Universal Serial Bus InterfaceMCF5253 Reference Manual, Rev. 1Freescale Semiconductor 24-77will automatically execute the value of Mult transactions. The host controller will not execute all Multtransactions if:• The endpoint is an OUT and Transaction n Length goes to zero before all the Mult transactionshave executed (ran out of data), or• The endpoint is an IN and the endpoint delivers a short packet, or an error occurs on a transactionbefore Mult transactions have been executed. The end of micro-frame may occur before all of thetransaction opportunities have been executed. When this happens, the transfer state of the transferdescription is advanced to reflect the progress that was made, the result written back to the iTD andthe host controller proceeds to processing the next micro-frame.24.9.8.2 Software Operational Model for iTDsA client buffer request to an isochronous endpoint may span 1 to N micro-frames. When N is larger thanone, the system software may have to use multiple iTDs to read or write data with the buffer (if N is largerthan eight, it must use more than one iTD).Figure 24-48 illustrates the simple model of how a client buffer is mapped by the system software to theperiodic schedule (that is, the periodic frame list and a set of iTDs). On the right is the client descriptionof its request. The description includes a buffer base address plus additional annotations to identify whichportions of the buffer should be used with each bus transaction. In the middle is the iTD data structuresused by the system software to service the client request. Each iTD can be initialized to service up to 24transactions, organized into eight groups of up to three transactions each. Each group maps to onemicro-frame's worth of transactions. The EHCI controller does not provide per-transaction results withina micro-frame. It treats the per-micro-frame transactions as a single logical transfer. On the left is the hostcontroller’s frame list. The system software establishes references from the appropriate locations in theframe list to each of the appropriate iTDs. If the buffer is large, then the system software can use a smallset of iTDs to service the entire buffer. The system software can activate the transaction description records(contained in each iTD) in any pattern required for the particular data stream.
