32-14
MPC860 PowerQUICC UserÕs Manual
MOTOROLA
Part V. The Communications Processor Module
2
C TxBD status and control bits.
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
0
R
Ñ
W
I
L
S
Ñ
NAK
UN
CL
2
Data Length
4
Tx Buffer Pointer
6
Figure 32-14. I
2
C Transmit Buffer Descriptor (TxBD)
Table 32-10. I
2
C TxBD Status and Control Bits
Bits
Name
Description
0
R
Ready.
0 The buffer is not ready to be sent. This BD or its buffer can be modiÞed. The CPM clears R after the
buffer is sent or an error occurs.
1 The buffer is ready for transmission or is being sent. The BD cannot be modiÞed once R is set.
1
Ñ
Reserved and should be cleared.
2
W
Wrap (last BD in TxBD table).
0 Not the last BD in the table.
1 Last BD in the table. After this buffer is used, the CPM transmits data using the BD pointed to by
TBASE (top of the table). The number of BDs in this table is determined only by the W bit and overall
space constraints of the dual-port RAM.
3
I
Interrupt.
0 No interrupt is generated after this buffer is serviced.
1 I2CER[TXB] or I2CER[TXE] is set when the buffer is serviced. If enabled, an interrupt occurs.
4
L
Last.
0 This buffer does not contain the last character of the message.
1 This buffer contains the last character of the message. After sending this buffer, the transmitter
generates a stop condition and deactivates. (Retrigger I2COM[STR] to initiate a new transmission.)
5
S
Generate start condition. Provides ability to send back-to-back messages on one I2COM[STR] trigger.
0 Do not send a start condition before the Þrst byte of the buffer.
1 Send a start condition before the Þrst byte of the buffer. (Used to separate messages.)
Note: If this BD is the Þrst one in a message when I2COM[STR] is triggered, a start condition is sent
regardless of the value of TxBD[S].
6Ð12 Ñ
Reserved and should be cleared.
13
NAK
No acknowledge. Indicates that the transmission was aborted because the last byte sent was not
acknowledged. The I
2
C controller updates NAK after the buffer is sent.
14
UN
Underrun. Indicates that the I
2
C controller encountered a transmitter underrun condition while sending
the associated buffer. The I
2
C controller updates UN after the buffer is sent.
15
CL
Collision. Indicates that transmission terminated because the transmitter was lost while arbitrating for
the bus. The I
2
C controller updates CL after the buffer is sent.
Содержание MPC860 PowerQUICC
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