NXP Semiconductors MCF5253, Reference Manual, Page: 603 / 648

Universal Serial Bus Interface
MCF5253 Reference Manual, Rev. 1
Freescale Semiconductor 24-141
24.11.4 Managing Queue Heads
The device queue head (dQH) points to the linked list of transfer tasks, each depicted by the device
Transfer Descriptor (dTD). An area of memory pointed to by ENDPOINTLISTADDR contains a group of
all dQH's in a sequential list as shown in
Figure 24-64 . The even elements in the list of dQH's are used for
receive endpoints (OUT/SETUP) and the odd elements are used for transmit endpoints (IN/INTERRUPT).
Device transfer descriptors are linked head to tail starting at the queue head and ending at a terminate bit.
Once the dTD has been retired, it will no longer be part of the linked list from the queue head. Therefore,
the software is required to track all transfer descriptors since pointers will no longer exist within the queue
head once the dTD is retired (see
Section 24.11.5.1, “Software Link Pointers ” ).
In addition to the current and next pointers and the dTD overlay examined in Section 24.11.3.3, “Device
Operational Model For Packet Transfers ” , the dQH also contains the following parameters for the
associated endpoint: Multipler, Maximum Packet Length, Interrupt On Setup. The complete initialization
of the dQH including these fields is demonstrated in the next section.
Ping
Ignore Ignore Ignore Ignore Ignore
Invalid
Ignore Ignore Ignore Ignore Ignore
1
Zero Length Packet
2
Force Bit Stuff Error
Table 24-87. Isochronous Endpoint Bus Response Matrix (continued)
Stall Not Primed Primed Underflow Overflow