Universal Serial Bus InterfaceMCF5253 Reference Manual, Rev. 124-50 Freescale Semiconductor24.8.3.2 iTD Transaction Status and Control ListDWords 1 through 8 are eight slots of transaction control and status. Each transaction description includes:• Status results field• Transaction length (bytes to send for OUT transactions and bytes received for IN transactions).• Buffer offset. The PG and Transaction n Offset fields are used with the buffer pointer list toconstruct the starting buffer address for the transaction.The host controller uses the information in each transaction description plus the endpoint informationcontained in the first three DWords of the Buffer Page Pointer list, to execute a transaction on the USB.2–1 Typ This field indicates to the Host Controller whether the item referenced is an iTD, siTD or a QH. This allowsthe Host Controller to perform the proper type of processing on the item after it is fetched. Value encodingsare:00 iTD (isochronous transfer descriptor)01 QH (queue head)10 siTD (split transaction isochronous transfer descriptor11 FSTN (frame span traversal node)0 T Terminate1 Link Pointer field is not valid0 Link Pointer field is validTable 24-39. iTD Transaction Status and ControlBit Name Description31–28 Status This field records the status of the transaction executed by the host controller for this slot. This field isa bit vector with the following encoding:31 Active. Set by the software to enable the execution of an isochronous transaction by the HostController. When the transaction associated with this descriptor is completed, the Host Controllersets this bit to zero indicating that a transaction for this element should not be executed when it isnext encountered in the schedule.30 Data Buffer Error. Set by the Host Controller during status update to indicate that the Host Controlleris unable to keep up with the reception of incoming data (overrun) or is unable to supply data fastenough during transmission (underrun). If an overrun condition occurs, no action is necessary.29 Babble Detected. Set by the Host Controller during status update when” babble” is detected duringthe transaction generated by this descriptor.28 Transaction Error (XactErr). Set by the Host Controller during status update in the case where thehost did not receive a valid response from the device (Time-out, CRC, Bad PID, etc.). This bit mayonly be set for isochronous IN transactions.27–16 Transaction nLengthFor an OUT, this field is the number of data bytes the host controller will send during the transaction.The host controller is not required to update this field to reflect the actual number of bytes transferredduring the transfer. For an IN, the initial value of the endpoint to deliver. During the status update, thehost controller writes back the field is the number of bytes the host expects the number of bytessuccessfully received. The value in this register is the actual byte count (for example, 0 zero lengthdata, 1 one byte, 2 two bytes, etc.). The maximum value this field may contain is 0xC00 (3072).15 ioc Interrupt on complete. If this bit is set, it specifies that when this transaction completes, the HostController should issue an interrupt at the next interrupt threshold.Table 24-38. Next Schedule Element Pointer (continued)Bit Name Description
