Universal Serial Bus InterfaceMCF5253 Reference Manual, Rev. 1Freescale Semiconductor 24-71each micro-frame, if the periodic schedule is enabled (see) then the host controller must execute from theperiodic schedule before executing from the asynchronous schedule. It will only execute from theasynchronous schedule after it encounters the end of the periodic schedule. The host controller traversesthe periodic schedule by constructing an array offset reference from the PERIODICLISTBASE and theFRINDEX registers (see Figure 24-43). It fetches the element and begins traversing the graph of linkedschedule data structures.The end of the periodic schedule is identified by a next link pointer of a schedule data structure having itsT-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 theperiodic schedule and transitions immediately to traversing the asynchronous schedule. Once thistransition is made, the host controller executes from the asynchronous schedule until the end of themicro-frame.Figure 24-43. Derivation of Pointer into Frame List ArrayWhen the host controller determines that it is time to execute from the asynchronous list, it uses theoperational register ASYNCLISTADDR to access the asynchronous schedule, as shown in Figure 24-44.Figure 24-44. General Format of Asynchronous Schedule ListThe ASYNCLISTADDR register contains a physical memory pointer to the next queue head. When thehost controller makes a transition to executing the asynchronous schedule, it begins by reading the queuehead referenced by the ASYNCLISTADDR register. The software must set queue head horizontal pointerT-bits to a zero for queue heads in the asynchronous schedule.See Section 24.9.9, “Asynchronous Schedule” for complete operational details.Periodic FrameList31 12 11 2 1 031 12 13 12 3 2 0DWord-AlignedPeriodic Frame List ElementAddressPeriodic Frame List BaseAddress Frame Index Register•••USBSTSOperationalRegistersUSBCMDAsyncListAddr••••••H
