routing example for a type-III PCB for a DDR5 design. Altera recommends that you route Data Group signals such as DQ, DM and DQS on shallow layers as stripline, with the least Z-height via transition to avoid vertical crosstalk for high performance.

Minimal stub effect or back drill is recommended but not mandatory to avoid high reflection for maximum data rate performance for a DDR5 interface. You can run simulation to determine whether to use backdrill. Long via stubs will affect the intersymbol interference (ISI) of the channel, but the impact of ISI is less than the impact of crosstalk for maximum performance.

Figure 33. Suggested Routings

In the above figure, case A routing is suggested for DDR5 Data Group signals over case B, to support maximum data rate. If data signals are routed on deeper layers (as in case B, with long via and short stub), the impact of crosstalk is significant and causes reduced data rate and performance.

To minimize crosstalk horizontally between the signals on the same layer, PCB designers must maintain adequate signal trace-to-trace (edge to edge) space with a minimum spacing of 3 x H separation distance, where H is the dielectric thickness to the closest reference plane, as illustrated below. Refer to the routing guideline sections for detailed spacing requirements for each signal group.

Figure 34. Minimum Trace-to-Trace Separation Distance