Information

Universal Serial Bus Interface
MPC8308 PowerQUICC II Pro Processor Reference Manual, Rev. 1
13-88 Freescale Semiconductor
host controller automatically moving to the next page pointer (that is, C_Page) when necessary. There are
three conditions for how the host controller handles C_Page.
• The current transaction does not span a page boundary. The value of C_Page is not adjusted by the
host controller.
• The current transaction does span a page boundary. The host controller must detect the page cross
condition and advance to the next buffer while streaming data to/from the USB.
• The current transaction completes on a page boundary (that is, the last byte moved for the current
transaction is the last byte in the page for the current page pointer). The host controller must
increment C_Page before writing back status for the transaction.
Note that the only valid adjustment the host controller may make to C_Page is to increment by one.
13.6.10.2 Adding Interrupt Queue Heads to the Periodic Schedule
The link path(s) from the periodic frame list to a queue head establishes in which frames a transaction can
be executed for the queue head. Queue heads are linked into the periodic schedule so they are polled at the
appropriate rate. System software sets a bit in a queue head's S-Mask to indicate which microframe within
a 1 millisecond period a transaction should be executed for the queue head. Software must ensure that all
queue heads in the periodic schedule have S-Mask set to a non-zero value. An S-mask with a zero value
in the context of the periodic schedule yields undefined results.
If the desired poll rate is greater than one frame, system software can use a combination of queue head
linking and S-Mask values to spread interrupts of equal poll rates through the schedule so that the periodic
bandwidth is allocated and managed in the most efficient manner possible. Some examples are illustrated
in Table 13-66.
13.6.10.3 Managing Transfer Complete Interrupts from Queue Heads
The host controller sets an interrupt to be signaled at the next interrupt threshold when the completed
transfer (qTD) has an Interrupt on Complete (IOC) bit set, or whenever a transfer (qTD) completes with a
short packet. If system software needs multiple qTDs to complete a client request (that is, like a control
transfer) the intermediate qTDs do not require interrupts. System software may only need a single interrupt
to notify it that the complete buffer has been transferred. System software may set IOC's to occur more
frequently. A motivation for this may be that it wants early notification so that interface data structures can
be re-used in a timely manner.
Table 13-66. Example Periodic Reference Patterns for Interrupt Transfers
Frame #
Reference
Sequence
Description
0, 2, 4, 6, 8, ....
S-Mask = 0x01
A queue head for the bInterval of 2 milliseconds (16 microframes) is linked into the periodic schedule so
that it is reachable from the periodic frame list locations indicated in the previous column. In addition, the
S-Mask field in the queue head is set to 0x01, indicating that the transaction for the endpoint should be
executed on the bus during microframe 0 of the frame.
0, 2, 4, 6, 8, ...
S-Mask = 0x02
Another example of a queue head with a bInterval of 2 milliseconds is linked into the periodic frame list at
exactly the same interval as the previous example. However, the S-Mask is set to 0x02 indicating that the
transaction for the endpoint should be executed on the bus during microframe 1 of the frame.