Before you layout a circuit, it's generally a good idea to understand what is happening in your schematic. And before you lay out a schematic, it's generally a good idea to understand what is happening in your block diagram.
"This book is a bit technical. If you are a beginner, it's okay that you don't understand everything that follows -- your first project will still be successful. Try to pick up as much as you can, and ask questions in the forums!"
3. Data Bus Overview
The Universal Serial Bus specification relies on differential pair signaling to transfer data along its length at high speeds. A differential pair consists of two digital signal lines that have the same amplitude but opposite polarity. Our circuit does not necessarily require special handling of the differential pair signals, but we will talk about them nonetheless. And since this is not a course on signal integrity, I'll spare you the physics for now. But feel free to ask questions in the forums.
For a differential pair to work as intended, the individual traces need to have consistent width and have consistent spacing to other conductors, and the two traces should remain as identical as possible -- following an identical path and have identical length. The general idea is that a Low→High transition needs to propagate along the wire parallel to and at the same time as a High→Low transition. The electric fields are confined tightly between the two traces if the two signals travel the same distance over the same dielectric.
This animation shows the electric fields surrounding two opposite polarity pulses (red and green cylinders) moving through two parallel wires. As long as the opposite polarity pulses are timed properly, the electric fields remain tightly confined to the area around the wires. (Similar graphics at AllAboutCircuits.com)
If the traces/wires are not the same lengths, or if the impedance changes in the surrounding environment, the symmetry between the two wires is lost, and one transition will lead or lag the other. When the two wires have the same polarity, the electric fields repel and can spread further from the wires.
This animation shows the electric fields surrounding two similar polarity pulses (red and green cylinders) moving through two parallel wires. The electric field lines here are repulsive.
Electromagnetic interference is the result of rapidly changing electromagnetic fields. If the field lines in the first animation switch to look like the second animation, or back again, the electromagnetic field disturbance will propagate outwards from your traces to other areas of your circuit, or into the broader environment.
So the purpose of differential-pair routing is to ensure that the two wires stay as symmetric as possible over as equal a distance and as short a distance as possible.
In Our Circuit
In our circuit, we have a USB2.0 connection between the SAMD11, ESD Diodes, and the MicroUSB. USB2.0 doesn't have terrifically fast rise/fall times, so as long as we make a minimal effort to route the traces properly, we should be okay. We don't even really need to concern ourselves with an impedance controlled stackup.
Parts and pins have been selected to ensure you can route the two nets USB_DP and USB_DM in a single layer from the SAMD11 to the MicroUSB connector straight through D501, the ESD protection diodes.