22-12
MPC860 PowerQUICC UserÕs Manual
MOTOROLA
Part V. The Communications Processor Module
¥
The word at 0x4 (buffer pointer) points to the beginning of the buffer in
memory (internal or external).
Ñ For an RxBD, the value must be even.
Ñ For a TxBD, this pointer can be even or odd.
Shown in Figure 22-6, the format of Tx and Rx BDs is the same in each SCC mode. Only
the status and control bits differ for each protocol.
For frame-oriented protocols, a message can reside in as many buffers as necessary. Each
buffer has a maximum length of 65,535 bytes. The CP does not assume that all buffers of a
single frame are currently linked to the BD table. The CP does assume, however, that the
unlinked buffers are provided by the core in time to be sent or received; otherwise, an error
condition is reportedÑan underrun error when sending and a busy error when receiving.
Figure 22-7 shows the SCC BD table and buffer structure.
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
0
Status and Control
2
Data Length
4
High-Order Buffer Pointer
6
Low-Order Buffer Pointer
Figure 22-6. SCC Buffer Descriptors (BDs)
Содержание MPC860 PowerQUICC
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