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Zynq-7000 AP SoC Technical Reference Manual www.xilinx.com 466
UG585 (v1.11) September 27, 2016
Chapter 15: USB Host, Device, and OTG Controller
A 0 value in this field, in combination with existing in the periodic frame list has undefined results.
This field should have only one bit set to 1 at any given time. Having more than one bit set will result
in undefined results.
QH DWord 3: Current qTD Pointer
The DWord 3 of a Queue Head contains a pointer to the source qTD currently associated with the
overlay. The host controller uses this pointer to write back the overlay area into the source qTD after
the transfer is complete.
15.12.7 Transfer Overlay Area
The nine DWords in this area represent a transaction working space for the host controller. The
general operational model is that the host controller can detect whether the overlay area contains a
description of an active transfer. If it does not contain an active transfer, then it follows the Queue
Head Horizontal Link Pointer to the next queue head. The host controller will never follow the Next
Transfer Queue Element or Alternate Queue Element pointers unless it is actively attempting to
advance the queue. For the duration of the transfer, the host controller keeps the incremental status
of the transfer in the overlay area. When the transfer is complete, the results are written back to the
original queue element.
Table 15-43: USB Host QH DWord 3
Bits Description
31:5 Current Element Transaction Descriptor Link Pointer. Current qTD Pointer. This field contains the
address Of the current transaction being processed in this queue and corresponds to memory address
signals [31:5].
4:0 Reserved. Write 0.
Table 15-44: USB Host Transfer Overlay Descriptors
Bits Description
qTD
DWord
QH
DWord
Next qTD Pointer
04
31:5 Next qTD Pointer. This field contains the address of the next transaction being processed in
this queue and corresponds to memory address signals [31:5], respectively.
4:1 Reserved. Write 0.
0 Terminate transfer, T.
0: link to the Next qTD Pointer field; the address is valid.
1: end the transaction, the Next qTD Pointer field is not valid.