Communications Processor (CP)
MOTOROLA
MC68302 USER’S MANUAL
4-119
The first word of the Rx BD contains control and status bits. Bits 15–13 are written by the
user before the buffer is linked to the Rx BD table, and bits 1 and 3 are set by the IMP fol-
lowing message reception. Bit 15 is set by the M68000 core when the buffer is available to
the V.110 controller and is cleared by the V.110 controller after filling the buffer.
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 the 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 V.110 controller. The M68000 core should
not write to any fields of this BD after it sets this bit. The empty bit will remain set
while the V.110 controller 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 V.110
controller receives incoming data by placing it in the first BD in the table.
NOTE
The user is required to set the wrap bit in one of the first eight
BDs; otherwise, errant behavior may occur.
Bits 12–4, 2, 0—Reserved for future use.
SE—Synchronization Error
A frame with a synchronization error was received. A synchronization error is detected by
the V.110 controller when the MSB of a byte (except the all-zeros byte) is not one.
OV—Overrun
A receiver overrun occurred during message reception.
15
E
14
X
13
W
12
—
11
—
10
—
9
—
8
—
7
—
6
—
5
—
4
—
3
2
—
1
0
—
OFFSET + 0
OFFSET + 2
OFFSET + 4
SE
OV
DATA LENGTH
OFFSET +6
RX BUFFER POINTER (24-bits used, upper 8 bits must be 0)
Figure 4-40. V.110 Receive Buffer Descriptor