Information

Universal Serial Bus Interface
MPC8308 PowerQUICC II Pro Processor Reference Manual, Rev. 1
Freescale Semiconductor 13-47
The periodic frame list is a 4K-page aligned array of frame list link pointers. The length of the frame list
may be programmable. The programmability of the periodic frame list is exported to system software
through the HCCPARAMS register. If non-programmable, the length is 1024 elements. If programmable,
the length can be selected by system software as one of 256, 512, or 1024 elements. An implementation
must support all three sizes. Programming the size (that is, the number of elements) is accomplished by
system software writing the appropriate value into frame list size field in the USBCMD register.
Frame list link pointers direct the host controller to the first work item in the frame’s periodic schedule for
the current microframe. The link pointers are aligned on DWord boundaries within the frame list.
Figure 13-36 shows the format for the frame list link pointer.
Frame list link pointers always reference memory objects that are 32-byte aligned. The referenced object
may be an isochronous transfer descriptor for high-speed devices, a split-transaction isochronous transfer
descriptor (for full-speed isochronous endpoints), or a queue head (used to support high-, full- and
low-speed interrupt). System software should not place non-periodic schedule items into the periodic
schedule. The least-significant bits in a frame list pointer are used to key the host controller in as to the
type of object the pointer is referencing.
The least-significant bit is the T bit (bit 0). When this bit is set, the host controller never uses the value of
the frame list pointer as a physical memory pointer. The Typ field indicates the exact type of data structure
being referenced by this pointer. The value encodings for the Typ field are given in Table 13-39.
13.5.2 Asynchronous List Queue Head Pointer
The asynchronous transfer list (based at the ASYNCLISTADDR register) is where all the control and bulk
transfers are managed. Host controllers use this list only when it reaches the end of the periodic list, the
periodic list is disabled, or the periodic list is empty. Figure 13-37 shows the asynchronous schedule
organization.
31 543210
Frame List Link Pointer 00 Typ T
Figure 13-36. Frame List Link Pointer Format
Table 13-39. Typ Field Encodings
Typ Description
00 Isochronous transfer descriptor
01 Queue head
10 Split transaction isochronous transfer descriptor
11 Frame span traversal node