Universal Serial Bus InterfaceMCF5253 Reference Manual, Rev. 124-96 Freescale SemiconductorPointer. At the same time, it sets an internal flag indicating that it is now in Recovery Path mode (therecovery path is annotated in Figure 24-55 with a large dashed line). The host controller continuestraversing data structures on the recovery path and executing only those bus transactions as noted above,on the recovery path until it reaches Restore FSTN (Restore-N). Restore-N.Back Path Link Pointer.T-bitis set (definition of a Restore indicator), so the host controller exits Recovery Path mode by clearing theinternal Recovery Path mode flag and commences (restores) schedule traversal using the saved value ofthe Save-Place FSTN's Normal Path Link Pointer (for example, Save-N.Normal Path Link Pointer). Thenodes traversed during these micro-frames include: {83.0, 83.1, 83.2, Save-A, 82.2, 82.3, 42, 20 , Restore-N,43, 21, Restore-N, 10...}.In frame N+1 (micro-frames 2-7), when the host controller encounters Save-Path FSTN Save-N, itunconditionally follows Save-N.Normal Path Link Pointer. The nodes traversed during these micro-framesinclude: {83.0, 83.1, 83.2, Save-A, 4 3, 21, Restore-N, 10...}.24.9.12.2.3 Software Operational Model for FSTNsThe software must create a consistent, coherent schedule for the host controller to traverse. When usingFSTNs, the system software must adhere to the following rules:• Each Save-Place indicator requires a matching Restore indicator.The Save-Place indicator is an FSTN with a valid Back Path Link Pointer and T-bit equal to zero.Note that Back Path Link Pointer[Typ] field must be set to indicate the referenced data structure isa queue head. The Restore indicator is an FSTN with its Back Path Link Pointer[T] bit set.A Restore FSTN may be matched to one or more Save-Place FSTNs. For example, if the scheduleincludes a poll-rate 1 level, then the system software only needs to place a Restore FSTN at thebeginning of this list in order to match all possible Save-Place FSTNs.• If the schedule does not have elements linked at a poll-rate level of one, and one or moreSave-Place FSTNs are used, then the system software must ensure the Restore FSTN's NormalPath Link Pointer's T-bit is set, as this will be use to mark the end of the periodic list.• When the schedule does have elements linked at a poll rate level of one, a Restore FSTN must bethe first data structure on the poll rate one list. All traversal paths from the frame list converge onthe poll-rate one list. The system software must ensure that Recovery Path mode is exited beforethe host controller is allowed to traverse the poll rate level one list.• A Save-Place FSTN's Back Path Link Pointer must reference a queue head data structure. Thereferenced queue head must be reachable from the previous frame list location. In other words, ifthe Save-Place FSTN is reachable from frame list offset N, then the FSTN's Back Path Link Pointermust reference a queue head that is reachable from frame list offset N-1.The software should make the schedule as efficient as possible. What this means in this context is that thesoftware should have no more than one Save-Place FSTN reachable in any single frame. Note there willbe times when two (or more, depending on the implementation) could exist as full/low-speed footprintschange with bandwidth adjustments. This could occur, for example when a bandwidth rebalance causesthe system software to move the Save-Place FSTN from one poll rate level to another. During thetransition, the software must preserve the integrity of the previous schedule until the new schedule is inplace.
