An app note from Silicon Labs with design guidelines for implementing USB host and device applications using USB capable EFM32 microcontrollers. Link here. (PDF!)
This document will explain how to connect the USB pins of an EFM32 microcontroller, and will give general guidelines on PCB design for USB applications. First some quick rules-of-thumb for routing and layout are presented before a more detailed explanation follows.
The information in this document is meant to supplement the information already presented in Energy Micro application notes AN0002 Hardware Design Considerations and AN0016 Oscillator Design Considerations, and it is recommended to follow these guidelines as well.