
Communications Processor (CP)
MOTOROLA
MC68302 USER’S MANUAL
4-61
CD—Carrier Detect Lost
The carrier detect signal was negated during message reception.
Data Length
Data length contains the number of octets written by the CP into this BD's data buffer. It
is written by the CP once as the BD is closed.
NOTE
The actual amount of memory allocated for this buffer should be
greater than or equal to the contents of maximum receive buffer
length register (MRBLR).
Rx Buffer Pointer
The receive buffer pointer, which always points to the first location of the associated data
buffer, may be even or odd. The buffer may reside in either internal or external memory.
NOTE
For correct operation of the function codes, the upper 8 bits of
the pointer must be initialized to zero.
4.5.11.15 UART Transmit Buffer Descriptor (Tx BD)
Data is presented to the CP for transmission on an SCC channel by arranging it in buffers
referenced by the channel's Tx BD table. The CP confirms transmission (or indicates error
conditions) through the BDs to inform the M68000 core that the buffers have been serviced.
The Tx BD shown in Figure 4-22.
Figure 4-22. UART Transmit Buffer Descriptor
The first word of the Tx BD contains status and control bits. The following bits are prepared
by the user before transmission and set by the CP after the buffer has been transmitted.
R—Ready
0 = This buffer is not currently ready for transmission. The user is free to manipulate
the BD (or its associated buffer). The CP clears this bit after the buffer has been
transmitted or after an error condition has been encountered.
1 = The data buffer, which has been prepared for transmission by the user, has not
been transmitted or is currently transmitting. No fields of this BD may be written by
the user once this bit is set.
15
R
14
X
13
W
12
I
11
CR
10
A
9
P
8
—
7
—
6
—
5
—
4
—
3
—
2
—
1
—
0
OFFSET + 0
OFFSET + 2
CT
DATA LENGTH
OFFSET + 4
OFFSET +6
TX BUFFER POINTER (24-bits used, upper 8 bits must be 0)