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16.10.8.3 USB TRANSMIT BUFFER DESCRIPTOR. Data to be transmitted with the USB
is presented to the communication processor module by arranging it in buffers referenced
by the transmit (TX) buffer descriptor ring. The first word of the TX buffer descriptor contains
status and control bits.
BIT
0
1
2
OFFSET + 0
R
RES
W
OFFSET + 2
OFFSET + 4
OFFSET + 6
NOTE: You are only responsible for initializing the items in bold.
Note: The communication processor module sets all the status bits in this buffer
descriptor, but you must clear them before submitting the buffer descriptor to the
communication processor module.
R—Ready
0 = The data buffer associated with this buffer descriptor is not ready for transmission.
You are free to manipulate this buffer descriptor or its associated data buffer. The
communication processor module clears this bit after the buffer has been
transmitted or after an error condition is encountered.
1 = The data buffer, which you have prepared for transmission, has not been
transmitted or is currently being transmitted. You cannot write to the fields of this
buffer descriptor once this bit is set.
Bits 1, 10 and 15—Reserved
These bits are reserved and must be set to 0.
W—Wrap (Final Buffer Descriptor in Table)
0 = This is not the last buffer descriptor in the TX buffer descriptor table.
1 = This is the last buffer descriptor in the TX buffer descriptor table. After this buffer
has been used, the communication processor module will transmit incoming data
from the first buffer descriptor in the table (the buffer descriptor pointed to by
TBASEx). The number of TX buffer descriptors in this table is programmable and
is determined only by the W bit and the overall space constraints of the dual-port
RAM.
I—Interrupt
0 = No interrupt is generated after this buffer has been serviced.
1 = The TXB or TXEx bit in the USB event register is set when this buffer is serviced.
TXB and TXEx can cause interrupts if they are enabled.
MOTOROLA
3
4
5
6
7
I
L
TC
CNF LSP
DATA LENGTH
TX DATA BUFFER POINTER
MPC823e REFERENCE MANUAL
Communication Processor Module
8
9
10
11
12
PID
RES NAK STAL
TO
13
14
15
UN
RES
16-369
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