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June 2014 • The PCB Design Magazine 47 article RoUTINg DDR3 MEMoRY AND CPU FANoUT continues Figure 4: Choosing a correct via size can help save space for more tracks. vias. (You can be more liberal with vias as long as their lengths are taken into consideration for length matching). Creating DDR3 groups The first step before routing is to create the necessary signal groups, which can be done from the project's schematic. The specific method for doing this will depend on how constraints are managed in the software, but nets that are con- strained and grouped will follow certain rules during routing. An example of a system for signal grouping on the schematic is shown in Figure 2. Most ECAD tools have a "net class" feature for this kind of net grouping. After bringing the signal groups to the PCB, it's very useful to assign different colors to each group, to make routing easier to mentally fol- low, as shown in Figure 3. Once a distinct col- or has been chosen for each signal group (net class), it's time to fanout the DDR3 interface of the CPU. Choosing the correct via style for a particu- lar memory group and deciding how PCB layers

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