Freescale Semiconductor MPC8313E PowerQUICC II Pro Family Reference Manual page 1000

Universal Serial Bus Interface
after it encounters the end of the periodic schedule. The host controller traverses the periodic schedule by
constructing an array offset reference from the PERIODICLISTBASE and the FRINDEX registers (see
Figure 16-43). It fetches the element and begins traversing the graph of linked schedule data structures.
The end of the periodic schedule is identified by a next link pointer of a schedule data structure having its
T-bit set. When the host controller encounters a T-Bit set during a horizontal traversal of the periodic list,
it interprets this as an End-Of-Periodic-List mark. This causes the host controller to cease working on the
periodic schedule and transitions immediately to traversing the asynchronous schedule. Once this
transition is made, the host controller executes from the asynchronous schedule until the end of the
micro-frame.
31
31
Periodic Frame List Base
Address
When the host controller determines that it is time to execute from the asynchronous list, it uses the
operational register ASYNCLISTADDR to access the asynchronous schedule, as shown in
Operational
Registers
USBCMD
USBSTS
AsyncListAddr
Figure 16-44. General Format of Asynchronous Schedule List
The ASYNCLISTADDR register contains a physical memory pointer to the next queue head. When the
host controller makes a transition to executing the asynchronous schedule, it begins by reading the queue
head referenced by the ASYNCLISTADDR register. Software must set queue head horizontal pointer
T-bits to a zero for queue heads in the asynchronous schedule.
16.6.6 Periodic Schedule Frame Boundaries vs. Bus Frame Boundaries
The USB Specification Revision 2.0 requires that the frame boundaries (SOF frame number changes) of
the high-speed bus and the full- and low-speed bus(es) below USB 2.0 hubs be strictly aligned.
MPC8313E PowerQUICC™ II Pro Integrated Processor Family Reference Manual, Rev. 2
16-72
DWord-Aligned
Periodic Frame List Element
Address
12
Figure 16-43. Derivation of Pointer into Frame List Array
•
•
•
•
•
•
12 11 2 1 0
13 12 3 2
Frame Index Register
H
Periodic Frame
List
•
•
•
0
Figure
Freescale Semiconductor
16-44.