Scc Asynchronous Hdlc Txbds - Freescale Semiconductor MPC850 User Manual

Table 25-10. Asynchronous HDLC RxBD Status and Control Field
Bits Name
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.
The data length and buffer pointer fields are described in Section 21.3, "SCC Buffer
Descriptors (BDs)." 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.

25.15 SCC Asynchronous HDLC TxBDs

The CPM uses the TxBD, shown in Figure 25-8, to confirm transmissions and indicate error
conditions.
0 1
R
Offset + 0 —
Offset + 2
Offset + 4
Offset + 6
Table 25-11 describes the SCC asynchronous HDLC TxBD status and control fields.
Table 25-11. Asynchronous HDLC TxBD Status and Control Field
Bits Name
R
0 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 — Reserved, should be cleared.
W
2 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.
I
3 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.
L
4 Last.
0 Not the last buffer in the current frame.
1 Last buffer in the current frame. The proper CRC and closing flag are sent after the last byte.
5 — Reserved, should be cleared.
Descriptions (Continued)
2 3 4 5
W I L
—
Figure 25-8. SCC Asynchronous HDLC TxBDs
Descriptions
Chapter 25. SCC Asynchronous HDLC Mode and IrDA
SCC Asynchronous HDLC TxBDs
Description
6 7 8 9
CM
Data Length
Tx Buffer Pointer
Description
10 11 12 13
—
14 15
CT