Serial Management Controllers (SMCs)
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
29-17
29.3.11 SMC UART Transmit BD (TxBD)
Data is sent to the CPM for transmission on an SMC channel by arranging it in buffers referenced by
descriptors in the channel’s TxBD table. Using the BDs, the CP confirms transmission or indicates error
conditions so that the processor knows the buffers have been serviced.
Table 29-9
describes SMC UART TxBD status and control fields.
Data length represents the number of octets that the CP should transmit from this BD’s buffer. It is never
modified by the CP and normally is greater than zero. It can be zero if P is set, in which case only a
preamble is sent. If there are more than 8 bits in the UART character, data length should be even. For
example, to transmit three UART characters of 8-bit data, 1 start, and 1 stop, initialize the data length field
0
1
2
3
4
5
6
7
8
15
0
R
—
W
I
—
CM
P
—
2
Data Length
4
Tx Buffer Pointer
6
Figure 29-8. SMC UART Transmit BD (TxBD)
Table 29-9. SMC UART TxBD Status and Control Field Descriptions
Bits
Name
Description
0
R
Ready
0 The buffer is not ready for transmission; BD and its buffer can be altered. The CP clears R after
the buffer has been sent or an error occurs.
1 The buffer has not been completely sent. This BD must not be updated while R is set.
1
—
Reserved, should be cleared.
2
W
Wrap (final BD in the TxBD table)
0 Not the last BD in the table.
1 Last BD in the table. After this buffer is used, the CP transmits data from the first BD that TBASE
points to. The number of TxBDs in this table is determined only by the W bit.
3
I
Interrupt
0 No interrupt is generated after this buffer is serviced.
1 The SMCE[TX] is set when this buffer is serviced. TX can cause an interrupt if it is enabled.
4–5
—
Reserved, should be cleared.
6
CM
Continuous mode
0 Normal operation.
1 The CP does not clear R after this BD is closed and automatically retransmits the buffer when it
accesses this BD next.
7
P
Preamble
0 No preamble sequence is sent.
1 The UART sends one all-ones character before it sends the data so that the other end detects an
idle line before the data is received. If this bit is set and the data length of this BD is zero, only a
preamble is sent.
8–15
—
Reserved, should be cleared.
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