PCB Routing Guidelines - Part 13 (USB signals)

The first question that comes to mind must USB signals be routed as impedance controlled? To explain this, 

We have chips with USB2.0 compliant but operate on low speed and Full speed USB this meant even though we say USB2.0 they might actually be running at 1.5 Mbps and 12 Mbps rates. USB2.0 actually has a data rate of 480 Mbps which is called a High Speed USB. If we are using low speed and full speed USB, we might not need a impedance controlled board. Boards also work if you do not a continuous ground plane, So, a 2-layer board also works pretty fine without reference plane for 2-layers. This scenario entirely changes, if you are designing a board for compliance requirements. Then it is always preferred to have continuous ground plane. For a board which is designed for 480 Mbps, we need a  impedance controlled board.

While the above points are about the PCB Layout engineer perspective, both the circuit designers and PCB designers must note that the rise time of the Low speed and Full speed USB are in few nano sec (ns) while the High speed USB lines are in pico sec (ps). Obviously, signals in ps rise times need more attention by the designers.

Series resistors on USB lines:

Datasheet might recommend series resistors on the USB D+ and D- lines. Below is the snapshot of the USB specifications from the EFM32GG12 datasheet.

Place the series resistors as close as possible to the source.