Motorola PowerQUICC II MPC8280 Series Reference Manual page 888

USB Buffer Descriptor Ring
Tx buffer pointer (the third and fourth half words of a TxBD) always points to the first
location of the buffer in internal or external memory. The pointer may be even or odd.
27.6.3 USB Transmit Buffer Descriptor (Tx BD) for Host
The Tx BD described in this section is used when the packet-level interface is active. See
Section 27.5.1.1, "Packet-Level Interface," for more information.
Data to be transmitted with the USB to the CP by is arranged in buffers referenced by the
Tx BD ring. The first word of the Tx BD contains status and control bits.
0 1
OFFSET + 0 R —
OFFSET + 2
OFFSET + 4
OFFSET + 6
Figure 27-20. USB Transmit Buffer Descriptor (Tx BD)
1
Entries in boldface must be initialized by the user.
2
All fields should be prepared by the user before transmission.
Table 27-17 describes USB TxBD fields.
Offset Bit Name
0x00 0 R
1 —
2 W
3 I
4 L
27-30
Freescale Semiconductor, Inc.
2 3 4 5 6
W I L TC CNF LSP
TX DATA BUFFER POINTER
Table 27-18. USB Host Tx BD Fields
Ready
0 The data buffer associated with this BD is not ready for transmission. The user is
free to manipulate this BD or its associated data buffer. The CP clears this bit after
the buffer has been transmitted or after an error condition is encountered.
1 The data buffer, which has been prepared for transmission by the user, has not been
transmitted or is currently being transmitted. No fields of this BD may be written by
the user once this bit is set.
Reserved, should be cleared
Wrap (Final BD in Table)
0 This is not the last BD in the Tx BD table.
1 This is the last BD in the Tx BD table. After this buffer has been used, the CP will
send data using the first BD in the table (the BD pointed to by TBASEx). The
number of Tx BDs in this table is programmable, and is determined only by the Tx
BD[W] and the overall space constraints of the dual-port RAM.
Interrupt
0 No interrupt is generated after this buffer has been serviced.
1The TXB bit in the event register is set when this buffer is serviced. TXB can cause
an interrupt if it is enabled.
Last
0 Buffer does not contain the last byte of the message
1 Buffer contains the last byte of the message
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PID — NAK STAL
DATA LENGTH
Description
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TO UN —
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MOTOROLA