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MOTOROLA Chapter 6. Programming Model 6-27
PRELIMINARYÑSUBJECT TO CHANGE WITHOUT NOTICE
On transmit, an underrun occurs if the transmit FIFO empties of data before the end of the
frame. In this case, a bad CRC is appended to the partially transmitted data. In addition, the
UN bit is set in the last BD in the current frame. This situation can occur if the FEC cannot
access the 60x bus or if the next BD in the frame is unavailable.
Note: A software driver that sets TxBD[R] should then write to X_DES_ACTIVE.
2 W Wrap, written by user.
0 The next BD is found in the consecutive location
1 The next BD is found at the location deÞned in X_DES_START.
3 TO2 Transmit software ownership bit
This Þeld is available for use by software. This read/write bit is not modiÞed by hardware and its
value does not affect hardware.
4 L Last in frame, written by user.
0 The buffer is not the last in the transmit frame.
1 The buffer is the last in the transmit frame.
5 TC Tx CRC, written by user (valid if L = 1).
0 End transmission immediately after the last data byte.
1 Transmit the CRC sequence after the last data byte.
6 DEF Defer indication, written by FEC (valid if L = 1). Set when the FEC had to defer while trying to
transmit a frame. This bit is not set if a collision occurs during transmission.
7 HB Heartbeat error, written by FEC (valid if L = 1). Set to indicate that the collision input was not
asserted within the heartbeat window after transmission completed. HB can be set only if
X_CNTRL[HBC] = 1.
8 LC Late collision, written by FEC (valid if L = 1). Set to indicate that a collision occurred after 56 data
bytes were transmitted. The FEC terminates the transmission.
9 RL Retransmission limit, written by FEC (valid if L = 1). Set to indicate that the transmitter failed retry
limit + 1 attempts to send a message due to repeated collisions.
10Ð13 RC Retry count, written by FEC (valid if L = 1). Counts retries needed to successfully send this
frame. If RC = 0, the frame was sent correctly the Þrst time. If RC = 15, the frame was sent
successfully while the retry count was at its maximum value. If RL = 1, RC has no meaning.
14 UN Underrun, written by FEC (valid if L = 1). If set, the FEC encountered a transmit FIFO underrun
while sending one or more buffers associated with this frame. When a Tx FIFO underrun occurs,
transmission of the frame stops and an incorrect CRC is appended. Any remaining buffers
associated with this frame are accessed and dumped by the transmit logic.
15 CSL Carrier sense lost, written by FEC (valid if L = 1). Carrier sense dropped out or never asserted
during transmission of a frame without collision.
Offset+2 Data
length
Data length, written by user and never by the FEC. Indicates the number of octets the FEC
should send from this BDÕs buffer. The DMA engine uses bits 21Ð31. Bits 16Ð20 are ignored.
Offset+4 Tx
buffer
pointer
Tx buffer pointer A[0Ð31], written by user and never by the FEC. The transmit buffer pointer,
which contains the address of the associated buffer, may be even or odd. The buffer must reside
in external memory to the MPC860T.
Table 6-29. Transmit Buffer Descriptor (TxBD) Field Descriptions (Continued)
Bits Name Description
Fr
eescale S
emiconduct
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Freescale Semiconductor, Inc.
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