Freescale Semiconductor MCF52277 Reference Manual page 408

Universal Serial Bus Interface – On-The-Go Module
Table 20-43. Endpoint Capabilities/Characteristics (continued)
Field
29 Zero length termination select. This bit is ignored in isochronous transfers.
ZLT Clearing this bit enables the hardware to automatically append a zero length packet when the following
conditions are true:
• The packet transmitted equals maximum packet length
• The dTD has exhausted the field Total Bytes
After this the dTD retires. When the device is receiving, if the last packet length received equals the maximum
packet length and the total bytes is zero, it waits for a zero length packet from the host to retire the current dTD.
Setting this bit disables the zero length packet. When the device is transmitting, the hardware does not append
any zero length packet. When receiving, it does not require a zero length packet to retire a dTD whose last packet
was equal to the maximum packet length packet. The dTD is retired as soon as Total Bytes field goes to zero,
or a short packet is received.
0 Enable zero length packet (default).
1 Disable the zero length packet.
Note: Each transfer is defined by one dTD, so the zero length termination is for each dTD. In some software
application cases, the logic transfer does not fit into only one dTD, so it does not make sense to add a zero
length termination packet each time a dTD is consumed. On those cases we recommend to disable the
ZLT feature, and use software to generate the zero length termination.
28–27 Reserved. Reserved for future use and must be cleared.
26–16 Maximum packet length. This directly corresponds to the maximum packet size of the associated endpoint
Maximum (wMaxPacketSize). The maximum value this field may contain is 0x400 (1024).
Packet Length
15 Interrupt on setup (IOS). This bit used on control type endpoints indicates if USBSTS[UI] is set in response to a
IOS setup being received.
14–0 Reserved. Reserved for future use and must be cleared.
20.5.2.1.2 Current dTD Pointer (Offset = 0x4)
The device controller uses the current dTD pointer to locate transfer in progress. This word is for USB
OTG (hardware) use only and should not be modified by DCD software.
Field
31–5 Current dtd. This field is a pointer to the dTD represented in the transfer overlay area. This field is modified by the
Current dtd device controller to next dTD pointer during endpoint priming or queue advance.
4–0 Reserved. Reserved for future use and must be cleared.
20.5.2.1.3 Transfer Overlay (Offset = 0x8–0x20)
The seven longwords in the overlay area represent a transaction working space for the device controller.
The general operational model is that the device controller can detect whether the overlay area contains a
description of an active transfer. If it does not contain an active transfer, it does not read the associated
endpoint.
After an endpoint is readied, the dTD is copied into this queue head overlay area by the device controller.
Until a transfer expires, software must not write the queue head overlay area or the associated transfer
descriptor. When the transfer is complete, the device controller writes the results back to the original
transfer descriptor and advance the queue.
20-50
Description
Table 20-44. Current dTD Pointer
Description
MCF52277 Reference Manual, Rev. 1
Freescale Semiconductor