Information

Universal Serial Bus Interface
MPC8308 PowerQUICC II Pro Processor Reference Manual, Rev. 1
13-76 Freescale Semiconductor
linked first (for example, closest to the periodic frame list), followed by shorter poll rates, with queue
heads with a poll rate of one, on the very end.
Figure 13-47. Example Periodic Schedule
13.6.8 Managing Isochronous Transfers Using iTDs
The structure of an iTD is presented in isochronous (high-speed) transfer descriptor (iTD). There are four
distinct sections to an iTD:
• Next link pointer
— This is the first field.
— This field is for schedule linkage purposes only.
• Transaction descrption array
— This is an eight-element array.
— Each element represents control and status information for one microframe's worth of
transactions for a single high-speed isochronous endpoint.
• Buffer page pointer array
— This is a 7-element array of physical memory pointers to data buffers.
— These are 4K aligned pointers to physical memory.
• Endpoint capabilities
— This area utilizes the unused low-order 12 bits of the buffer page pointer array.
— Its fields are used across all transactions executed for this iTD, including endpoint addressing,
transfer direction, maximum packet size and high-bandwidth multiplier.
Last
Periodic has
End of
List Mark
•
•
•
8
A
A
A
A
A
A
4
1
1024, 512, or 256
Elements
Poll Rate: N ––> 1
Isochronous Transfer
Descriptor(s)
Periodic Frame List
•
•
•
• • •
A
Interrupt Queue
Heads
Poll Rate: 1