Here is an app note from Cypress with design guidelines for their EZ-USB FX1 and FX2LP USB peripheral ICs. They are basically 8051 microcontrollers with a built in USB serial interface engine.
While these devices are easy to use, sometimes getting a new board up and running is tricky due to the various details that need to be carefully followed to get the hardware functional. AN15456 not only helps catch potential problems before building a board, but also helps debug the hardware after the board is created. The goal is to get the board ready to download firmware and begin the firmware development.