Processor Users Manual

MOTOROLA Chapter 19. Serial Communications Controllers (SCCs) 19-11
Part IV. Communications Processor Module
¥ The word at offset + 0x4 (buffer pointer) points to the beginning of the buffer in
memory (internal or external).
Ñ For an RxBD, the value must be a multiple of four. (word-aligned)
Ñ For a TxBD, this pointer can be even or odd.
Shown in Figure 19-6, 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 CPM does not assume that all buffers of
a single frame are currently linked to the BD table. The CPM 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 19-7 shows the SCC BD table and buffer structure.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Offset + 0 Status and Control
Offset + 2 Data Length
Offset + 4 High-Order Buffer Pointer
Offset + 6 Low-Order Buffer Pointer
Figure 19-6. SCC Buffer Descriptors (BDs)