82801BA Datasheet, PDF (171/498 Pages) Intel Corporation – Intel 82801BA I/O Controller Hub 2 (ICH2) and Intel 82801BAM I/O Controller Hub 2 Mobile

Functional Description
5.16.1.2 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 5-16. The 4 different USB transfer types are supported by a small number of control bits in
the descriptor that the ICH2 interprets during operation. All Transfer Descriptors have the same
basic, 32-byte structure. During operation, the ICH2 hardware performs consistency checks on
some fields of the TD. If a consistency check fails, the ICH2 halts immediately and issues an
interrupt to the system. This interrupt cannot be masked within the ICH2.
Figure 5-16. Transfer Descriptor
31 30 29 28 27 26 25 24 23 21 20 19 18 16 15 14
Link Pointer
11 10 8 7
43 2 1 0
0 Vf Q T
R SPD C_ERR LS ISOISC
Status
R
ActLen
MaxLen
R D EndPt
Device Address
PID
Buffer Pointer
R = Reserved
ICH2 Read/Write
ICH2 Read Only
T fd
Table 5-57. 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.
3 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
2
the ICH2 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 ICH2 whether the item referenced by the link pointer is
another TD or a QH. This allows the ICH2 to perform the proper type of processing on the item after
1
it is fetched
1 = QH.
0 = TD.
Terminate (T). This bit informs the ICH2 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 ICH2 that there are no
more valid entries in the queue. A TD encountered outside of a queue context with the T bit set
0 _{informs the ICH2 that this is the last TD in the frame.}
1 = Link Pointer field not valid.
0 = Link Pointer field is valid.
82801BA ICH2 and 82801BAM ICH2-M Datasheet
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