
Communication Processor Module
MOTOROLA
MPC823 USER’S MANUAL
16-361
USB
COMMUNICATION
16
PROCESSOR
MODULE
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.
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, 6–12, and 15—Reserved
These bits are reserved and should 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.
BIT
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
OFFSET + 0
R
RES
WI
L
TC
CNF RES
PID
RES
NAK STAL
TO
UN
RES
OFFSET + 2
DATA LENGTH
OFFSET + 4
TX DATA BUFFER POINTER
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. You should clear all the status bits before submitting the buffer
descriptor to the communication processor module. For example, the parity error
bit is only set when a parity error occurs.