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Communications Processor (CP)
Rx Buffer Pointer
This pointer contains the address of the associated data buffer and may be even or odd.
The buffer may reside in either internal or external memory.
For correct operation of the function codes, the upper 8 bits of
the pointer must be initialized to zero.
4.5.14.11 DDCMP Transmit Buffer Descriptor (Tx BD)
Data is presented to the CP for transmission over an SCC channel by arranging it in buffers
referenced by the channel's Tx BD table. The CP confirms transmission (or indicates error
conditions) using the BDs to inform the M68000 core that the buffers have been serviced.
The Tx BD is shown in Figure 4-37.
15
14
OFFSET + 0
R
X
OFFSET + 2
OFFSET + 4
OFFSET +6
Figure 4-37. DDCMP Transmit Buffer Descriptor
The first word contains status and control bits. Bits 15–9 are prepared by the user before
transmission. Bits 1–0 are set by the DDCMP controller after the buffer has been transmit-
ted. Bit 15 is set by the user when the buffer and BD have been prepared and is cleared by
the DDCMP controller when the message is transmitted.
R—Ready
0 = This buffer is not currently ready for transmission. The user is free to manipulate
this BD (or its associated buffer). The DDCMP controller clears this bit after the
buffer has been completely transmitted (or after an error condition is encountered).
1 = The data buffer has been prepared for transmission by the user (but not yet trans-
mitted). No fields of this BD may be written by the user once this bit is set.
X—External Buffer
0 = The buffer associated with this BD is in internal dual-port RAM.
1 = The buffer associated with this BD is in external memory.
W—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 DDCMP
controller will transmit data from the first BD in the table.
The user is required to set the wrap bit in one of the first eight
BDs; otherwise, errant behavior may occur.
4-112
13
12
11
10
W
I
L
TC
TX BUFFER POINTER (24-bits used, upper 8 bits must be 0)
MC68302 USER'S MANUAL
NOTE
9
8
7
6
OL
—
—
—
DATA LENGTH
NOTE
5
4
3
2
—
—
—
—
MOTOROLA
1
0
UN
CT
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