10 scc hdlc transmit buffer descriptor (txbd), Scc hdlc transmit buffer descriptor (txbd) -11 – Motorola MPC8260 User Manual
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Chapter 21. SCC HDLC Mode
21-11
Part IV. Communications Processor Module
21.10 SCC HDLC Transmit Buffer Descriptor (TxBD)
The CP uses the TxBD, shown in Figure 21-6, to conÞrm transmissions and indicate error
conditions.
Table 21-8 describes HDLC TxBD status and control Þelds.
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Offset + 0
R
Ñ
W
I
L
TC
CM
Ñ
UN
CT
Offset + 2
Data Length
Offset + 4
Tx Buffer Pointer
Offset + 6
Figure 21-6. SCC HDLC Transmit Buffer Descriptor (TxBD)
Table 21-8. SCC HDLC TxBD Status and Control Field Descriptions
Bits
Name
Description
0
R
Ready.
0 The buffer is not ready for transmission. Both the buffer and the BD can be updated. The CP clears
R after the buffer is sent or an error is encountered.
1 The buffer has not been sent or is being sent and the BD cannot be updated.
1
Ñ
Reserved, should be cleared.
2
W
Wrap (last BD in TxBD table).
0 Not the last BD in the table.
1 Last BD in the BD table. After this buffer is used, the CP sends data using the BD pointed to by
TBASE. The number of TxBDs in this table is determined by TxBD[W] and the space constraints of
the dual-port RAM.
3
I
Interrupt.
0 No interrupt is generated after this buffer is processed.
1 SCCE[TXB] or SCCE[TXE] is set when this buffer is processed, causing interrupts if not masked.
4
L
Last.
0 Not the last buffer in the frame.
1 Last buffer in the frame.
5
TC
Tx CRC. Valid only when TxBD[L] = 1. Otherwise, it is ignored.
0 Transmit the closing ßag after the last data byte. This setting can be used to send a bad CRC after
the data for testing purposes.
1 Transmit the CRC sequence after the last data byte.
6
CM
Continuous mode.
0 Normal operation.
1 The CP does not clear TxBD[R] after this BD is closed allowing the buffer to be resent the next time
the CP accesses this BD. However, TxBD[R] is cleared if an error occurs during transmission,
regardless of CM.
7Р13 С
Reserved, should be cleared.
14
UN
Underrun. Set after the SCC sends a buffer and a transmitter underrun occurred.
15
CT
CTS lost. Indicates when CTS in NMSI mode or layer 1 grant is lost in GCI or IDL mode during frame
transmission. If data from more than one buffer is currently in the FIFO when this error occurs, the
HDLC writes CT in the current BD after sending the buffer.