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Bits
Name
15
R
14
TO1
13
W
12
TO2
11
L
10
TC
9
DEF
8
HB
7
LC
6
RL
5–2
RC
1
UN
0
CSL
Table 11-35. TxBD Field Descriptions
Ready. Written by the FEC (= 0) and user (= 1).
0 The data buffer associated with this BD is not ready for transmission. The user
is free to manipulate this BD or its associated data buffer. The FEC clears this bit
after the buffer has been transmitted or after an error condition is encountered.
1 The data buffer, which has been prepared for transmission by the user, has not
been transmitted or is currently being transmitted. No fields of this BD may be
written by the user once this bit is set.
Transmit software ownership bit.Reserved for use by software. This read/write bit
is not modified by hardware, nor does its value affect hardware.
Wrap. Written by the user.
0 The next buffer descriptor is found in the consecutive location.
1 The next buffer descriptor is found at the location defined in X_DES_START.
Transmit software ownership bit. Reserved for use by software. This read/write bit
is not modified by hardware, nor does its value affect hardware.
Last in frame. Written by the user.
0 The buffer is not the last in the transmit frame.
1 The buffer is the last in the transmit frame.
Tx CRC. Written by the user and is only valid if L = 1.
0 End transmission immediately after the last data byte.
1 Transmit the CRC sequence after the last data byte.
Defer indication. Written by the FEC and is only valid if L = 1. The FEC had to
defer while trying to transmit a frame. This bit is not set if a collision occurs during
transmission.
Heartbeat error. Written by the FEC and is valid only if L = 1. The collision input
was not asserted within the heartbeat window following the completion of
transmission. Cannot be set unless the HBC bit is set in the X_CNTRL register.
Late collision. Written by the FEC and is only valid if L = 1. A collision has
occurred after 56 data bytes are transmitted. The FEC terminates the
transmission.
Retransmission limit. Written by the FEC and is only valid if L = 1. The transmitter
has failed retry limit + 1 attempts to successfully transmit a message due to
repeated collisions on the medium.
Retry count. Written by the FEC and is valid only if L = 1. These four bits indicate
the number of retries required before this frame is successfully transmitted. If
RC = 0, then the frame was transmitted correctly the first time. If RC = 15, then
the frame was transmitted successfully while the retry count was at its maximum
value. If RL is set, then RC has no meaning.
Underrun. Written by the FEC and is only valid if L = 1. The FEC encountered a
transmit FIFO underrun while transmitting one or more of the data buffers
associated with this frame. When transmit FIFO underrun occurs, transmission of
the frame stops, and an incorrect CRC is appended. The remaining buffer(s)
associated with this frame are sent as a DMA and dumped by the transmit logic.
Carrier sense lost. Written by the FEC and is valid only if L = 1. Carrier sense
dropped out or never asserted during transmission of a frame without collision.
Chapter 11. Ethernet Module
Description
Buffer Descriptors
11-37
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