Universal Serial Bus InterfaceMCF5253 Reference Manual, Rev. 1Freescale Semiconductor 24-6124.8.5.4 qTD Buffer Page Pointer ListThe last five DWords of a queue element transfer descriptor is an array of physical memory addresspointers. These pointers reference the individual pages of a data buffer.The system software initializes Current Offset field to the starting offset into the current page, wherecurrent page is selected via the value in the C_Page field.24.8.6 Queue HeadFigure 24-41 shows the queue head structure.Table 24-54. qTD Buffer PointerBit Name Description31–12 Buffer Pointer(page n)Each element in the list is a 4K page aligned physical memory address. The lower 12 bits in each pointerare reserved (except for the first one), as each memory pointer must reference the start of a 4K page.The field C_Page specifies the current active pointer. When the transfer element descriptor is fetched,the starting buffer address is selected using C_Page (similar to an array index to select an arrayelement). If a transaction spans a 4K buffer boundary, the host controller must detect the page-spanboundary in the data stream, increment C_Page and advance to the next buffer pointer in the list, andconclude the transaction via the new buffer pointer.11–0 Current Offset(Page 0)/–(Pages 1-4)This field is reserved in all pointers except the first one (that is, Page 0). The host controller should ignoreall reserved bits. For the page 0 current offset interpretation, this field is the byte offset into the currentpage (as selected by C_Page). The host controller is not required to write this field back when the qTDis retired. The software should ensure the reserved fields are initialized to zeros.31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 OffsetQueue Head Horizontal Link Pointer 00 Typ T 0x00RL C Maximum Packet Length H dtc EPS EndPt I Device Address 0x0411 Offsets 0x04 through 0x0B contain the static endpoint state.Mult Port Number Hub Addr μFrame C-mask μFrame S-mask 0x081Current qTD Pointer22 Host controller read/write; all others read-only.00000 0x0CNext qTD Pointer 2 0000 T 2 0x1033 Offsets 0x10 through 0x2F contain the transfer overlay.Alternate Next qTD Pointer2 NakCnt 2 T 2 0x143,44 Offsets 0x14 through 0x27 contain the transfer results.dt 1 Total Bytes to Transfer 2 ioc 2 C_Page2 Cerr 2 PIDCode 2 Status 2 0x183,4Buffer Pointer (Page 0) 2 Current Offset 2 0x1C3,4Buffer Pointer (Page 1) 2 0000 C-prog-mask 2 0x203,4Buffer Pointer (Page 2) 2 S-bytes 2 FrameTag2 0x243,4Buffer Pointer (Page 3) 2 0000_0000_0000 0x283Buffer Pointer (Page 4) 2 0000_0000_0000 0x2C 3Figure 24-41. Queue Head Layout
