Universal Serial Bus InterfaceMCF5253 Reference Manual, Rev. 124-78 Freescale SemiconductorFigure 24-48. Example Association of iTDs to Client Request BufferAs noted above, the client request includes a pointer to the base of the buffer and offsets into the buffer toannotate which buffer sections are to be used on each bus transaction that occurs on this endpoint. Thesystem software must initialize each transaction description in an iTD to ensure it uses the correct portionof the client buffer. For example, for each transaction description, the PG field is set to index the correctphysical buffer page pointer and the Transaction Offset field is set relative to the correct buffer pointerpage (for example, the same one referenced by the PG field). When the host controller executes atransaction it selects a transaction description record based on FRINDEX[2:0]. It then uses the currentPage Buffer Pointer (as selected by the PG field) and concatenates to the transaction offset field. The resultis a starting buffer address for the transaction. As the host controller moves data for the transaction, it mustwatch for a page wrap condition and properly advance to the next available Page Buffer Pointer. Thesystem software must not use the Page 6 buffer pointer in a transaction description where the length of thetransfer will wrap a page boundary. Doing so yields undefined behavior. The host controller hardware isnot required to alias the page selector to page zero. USB 2.0 isochronous endpoints can specify a periodgreater than one. The software can achieve the appropriate scheduling by linking iTDs into the appropriateframes (relative to the frame list) and by setting appropriate transaction description elements active bits toa one.24.9.8.2.1 Periodic Scheduling ThresholdThe Isochronous Scheduling Threshold field in the HCCPARAMS capability register is an indicator to thesystem 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 valueof this field indicates to the system software the minimum distance it can update isochronous data (relativeto the current location of the host controller execution in the periodic list) and still have the host controllerprocess them.Frame i+1Frame ListFrame iFrame i+2•••Frame i+nClientRequestClient BufferUSBTransaction•••••••••••••••iTD0iTD1iTDNInformation
