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Universal Serial Bus Interface
MCF5253 Reference Manual, Rev. 1
Freescale Semiconductor
24-75
Figure 24-47. Example Periodic Schedule
24.9.8
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:
•
The first field is the Next Link Pointer. This field is for schedule linkage purposes only.
•
Transaction description array. This area is an eight-element array. Each element represents control
and status information for one micro-frame's worth of transactions for a single high-speed
isochronous endpoint.
•
The buffer page pointer array 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. The fields in this area are used across all transactions executed for this iTD, including
endpoint addressing, transfer direction, maximum packet size and high-bandwidth multiplier.
24.9.8.1
Host Controller Operational Model for iTDs
The host controller uses FRINDEX register bits [12:3] to index into the periodic frame list. This means
that the host controller visits each frame list element eight consecutive times before incrementing to the
next periodic frame list element. Each iTD contains eight transaction descriptions, which map directly to
FRINDEX register bits [2:0]. Each iTD can span 8 micro-frames worth of transactions. When the host
controller fetches an iTD, it uses FRINDEX register bits [2:0] to index into the transaction description
array. If the active bit in the Status field of the indexed transaction description is cleared, the host controller
ignores the iTD and follows the Next pointer to the next schedule data structure.
When the indexed active bit is a one the host controller continues to parse the iTD. It stores the indexed
transaction description and the general endpoint information (device address, endpoint number, maximum
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
Содержание MCF5253
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