Information

Universal Serial Bus Interface
MPC8308 PowerQUICC II Pro Processor Reference Manual, Rev. 1
13-56 Freescale Semiconductor
13.5.5 Queue Element Transfer Descriptor (qTD)
This data structure is only used with a queue head. This data structure is used for one or more USB
transactions. This data structure is used to transfer up to 20,480 (5 ¥ 4096) bytes. The structure contains
two structure pointers used for queue advancement, a DWord of transfer state, and a five-element array of
data buffer pointers. This structure is 32 bytes (or one 32-byte cache line). This data structure must be
physically contiguous.
The buffer associated with this transfer must be virtually contiguous. The buffer may start on any byte
boundary. A separate buffer pointer list element must be used for each physical page in the buffer,
regardless of whether the buffer is physically contiguous.
Host controller updates (host controller writes) to stand-alone qTDs only occur during transfer retirement.
References in the following bit field definitions of updates to the qTD are to the qTD portion of a queue
head.
Figure 13-40 shows the queue element transfer descriptors.
Queue element transfer descriptors must be aligned on 32-byte boundaries.
4–1 — Reserved, should be cleared. This field is reserved for future use. It should be cleared.
0 T Terminate
0 siTD Back Pointer field is valid
1 siTD Back Pointer field is not valid
313029282726252423222120191817161514131211109876543210offset
Next qTD Pointer 0000 T 0x00
Alternate Next qTD Pointer
0000 T 0x04
dt
1
1
Host controller read/write; all others read-only.
Total Bytes to Transfer
1
ioc C_Page
1
Cerr
1
PID
Code
Status
1
0x08
Buffer Pointer (Page 0) Current Offset
1
0x0C
Buffer Pointer (Page 1)
0000_0000_0000 0x10
Buffer Pointer (Page 2)
0000_0000_0000 0x14
Buffer Pointer (Page 3)
0000_0000_0000 0x18
Buffer Pointer (Page 4)
0000_0000_0000 0x1C
Figure 13-40. Queue Element Transfer Descriptor (qTD)
Table 13-52. siTD Back Link Pointer (continued)
Bits Name Description