Universal Serial Bus InterfaceMCF5253 Reference Manual, Rev. 124-72 Freescale Semiconductor24.9.6 Periodic Schedule Frame Boundaries vs. Bus Frame BoundariesThe USB Specification Revision 2.0 requires that the frame boundaries (SOF frame number changes) ofthe high-speed bus and the full- and low-speed bus(es) below USB 2.0 hubs be strictly aligned.Super-imposed on this requirement is that USB 2.0 hubs manage full- and low-speed transactions via amicro-frame pipeline (see start- (SS) and complete- (CS) splits illustrated in Figure 24-45). A simple,direct projection of the frame boundary model into the host controller interface schedule architecturecreates tension (complexity for both hardware and software) between the frame boundaries and thescheduling mechanisms required to service the full- and low-speed transaction translator periodicpipelines.Figure 24-45. Frame Boundary Relationship between HS Bus and FS/LS BusThe simple projection, as Figure 24-45 illustrates, introduces frame-boundary wrap conditions forscheduling on both the beginning and end of a frame. In order to reduce the complexity for hardware andsoftware, the host controller is required to implement a one micro-frame phase shift for its view of frameboundaries. The phase shift eliminates the beginning of frame and frame-wrap scheduling boundaryconditions.The implementation of this phase shift requires that the host controller use one register value for accessingthe periodic frame list and another value for the frame number value included in the SOF token. These twovalues are separate, but tightly coupled. The periodic frame list is accessed via the Frame List IndexRegister (FRINDEX). Bits FRINDEX[2:0], represent the micro-frame number. The SOF value is coupledto the value of FRINDEX[13:3]. Both FRINDEX[13:3] and the SOF value are incremented based onFRINDEX[2:0]. It is required that the SOF value be delayed from the FRINDEX value by onemicro-frame. The one micro-frame delay yields a host controller periodic schedule and bus frameboundary relationship as illustrated in Figure 24-46. This adjustment allows the software to triviallyschedule the periodic start and complete-split transactions for full-and low-speed periodic endpoints, usingthe natural alignment of the periodic schedule interface.Figure 24-46 illustrates how periodic schedule data structures relate to schedule frame boundaries and busframe boundaries. To aid the presentation, two terms are defined. The host controller's view of the1-millisecond boundaries is called H-Frames. The high-speed bus's view of the 1-millisecond boundariesis called B-Frames.FS/LS BusHS Bus SS7CSCSCSCSCSCSCSCS076543210FrameBoundarySSMicro-FrameNumbers 1
