MOTOROLA
Chapter 26. SCC Asynchronous HDLC Mode and IrDA
26-13
Part V. The Communications Processor Module
The data length and buffer pointer Þelds are described in Section 22.2, ÒSCC Buffer
Descriptors (BDs).Ó Because asynchronous HDLC is a frame-based protocol, RxBD[Data
Length] of the last buffer of a frame contains the total number of frame bytes, including the
2 or 4 bytes for CRC.
26.15 SCC Asynchronous HDLC TxBDs
The CPM uses the TxBD, shown in Figure 26-8, to conÞrm transmissions and indicate error
conditions.
Table 26-11 describes the SCC asynchronous HDLC TxBD status and control Þelds.
14
OV
Overrun. Set when a receiver overrun occurs during frame reception.
15
CD
Carrier detect lost. Set when CD is negated during frame reception.
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
0
R
Ñ
W
I
L
Ñ
CM
Ñ
CT
2
Data Length
4
Tx Buffer Pointer
6
Figure 26-8. SCC Asynchronous HDLC TxBDs
Table 26-11. Asynchronous HDLC TxBD Status and Control Field Descriptions
Bits
Name
Description
0
R
Ready.
0 The buffer is not ready for transmission; the BD and the buffer can be updated. The CPM clears R
after the buffer is sent or after an error condition.
1 The buffer is ready but is not sent or is being sent. Do not update the BD while R =1.
1
Ñ
Reserved, should be cleared.
2
W
Wrap (last BD in table).
0 Not the last BD in the table.
1 The last BD in the table. After this buffer is used, the CPM sends incoming data using the BD
pointed to by TBASE. The number of TxBDs in this table are determined only by the W bit and overall
space constraints of the dual-port RAM.
3
I
Interrupt.
0 SCCE[TXB] is not set after this buffer is sent.
1 SCCE[TXB] is set when this buffer is sent by the asynchronous HDLC controller.
4
L
Last.
0 Not the last buffer in the current frame.
1 Last buffer in the current frame. The proper CRC and closing ßag are sent after the last byte.
5
Ñ
Reserved, should be cleared.
Table 26-10. Asynchronous HDLC RxBD Status and Control Field Descriptions
(Continued)
Bits
Name
Description
Содержание MPC860 PowerQUICC
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