Question about Line Routing on the BeagleBoard schematics

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1

On the Beagleboard schematics, the lines routed between pins are not
always taking the shortest path between the pins.Is there a set reason
why this is? Some reasons i came up with include:

Different line lengths to create timing differences between lines.
Negating any EMF or RF interference from other lines.
Negating any inductance.

Is there a specific reason or were the designers just board and did
something different?

2

Are you talking about the schematic or the PCB? If you are talking about the PCB there are several reasons. Just to name a few:

  1. The shortest path was not available when the autorouter ran because other traces were there first.
  2. Some of the traces are matched lengths, like DSS traces. You want them the same things so that the clock edges line up with the data.
  3. The shortest length is not a goal that is always sought after. It is more important not to run certain signals next to other signals that could interfere with each other. It is better to keep them apart.
  4. Some signals need to run over the ground planes so the traces have to follow the ground planes.

Gerald

3

the PCB lines sorry. Is there a way to distinguish lines that were
just autorouted and those actually requiring to be routed say over the
ground plane?

4

No there is not other than to look at the PCB layout and see which leads are actually over a ground plane over the entire route of the trace. The EHCI traces and DSS traces are the highest speed traces on the board and those are over a ground pane. The USB signal traces are also very high speed, so they are handled that way as well. These are common and basic rules that people follow. These requirements are typically specified by the creator of the schematic and listed on the schematic. That tells the layout person what they need to do. Some people will specify a conenction in th eschematic and tag it with an impedance that they layout person needs to maintian for a trace as well.

Gerald