Motorola MC68302 User Manual page 209

15 14 13 12 11 10
OFFSET '
Q
E
I
x
I
w
I - I - I - I - I - 1 - I - I - J - 1
SE
I - J
ov
J -
OFFSET+ 2 DATA LENGTH
OFFSET +4
OFFSET +6
4-106
RX BUFFER POINTER
Figure 4-21. V.110 Receive Buffer Descriptor
interrupt, and starts to receive data in the next buffer after any of the following
events:
Receiving of 10 bytes (80-bit frame)
Detecting of an error
Issuing the ENTER HUNT MODE command
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 following 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
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 con-
troller. The M68000 core should not write to any fields of this BD
when this bit is set. The empty bit will remain set while the V.110
controller is currently filling the buffer with received data.
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.
X - External Buffer
1 =The buffer associated with this BD is in external memory.
0 =The buffer associated with this BD is in internal dual-port RAM.
W - Wrap (Final BD in 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.
0 =This is not the last BD in the Rx BD table.
NOTE
The user is required to set the wrap bit in one of the first eight BDs;
otherwise, errant behavior may occur.
MC68302 USER'S MANUAL
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