Universal Serial Bus InterfaceMCF5253 Reference Manual, Rev. 124-126 Freescale SemiconductorTable 24-78. dTD TokenBit Description31 Reserved. Bit reserved for future use and should be cleared.30–16 Total Bytes. This field specifies the total number of bytes to be moved with this transfer descriptor. This field isdecremented by the number of bytes actually moved during the transaction and only on the successful completion ofthe transaction.The maximum value the software may store in the field is 5*4K(5000H). This is the maximum number of bytes 5 pagepointers can access. Although it is possible to create a transfer up to 20K this assumes the 1st offset into the first pageis 0. When the offset cannot be predetermined, crossing past the 5th page can be guaranteed by limiting the total bytesto 16K**. Therefore, the maximum recommended transfer is 16K(4000H).If the value of the field is zero when the host controller fetches this transfer descriptor (and the active bit is set), thedevice controller executes a zero-length transaction and retires the transfer descriptor.It is not a requirement for IN transfers that Total Bytes To Transfer be an even multiple of Maximum Packet Length. Ifthe software builds such a transfer descriptor for an IN transfer, the last transaction will always be less that MaximumPacket Length.15 Interrupt On Complete (IOC). This bit is used to indicate if USBINT is to be set in response to device controller beingfinished with this dTD.14–12 Reserved. Bits reserved for future use and should be cleared.11–10 Multiplier Override (MultO). This field can be used for transmit ISO's (that is, ISO-IN) to override the multiplier in theQH. This field must be zero for all packet types that are not transmit-ISO.Example:If QH.multiplier = 3; Maximum packet size = 8; Total Bytes = 15; MultiO = 0 [default]Three packets are sent: {Data2(8); Data1(7); Data0(0)}If QH.multiplier = 3; Maximum packet size = 8; Total Bytes = 15; MultiO = 2Two packets are sent: {Data1(8); Data0(7)}For maximal efficiency, the software should compute MultO = greatest integer of (Total Bytes / Max. Packet Size)except for the case when Total Bytes = 0; then MultO should be 1.Note: Non-ISO and Non-TX endpoints must set MultO= 00.9–8 Reserved. Bits reserved for future use and should be cleared.7–0 Status. This field is used by the Device Controller to communicate individual command execution states back to theDevice Controller software. This field contains the status of the last transaction performed on this qTD. The bitencodings are:Bit Status Field Description7 Active.6 Halted.5 Data Buffer Error.3 Transaction Error.4,2,0 Reserved.
