The USB controller engine transfers data packets arriving from the USB host into the USB buffers and also transmits valid data from the buffers to the USB host. The controller engine has dedicated, fixed buffer space for input endpoint 0 and output endpoint 0, which are the default USB endpoints for control transfers.
The 14 remaining endpoints (seven input and seven output) may have one or more USB buffers assigned to them. All the buffers are located in the USB buffer memory. This memory is implemented as "multiport" memory, in that it can be accessed both by the USB buffer manager and also by the CPU and DMA. Each endpoint has a dedicated set of descriptor registers that describe the use of that endpoint (see Figure 42-6). Configuration of each endpoint is performed by setting its descriptor registers. These data structures are located in the USB buffer memory and contain address pointers to the next memory buffer for receiving or transmitting.
Assigning one or two data buffers to an endpoint, of up to 64 bytes, requires no further software involvement after configuration. If more than two buffers per endpoint are desired, however, software must change the address pointers on the fly during a receive or transmit process. Synchronization of empty and full buffers is done using validation flags. All events are indicated by flags and fire a vector interrupt when enabled. Transfer event indication can be enabled separately.