Information

Universal Serial Bus Interface
MPC8308 PowerQUICC II Pro Processor Reference Manual, Rev. 1
Freescale Semiconductor 13-75
Software is allowed to write to FRINDEX. Section 13.3.2.4, “Frame Index Register (FRINDEX),”
provides the requirements that software should adhere when writing a new value in FRINDEX.
13.6.7 Periodic Schedule
The periodic schedule traversal is enabled or disabled through USBCMD[PSE] (periodic schedule enable).
If USBCMD[PSE] is cleared, then the host controller simply does not try to access the periodic frame list
via the PERIODICLISTBASE register. Likewise, when USBCMD[PSE] is a one, then the host controller
does use the PERIODICLISTBASE register to traverse the periodic schedule. The host controller will not
react to modifications to USBCMD[PSE] immediately. In order to eliminate conflicts with split
transactions, the host controller evaluates USBCMD[PSE] only when FRINDEX[2–0] is zero. System
software must not disable the periodic schedule if the schedule contains an active split transaction work
item that spans the 0b000 microframe. These work items must be removed from the schedule before
USBCMD[PSE] is cleared. USBSTS[PS] (periodic schedule status) indicates status of the periodic
schedule. System software enables (or disables) the periodic schedule by setting (or clearing)
USBCMD[PSE]. Software then can poll USBSTS[PS] to determine when the periodic schedule has made
the desired transition. Software must not modify USBCMD[PSE] unless the value of USBCMD[PSE]
equals that of USBSTS[PS].
The periodic schedule is used to manage all isochronous and interrupt transfer streams. The base of the
periodic schedule is the periodic frame list. Software links schedule data structures to the periodic frame
list to produce a graph of scheduled data structures. The graph represents an appropriate sequence of
transactions on the USB. Figure 13-47 illustrates isochronous transfers (using iTDs and siTDs) with a
period of one are linked directly to the periodic frame list. Interrupt transfers (are managed with queue
heads) and isochronous streams with periods other than one are linked following the period-one
iTD/siTDs. Interrupt queue heads are linked into the frame list ordered by poll rate. Longer poll rates are
Table 13-65. Operation of FRINDEX and SOFV (SOF Value Register)
Current Next
FRINDEX[13–3] SOFV FRINDEX[2–0] FRINDEX[13–3] SOFV FRINDEX[2–0]
NN111N+1N000
N+1 N 000 N+1 N+1 001
N+1 N+1 001 N+1 N+1 010
N+1 N+1 010 N+1 N+1 011
N+1 N+1 011 N+1 N+1 100
N+1 N+1 100 N+1 N+1 101
N+1 N+1 101 N+1 N+1 110
N+1 N+1 110 N+1 N+1 111