Software Operational Model For Itds - Freescale Semiconductor MPC8313E Family Reference Manual

Universal Serial Bus Interface
updates the appropriate record in the iTD and moves to the next schedule data structure. The maximum
sized transaction supported is 3 × 1024 bytes.
For IN transfers, the host controller issues Mult transactions. It is assumed that software has properly
initialized the iTD to accommodate all possible data. During each IN transaction, the host controller must
use Maximum Packet Size to detect packet babble errors. The host controller keeps the sum of bytes
received in the Transaction n Length field.
After all transactions for the endpoint have completed for the microframe, Transaction n Length contains
the total bytes received. The following actions can occur:
• If the final value of Transaction n Length is less than the value of Maximum Packet Size, less data
was received than was allowed for from the associated endpoint. This short packet condition does
not set USBSTS[UI] (USB interrupt). The host controller does not detect this condition.
• If the device sends more than Transaction n Length or Maximum Packet Size bytes (whichever is
less), the host controller sets the Babble Detected bit and clears the Active bit. Note, that the host
controller is not required to update the iTD field Transaction n Length in this error scenario.
• If the Mult field is greater than one, the host controller automatically executes the value of Mult
transactions. The host controller does not execute all Mult transactions in the following cases:
— The endpoint is an OUT and Transaction n Length goes to zero before all the Mult transactions
have executed (ran out of data).
— The endpoint is an IN and the endpoint delivers a short packet, or an error occurs on a
transaction before Mult transactions have been executed.
The end of microframe may occur before all of the transaction opportunities have been executed.
When this happens, the transfer state of the transfer description is advanced to reflect the progress
that was made; the result is written back to the iTD; and the host controller proceeds to processing
the next microframe.
16.6.8.2

Software Operational Model for iTDs

A client buffer request to an isochronous endpoint may span 1 to N microframes. When N is larger than
one, system software may have to use multiple iTDs to read or write data with the buffer (if N is larger
than eight, it must use more than one iTD).
MPC8313E PowerQUICC II Pro Integrated Processor Family Reference Manual, Rev. 3
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