SCC Ethernet Mode
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
27-19
27.19 SCC Ethernet Transmit Buffer Descriptor
Data is sent to the ethernet controller for transmission on an SCC channel by arranging it in buffers
referenced by the channel TxBD table. The ethernet controller uses TxBDs to confirm transmission or
indicate errors so the core knows buffers have been serviced.
Table 27-9
describes TxBD status and control
fields.
0
1
2
3
4
5
6
7
8
9
10
13
14
15
0
R
PAD
W
I
L
TC
DEF
HB
LC
RL
RC
UN
CSL
2
Data Length
4
Tx Data Buffer Pointer
6
Figure 27-8. SCC Ethernet TxBD
Table 27-9. SCC Ethernet TxBD Status and Control Field Descriptions
Bits
Name
Description
0
R
Ready
0 The buffer is not ready for transmission. The user can update this BD or its data buffer. The CPM
clears R after the buffer has been sent or after an error occurs.
1 The user-prepared buffer has not been sent or is currently being sent. Do not modify this BD.
1
PAD
Short frame padding. Valid only when L is set. Otherwise, it is ignored.
0 Do not add PADs to short frames.
1 Add PADs to short frames. Pad bytes are inserted until the length of the sent frame equals the
MINFLR and they are stored in PADs in the parameter RAM.
2
W
Wrap (final BD in table)
0 Not the last BD in the table.
1 Last BD in the table. After this buffer is used, the CPM receives incoming data into the first BD
that TBASE points to in the table. The number of TxBDs in this table is determined only by the W
bit.
Note: The TxBD table must contain more than one BD in ethernet mode.
3
I
Interrupt
0 No interrupt is generated after this buffer is serviced.
1 SCCE[TXB] or SCCE[TXE] is set after this buffer is serviced. These bits can cause interrupts if
they are enabled.
4
L
Last
0 Not the last buffer in the transmit frame.
1 Last buffer in the transmit frame.
5
TC
Tx CRC. Valid only when L = 1. Otherwise, it is ignored.
0 End transmission immediately after the last data byte.
1 Transmit the CRC sequence after the last data byte.
6
DEF
Defer indication. The frame was deferred before being sent successfully, that is, the transmitter had
to wait for carrier sense before sending because the line was busy. This is not a collision indication;
collisions are indicated in RC.
7
HB
Heartbeat. Set when the collision input was not asserted within 20 transmit clocks after
transmission. HB cannot be set unless PSMR[HBC] = 1. The SCC writes HB after it finishes sending
the buffer.
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