Universal Serial Bus (USB)
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
31-25
The receive buffer pointer always points to the first location of the associated buffer. The pointer must be
divisible by 4. The buffer may reside in either internal or external memory.
31.12.2 USB Transmit Buffer Descriptor (TxBD) for Function
The data that the USB function wishes to transmit to the host, is arranged in buffers referenced by the
TxBD ring. The first word of the TxBD contains status and control bits.
Table 31-15
describes USB TxBD fields.
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
0
R
—
W
I
L
TC
CNF
PID
—
TO
UN
—
2
DATA LENGTH
4
TX DATA BUFFER POINTER
6
Figure 31-20. USB Transmit Buffer Descriptor (TxBD)
1,2
1
Entries in
boldface
must be initialized by the user.
2
All fields should be prepared by the user before transmission.
Table 31-15. USB Function TxBD Fields
Offset
Bit
Name
Description
0x00
0
R
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.
1
—
Reserved, should be cleared.
2
W
Wrap (Final BD in Table)
0 This is not the last BD in the TxBD table.
1 This is the last BD in the TxBD 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
TxBDs in this table is programmable, and is determined only by the TxBD[W] and the
overall space constraints of the dual-port RAM.
3
I
Interrupt
0 No interrupt is generated after this buffer has been serviced.
1The TXB or TXE bit in the event register is set when this buffer is serviced. TXB and
TXE can cause interrupts if they are enabled.
4
L
Last
0 Buffer does not contain the last byte of the message.
1 Buffer contains the last byte of the message.
5
TC
Transmit CRC. Valid only when the L bit is set; otherwise it is ignored. Prepare TC before
sending data.
0 Transmit end-of-packet after the last data byte. This setting can be used for testing
purposes to send a bad CRC after the data.
1 Transmit the CRC sequence after the last data byte.
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