NXP Semiconductors MCF5253, Reference Manual, Page: 540 / 648

Universal Serial Bus Interface
MCF5253 Reference Manual, Rev. 1
24-78 Freescale Semiconductor
Figure 24-48. Example Association of iTDs to Client Request Buffer
As noted above, the client request includes a pointer to the base of the buffer and offsets into the buffer to
annotate which buffer sections are to be used on each bus transaction that occurs on this endpoint. The
system software must initialize each transaction description in an iTD to ensure it uses the correct portion
of the client buffer. For example, for each transaction description, the PG field is set to index the correct
physical buffer page pointer and the Transaction Offset field is set relative to the correct buffer pointer
page (for example, the same one referenced by the PG field). When the host controller executes a
transaction it selects a transaction description record based on FRINDEX[2:0]. It then uses the current
Page Buffer Pointer (as selected by the PG field) and concatenates to the transaction offset field. The result
is a starting buffer address for the transaction. As the host controller moves data for the transaction, it must
watch for a page wrap condition and properly advance to the next available Page Buffer Pointer. The
system software must not use the Page 6 buffer pointer in a transaction description where the length of the
transfer will wrap a page boundary. Doing so yields undefined behavior. The host controller hardware is
not required to alias the page selector to page zero. USB 2.0 isochronous endpoints can specify a period
greater than one. The software can achieve the appropriate scheduling by linking iTDs into the appropriate
frames (relative to the frame list) and by setting appropriate transaction description elements active bits to
a one.
24.9.8.2.1 Periodic Scheduling Threshold
The Isochronous Scheduling Threshold field in the HCCPARAMS capability register is an indicator to the
system software as to how the host controller pre-fetches and effectively caches schedule data structures.
It is used by the system software when adding isochronous work items to the periodic schedule. The value
of this field indicates to the system software the minimum distance it can update isochronous data (relative
to the current location of the host controller execution in the periodic list) and still have the host controller
process them.
Frame i+1
Frame List
Frame i
Frame i+2
•
•
•
Frame i+n
Client
Request
Client Buffer
USB
Transaction
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
iTD
0
iTD
1
iTD
N
Information