Universal Serial Bus InterfaceMCF5253 Reference Manual, Rev. 124-62 Freescale Semiconductor24.8.6.1 Queue Head Horizontal Link PointerThe first DWord of a Queue Head contains a link pointer to the next data object to be processed after anyrequired processing in this queue has been completed, as well as the control bits defined below.This pointer may reference a queue head or one of the isochronous transfer descriptors. It must notreference a queue element transfer descriptor.24.8.6.2 Endpoint Capabilities/CharacteristicsThe second and third DWords of a Queue Head specifies static information about the endpoint. Thisinformation does not change over the lifetime of the endpoint. There are three types of information in thisregion:• Endpoint Characteristics. These are the USB endpoint characteristics including addressing,maximum packet size, and endpoint speed.• Endpoint Capabilities. These are adjustable parameters of the endpoint. They effect how theendpoint data stream is managed by the host controller.• Split Transaction Characteristics. This data structure is used to manage full- and low-speed datastreams for bulk, control, and interrupt via split transactions to USB2.0 Hub TransactionTranslator. There are additional fields used for addressing the hub and scheduling the protocoltransactions (for periodic).The host controller must not modify the bits in this region.Table 24-55. Queue Head DWord 0Bit Name Description31–5 QHLP Queue Head Horizontal Link Pointer. This field contains the address of the next data object to be processed inthe horizontal list and corresponds to memory address signals [31–5], respectively.4–3 – Reserved. These bits must be written as zeros.2–1 Typ This field indicates to the hardware whether the item referenced by the link pointer is an iTD, siTD or a QH. Thisallows the host controller to perform the proper type of processing on the item after it is fetched.00 iTD (isochronous transfer descriptor)01 QH (queue head)10 siTD (split transaction isochronous transfer descriptor)11 FSTN (frame span traversal node)0 T Terminate.1 Last QH (pointer is invalid).0 Pointer is valid.If the queue head is in the context of the periodic list, a one bit in this field indicates to the host controller that thisis the end of the periodic list. This bit is ignored by the host controller when the queue head is in theAsynchronous schedule. The software must ensure that queue heads reachable by the host controller alwayshave valid horizontal link pointers.
