7.4.1. General Layout Routing Guidelines
For maximum channel margin, you should consider the following general routing optimizations during the layout design phase:
- When routing the memory interface, ensure that there are solid ground reference planes without any plane splits or voids, to ensure an uninterrupted current return path.
- For signal vias in layer transitions, you must place ground stitching vias close by, within 80 mil in distance (closer is better), and in between signal vias, to minimize crosstalk among signal vias. Avoid any unnecessary signal layer transitions to minimize crosstalk, loss, and skews.
- Trace impedance plays an important role in signal integrity; board designers must follow impedance recommendations for each signal group and configuration according to the guidelines in this document. If you use a different stackup than the reference stackup in the PCB design, you must tune the trace width and geometries to achieve the impedance recommendations.
- Intel® recommends using 45-degree angles (not sharp 90-degree corners) when routing signal turns. Use 3×h spacing for serpentine routing, where h is the height or distance from the trace to the nearest GND reference plane.
- Avoid referencing a signal to both power and ground planes at the same time (dual referencing), for signal return paths. When this cannot be avoided, ensure that the closer reference plane is solid ground, and the far side power plane is not noisy.
- Avoid routing two internal signal layers adjacent to each other (dual stripline routing). When this cannot be avoided, use angled routing between two signal layers to minimize crosstalk and coupling between the layers.
- Follow time-domain length and skew matching rules to ensure that your interface meets timing requirements. You should route signals from the same byte or group together on the same layer to avoid any out-of-phase crosstalk caused by varying layer transition lengths.
- To optimize memory interface margins, Intel® recommends routing address, command, and control signals on shallow layers. (Shallow layers are those above the PCB core where via transition lengths are short.)
- For boards thicker than 65 mil, Intel® recommends alternating adjacent FPGA EMIF BGA/ball rows with deep and shallow board via transitions to minimize crosstalk between adjacent bytes. This method is illustrated in the following figure:
Figure 118. Recommended alternate adjacent via transitions to avoid crosstalk between adjacent bytes
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