Universal Serial Bus InterfaceMCF5253 Reference Manual, Rev. 124-58 Freescale Semiconductor24.8.5.3 qTD TokenThe third DWord of a queue element transfer descriptor contains most of the information the hostcontroller requires to execute a USB transaction (the remaining endpoint-addressing information isspecified in the queue head). Note that some of the field descriptions in Table 24-53 reference fieldsdefined in the queue head. See Section 24.8.6, “Queue Head,” for more information on these fields.4–1 – Reserved. These bits are reserved and their value has no effect on operation.0 T Terminate. This bit indicates to the Host Controller that there are no more valid entries in the queue.0 Pointer is valid (points to a valid Transfer Element Descriptor).1 Pointer is invalid.Table 24-53. qTD Token (DWord 2)Bit Name Description31 dt Data Toggle. This is the data toggle sequence bit. The use of this bit depends on the setting of the DataToggle Control bit in the queue head.30–16 Total Bytesto TransferTotal Bytes to Transfer. This field specifies the total number of bytes to be moved with this transferdescriptor. This field is decremented by the number of bytes actually moved during the transaction, onlyon the successful completion of the transaction. The maximum value the software may store in this field is5 × 4K (0x5000). This is the maximum number of bytes 5 page pointers can access. If the value of thisfield is zero when the host controller fetches this transfer descriptor (and the active bit is set), the hostcontroller executes a zero-length transaction and retires the transfer descriptor. It is not a requirement forOUT transfers that Total Bytes To Transfer be an even multiple of QH[Maximum Packet Length]. If thesoftware builds such a transfer descriptor for an OUT transfer, the last transaction will always be less thanQH[Maximum Packet Length]. Although it is possible to create a transfer up to 20K this assumes the pageis 0. When the offset cannot be predetermined, crossing past the 5th page can be guaranteed by limitingthe total bytes to 16K. Therefore, the maximum recommended transfer is 16K(0x4000).15 ioc Interrupt On Complete. If this bit is set, it specifies that when this qTD is completed, the Host Controllershould issue an interrupt at the next interrupt threshold.14–12 C_Page Current Page. This field is used as an index into the qTD buffer pointer list. Valid values are in the range0x0 to 0x4. The host controller is not required to write this field back when the qTD is retired.Table 24-52. qTD Alternate Next Element Transfer Pointer (DWord 1) (continued)Bit Name Description