Universal Serial Bus InterfaceMCF5253 Reference Manual, Rev. 1Freescale Semiconductor 24-85One queue head is used to manage the data stream for one endpoint. The queue head structure containsstatic endpoint characteristics and capabilities. It also contains a working area from where individual bustransactions for an endpoint are executed. Each qTD represents one or more bus transactions, which isdefined in the context of the EHCI specification as a transfer.The general processing model for the host controller's use of a queue head is simple:• Read a queue head• Execute a transaction from the overlay area• Write back the results of the transaction to the overlay area• Move to the next queue headIf the host controller encounters errors during a transaction, the host controller will set one of the errorreporting bits in the queue head's Status field. The Status field accumulates all errors encountered duringthe execution of a qTD (that is, the error bits in the queue head Status field are sticky until the transfer(qTD) has completed). This state is always written back to the source qTD when the transfer is complete.On transfer (for example, buffer or halt conditions) boundaries, the host controller must auto-advance(without software intervention) to the next qTD. Additionally, the hardware must be able to halt the queueso no additional bus transactions will occur for the endpoint and the host controller will not advance thequeue.24.9.10.1 Buffer Pointer List Use for Data Streaming with qTDsA qTD has an array of buffer pointers, which is used to reference the data buffer for a transfer. The EHCIspecification requires that the buffer associated with the transfer be virtually contiguous. This means thatif the buffer spans more than one physical page, it must obey the following rules:• The first portion of the buffer must begin at some offset in a page and extend through the end ofthe page.• The remaining buffer cannot be allocated in small chunks scattered around memory. For each 4Kchunk beyond the first page, each buffer portion matches to a full 4K page. The final portion, whichmay only be large enough to occupy a portion of a page, must start at the top of the page and becontiguous within that page.Figure 24-51 illustrates these requirements.
