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Part IV. Communications Processor Module
than 8 bits, the data length should be even. For example, to send three characters
of 8-bit data, 1 start, and 1 stop, the data length Þeld should be initialized to 3.
However, to send three characters of 9-bit data, the data length Þeld should be
initialized to 6 since the three 9-bit data Þelds occupy three half-words in
memory. The CP never modiÞes this Þeld.
¥ The word at offset + 4 points to the beginning of the buffer.
Ñ For an RxBD, the pointer must be even and can point to internal or external
memory.
Ñ For a TxBD, the pointer can be even or odd, unless the character exceeds 8 bits,
for which it must be even. The buffer can be in internal or external memory.
33.7.1.1 SPI Receive BD (RxBD)
The CP uses RxBDs to report on each received buffer. It closes the current buffer, generates
a maskable interrupt, and starts receiving data in the next buffer once the current buffer is
full. The CP also closes the buffer when the SPI is conÞgured as a slave and SPISEL is
negated, indicating that reception stopped. The core should write RxBD bits before the SPI
is enabled. The format of an RxBD is shown in Figure 33-11.
0
1
Offset + 0
E
Ñ
Offset + 2
Offset + 4
Offset + 6
Table 33-8 describes the RxBD status and control Þelds.
Table 33-8. SPI RxBD Status and Control Field Descriptions
Bits Name
0
E
Empty.
0 The buffer is full or stopped receiving because of an error. The core can examine or write to any Þelds
of this RxBD, but the CP does not use this BD while E = 0.
1 The buffer is empty or reception is in progress. The CP owns this RxBD and its buffer. Once E is set,
the core should not write any Þelds of this RxBD.
1
Ñ
Reserved, should be cleared.
2
W
Wrap (last BD in table).
0 Not the last BD in the RxBD table.
1 Last BD in the RxBD table. After this buffer is used, the CP receives incoming data using the BD
pointed to by RBASE (top of the table). The number of BDs in this table is determined only by the W
bit and overall space constraints of the dual-port RAM.
3
I
Interrupt.
0 No interrupt is generated after this buffer is Þlled.
1 SPIE[RXB] is set when this buffer is full, indicating the need for the core to process the buffer.
SPIE[RXB] causes an interrupt if not masked.
33-14
2
3
4
5
W
I
L
Ñ
CM
Figure 33-11. SPI RxBD
MPC8260 PowerQUICC II UserÕs Manual
6
7
8
9
10
Ñ
Data Length
Rx Buffer Pointer
Description
11
12
13
14
15
OV
ME
MOTOROLA
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