Freescale Semiconductor MCF54455 Reference Manual page 261

Universal Serial Bus Interface – On-The-Go Module
Field
31–5 Next dTD pointer. This field contains the physical memory address of the next dTD to be processed. The field
Next dTD corresponds to memory address signals [31:5], respectively.
pointer
4–1 Reserved. Reserved for future use and must be cleared.
0 Terminate. This bit indicates to the device controller no more valid entries exist in the queue.
T 0=Pointer is valid (points to a valid transfer element descriptor).
1=pointer is invalid.
10.5.2.2.2 dTD Token (Offset = 0x4)
The dTD token is used to specify attributes for the transfer including the number of bytes to read or write
and the status of the transaction.
Field
31 Reserved. Reserved for future use and must be cleared.
30–16 Total bytes. This field specifies the total number of bytes moved with this transfer descriptor. This field decrements
Total Bytes by the number of bytes actually moved during the transaction and only on the successful completion of the
transaction.
The maximum value software may store in the field is 5*4K(0x5000). This is the maximum number of bytes 5 page
pointers can access. Although possible to create a transfer up to 20K, this assumes the first offset into the first
page is 0. When the offset cannot be predetermined, crossing past the fifth page can be guaranteed by limiting
the total bytes to 16K**. Therefore, the maximum recommended transfer is 16K (0x4000).
Note: Larger transfer sizes can be supported, but require disabling ZLT and using multiple dTDs.
If the value of the field is 0 when the host controller fetches this transfer descriptor (and the active bit is set), the
device controller executes a zero-length transaction and retires the transfer descriptor.
For IN transfers it is not a requirement for total bytes to transfer be an even multiple of the maximum packet length.
If software builds such a transfer descriptor for an IN transfer, the last transaction is always less than maximum
packet length.
For OUT transfers the total bytes must be evenly divisible by the maximum packet length.
15 Interrupt on complete. Indicates if USBSTS[UI] is set in response to device controller finished with this dTD.
IOC
14–12 Reserved. Reserved for future use and must be cleared.
10-54
Table 10-47. Next dTD Pointer
Description
Table 10-48. dTD Token
Description
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