MCF5272CVM66 Datasheet, PDF (256/544 Pages) Freescale Semiconductor, Inc – MCF5272 ColdFire® Integrated Microprocessor

Ethernet Module
Table 11-35. TxBD Field Descriptions (continued)
Bits
Name
Description
8
HB
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 TCR register.
7
LC
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.
6
RL
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.
5–2
RC
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.
1
UN
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.
0
CSL
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.
-
Data Length Written by the user. Data length is the number of octets the FEC should transmit from this BD’s
data buffer. It is never modified by the FEC. Bits [10–0] are used by the DMA engine; bits [15–11]
are ignored.
-
Tx Buffer Pointer Written by the user. The transmit buffer pointer, which contains the address of the associated data
buffer, may be even or odd. The buffer must reside in memory external to the FEC. This value is
never modified by the FEC.
An underrun occurs when, during transmit, the transmit FIFO empties before the end of the frame. Then,
a bad CRC is appended to the partially transmitted data. In addition, the UN bit is set in the last BD for the
current frame. This situation can occur if the FEC cannot access an internal bus, or if the next BD in the
frame is not available.
NOTE
Anytime the software driver sets an R bit in a transmit descriptor, the driver
should immediately write to TDAR.
11-38
MCF5272 ColdFire^® Integrated Microprocessor User’s Manual, Rev. 3
Freescale Semiconductor