
Functional Description
5-76
82801AA and 82801AB Datasheet
5.15.1.2
Transfer Descriptor (TD)
Transfer Descriptors (TDs) express the characteristics of the transaction requested on USB by a
client. TDs are always aligned on 16-byte boundaries, and the elements of the TD are shown in
Figure 5-13
. The 4 different USB transfer types are supported by a small number of control bits in
the descriptor that the ICH interprets during operation. All Transfer Descriptors have the same
basic, 32-byte structure. During operation, the ICH hardware performs consistency checks on some
fields of the TD. If a consistency check fails, the ICH halts immediately and issues an interrupt to
the system. This interrupt cannot be masked within the ICH.
Figure 5-13. Transfer Descriptor
Table 5-55. TD Link Pointer
Bit
Description
31:4
Link Pointer (LP).
Bits [31:4] Correspond to memory address signals [31:4], respectively. This field
points to another TD or QH.
Reserved. Must be 0 when writing this field.
Depth/Breadth Select (VF).
This bit is only valid for queued TDs and indicates to the hardware
whether it should process in a depth first or breadth first fashion. When set to depth first, it informs
the ICH to process the next transaction in the queue rather than starting a new queue.
1 = Depth first.
0 = Breadth first.
QH/TD Select (Q).
This bit informs the ICH whether the item referenced by the link pointer is
another TD or a QH. This allows the ICH to perform the proper type of processing on the item after it
is fetched
1 = QH.
0 = TD.
Terminate (T).
This bit informs the ICH that the link pointer in this TD does not point to another valid
entry. When encountered in a queue context, this bit indicates to the ICH that there are no more
valid entries in the queue. A TD encountered outside of a queue context with the T bit set informs
the ICH that this is the last TD in the frame.
1 = Link Pointer field not valid.
0 = Link Pointer field is valid.
3
2
1
0
Q
Vf
0
0
1
2
3
4
31
Link Pointer
MaxLen
Buffer Pointer
T
7
8
15
16
23
24
10
11
14
18
19
20
R D
Status
R
ActLen
EndPt
Device Address
PID
R
IOC
ISO
LS
C_ERR
SPD
25
26
27
28
29
30
21
R=Reserved
ICH Read/Write
ICH Read Only