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Communications Processor (CP)
4-130
MC68302 USER’S MANUAL
MOTOROLA
Bits 11–6—Reserved for future use; should be written with zero.
COMMON SCC MODE BITS—See 4.5.3 SCC Mode Register (SCM) for a description of the
DIAG1, DIAG0, ENR, ENT, MODE1, and MODE0 bits.
4.5.16.8 Transparent Receive Buffer Descriptor (RxBD)
The CP reports information about the received data for each buffer using BD. The Rx BD is
shown in Figure 4-42. The CP closes the current buffer, generates a maskable interrupt, and
starts to receive data into the next buffer after one of the following events:
Detecting an error
Detecting a full receive buffer
Issuing the ENTER HUNT MODE command
Figure 4-42. Transparent Receive Buffer Descriptor
The first word of the Rx BD contains control and status bits.
E—Empty
0 = The data buffer associated with this BD has been filled with received data, or data
reception has been aborted due to an error condition. The M68000 core is free to
examine or write to any fields of this BD.
1 = The data buffer associated with this BD is empty. This bit signifies that the BD and
its associated buffer are available to the CP. After it sets this bit, the M68000 core
should not write to any fields of this BD when this bit is set. The empty bit will re-
main set while the CP is currently filling the buffer with received data.
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 Rx BD table.
1 = This is the last BD in the Rx BD table. After this buffer has been used, the CP will
receive incoming data into the first BD in the table. Setting this bit allows the use
of fewer than eight BDs to conserve internal RAM.
NOTE
The user is required to set the wrap bit in one of the first eight
BDs; otherwise, errant behavior may occur.
15
E
14
X
13
W
12
I
11
—
10
—
9
—
8
—
7
—
6
—
5
—
4
—
3
—
2
—
1
0
OFFSET + 0
OFFSET + 2
OFFSET + 4
OV
CD
DATA LENGTH
OFFSET +6
RX BUFFER POINTER (24-bits used, upper 8 bits must be 0)