Information

Universal Serial Bus Interface
MPC8308 PowerQUICC II Pro Processor Reference Manual, Rev. 1
Freescale Semiconductor 13-57
13.5.5.1 Next qTD Pointer
The first DWord of an element transfer descriptor is a pointer to another transfer element descriptor.
Table 13-53 describes the qTD next element transfer pointer.
13.5.5.2 Alternate Next qTD Pointer
The second DWord of a queue element transfer descriptor is used to support hardware-only advance of the
data stream to the next client buffer on short packet. To be more explicit the host controller will always use
this pointer when the current qTD is retired due to short packet. Table 13-54 describes the alternate qTD
next element transfer pointer.
13.5.5.3 qTD Token
The third DWord of a queue element transfer descriptor contains most of the information the host
controller requires to execute a USB transaction (the remaining endpoint-addressing information is
Table 13-53. qTD Next Element Transfer Pointer (DWord 0)
Bits Name Description
31–5 Next qTD
Pointer
This field contains the physical memory address of the next qTD to be processed and corresponds to
memory address signals [31:5], respectively.
4–1 — Reserved, should be cleared. These bits are reserved and their value has no effect on operation.
0 T Terminate. Indicates to the host controller that there are no more valid entries in the queue.
0 Pointer is valid (points to a valid transfer element descriptor)
1 Pointer is invalid
Table 13-54. qTD Alternate Next Element Transfer Pointer (DWord 1)
Bits Name Description
31–5 Alternate Next
qTD Pointer
This field contains the physical memory address of the next qTD to be processed in the event that
the current qTD execution encounters a short packet (for an IN transaction). The field corresponds
to memory address signals [31:5], respectively.
4–1 — Reserved, should be cleared. These bits are reserved and their value has no effect on operation.
0 T Terminate. Indicates to the host controller that there are no more valid entries in the queue.
0 Pointer is valid (points to a valid transfer element descriptor)
1 Pointer is invalid