Programming the USB controller is accomplished through two direct registers that are mapped to the SFR space of the MCU. These registers are called the USB address register (USB0ADR) and the USB data register (USB0DAT). Access to all of the USB registers and the endpoint FIFOs are through an indexing scheme where the USB0ADR register points to the associated register for access and the data register is used to pass the data to the controller. In the example shown in this slide, to write to the common registers the USB0ADR register would set the MUX to where the yellow line is pointing. After the USBADR register is set the data written to the USB0DAT register would then be written to the common register. To access the endpoint registers the index register must first be written to set the MUX to point to the channel highlighted by the red arrow. Then data written will be passed to the associated endpoint pointed to by the index register. Silicon Labs code examples provide routines that handle all of the indirect access which simplifies the implementation.