Freescale Semiconductor PowerQUICC MPC885 Reference Manual page 850

Universal Serial Bus (USB)
• Flexible data buffers with multiple buffers per frame
• Automatic retransmission upon transmit error
The USB host controller features are as follows:
• Supports control, bulk, interrupt, and isochronous data transfers
• CRC16 generation and checking
• NRZI encoding/decoding with bit stuffing
• Supports both 12- and 1.5-Mbps data rates (automatic generation of preamble token and data rate
configuration). Note that low-speed operation requires an external hub.
• Flexible data buffers with multiple buffers per frame
• Supports local loopback mode for diagnostics (12 Mbps only)
31.4 Host Controller Limitations
The following tasks are not supported by the hardware and must be implemented in software:
• CRC5 generation for tokens. (Because CRC5 is calculated on 11 bits, this task should not impose
much software overhead.)
• Retransmission after an error and error recovery
• Generation and transmission of an SOF (start of frame) token every 1 ms
• Scheduling the various transfers within and between frames
Because the MPC885 USB host controller does not integrate the root hub, an external hub is required when
more than one device is connected to the host. An external hub is also required for low-speed operation.
Also, note that the host controller programming model is similar to the function endpoint programming
model, but does not conform to the open host controller interface (OHCI) or universal host controller
interface (UHCI) standards in which software drivers are hardware-independent.
31.5 USB Controller Pin Functions and Clocking
As shown in Figure 31-1, the USB controller interfaces to the USB bus through a differential line driver
and differential line receiver. The OE (output enable) signal enables the line driver when the USB
controller transmits on the bus.
31-2
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor