Qtd Buffer Page Pointer List - Freescale Semiconductor MPC8313E Family Reference Manual

Universal Serial Bus Interface
Bits Name
16.5.5.4

qTD Buffer Page Pointer List

The last five DWords of a queue element transfer descriptor make up an array of physical memory address
pointers. These pointers reference the individual pages of a data buffer.
System software initializes the Current Offset field to the starting offset into the current page, where
current page is selected with the value in the C_Page field.
Table 16-56 describes the qTD buffer pointer.
Bits Name
31–12 Buffer Pointer
(page n )
11–0 Current Offset
(Page 0)/
—
(Pages 1–4)
MPC8313E PowerQUICC II Pro Integrated Processor Family Reference Manual, Rev. 3
16-62
Table 16-55. qTD Token (DWord 2) (continued)
1 Split transaction state (SplitXstate). This bit is ignored by the host controller
unless the QH[EPS] field indicates a full- or low-speed endpoint. When a full-
or low-speed device, the host controller uses this bit to track the state of the
split- transaction. The functional requirements of the host controller for
managing this state bit and the split transaction protocol depends on whether
the endpoint is in the periodic or asynchronous schedule. The bit encodings
are:
0 Do start split. This value directs the host controller to issue a start split
transaction to the endpoint.
1 Do complete split. This value directs the host controller to issue a Complete
split transaction to the endpoint.
0 Ping state (P)/ERR. If the QH[EPS] field indicates a high-speed device and
the PID Code indicates an OUT endpoint, then this is the state bit for the Ping
protocol. The bit encodings are:
0 Do OUT. This value directs the host controller to issue an OUT PID to the
endpoint.
1 Do Ping. This value directs the host controller to issue a PING PID to the
endpoint.
If the QH[EPS] field does not indicate a high-speed device, then this field is
used as an error indicator bit. It is set by the host controller whenever a
periodic split-transaction receives an ERR handshake.
Table 16-56. qTD Buffer Pointer
Each element in the list is a 4K page aligned physical memory address. The lower 12 bits in each
pointer are 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 array element). If a transaction spans a 4K buffer boundary, the host controller must
detect the page-span boundary in the data stream, increment C_Page and advance to the next
buffer pointer in the list, and conclude the transaction via the new buffer pointer.
Reserved in all pointers except the first one (that is, Page 0). The host controller should ignore all
reserved bits. For the page 0 current offset interpretation, this field is the byte offset into the current
page (as selected by C_Page). The host controller is not required to write this field back when the
qTD is retired. Software should ensure the reserved fields are initialized to zeros.
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