NXP Semiconductors MCF5253, Reference Manual

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Universal Serial Bus Interface
MCF5253 Reference Manual, Rev. 1
24-124 Freescale Semiconductor
24.10.1.2 Transfer Overlay
The seven DWords 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, then it will not read the associated
endpoint.
After an endpoint is readied, the dTD will be copied into this queue head overlay area by the device
controller. Until a transfer is expired, the software must not write the queue head overlay area or the
associated transfer descriptor. When the transfer is complete, the device controller will write the results
back to the original transfer descriptor and advance the queue.
See dTD for a description of the overlay fields.
24.10.1.3 Current dTD Pointer
The current dTD pointer is used by the device controller to locate the transfer in progress. This word is for
USB_DR (hardware) use only and should not be modified by DCD software.
24.10.1.4 Set-Up Buffer
The set-up buffer is dedicated storage for the 8-byte data that follows a set-up PID.
NOTE
Each endpoint has a TX and an RX dQH associated with it, and only the RX
queue head is used for receiving setup data packets.
29 zlt Zero length termination select. This bit is used to indicate when a zero length packet is used to terminate
transfers where to total transfer length is a multiple. This bit is not relevant for Isochronous transfers.
0 Enable zero length packet to terminate transfers equal to a multiple of the Maximum Packet Length.
(default).
1 Disable the zero length packet on transfers that are equal in length to a multiple Maximum Packet
Length.
28–27 – Reserved. These bit reserved for future use and should be cleared.
26–16 Maximum
Packet
Length
Maximum packet length. This directly corresponds to the maximum packet size of the associated endpoint
(wMaxPacketSize). The maximum value this field may contain is 0x400 (1024).
15 ios Interrupt on setup (IOS). This bit is used on control type endpoints to indicate if USBINT is set in response
to a setup being received.
14–0 Reserved. Bits reserved for future use and should be cleared.
Table 24-75. Current dTD Pointer
Bit Description
31–5 Current dtd. This field is a pointer to the dTD that is represented in the transfer overlay area. This field will be
modified by the Device Controller to next dTD pointer during endpoint priming or queue advance.
4–0 Reserved. Bit reserved for future use and should be cleared.
Table 24-74. Endpoint Capabilities/Characteristics (continued)
Bit Name Description