Motorola MPC8260 User Manual

24-14

MOTOROLA

If a collision occurs while a frame is being received, reception stops. This error is reported
only in the BD if the length of the frame exceeds MINFLR or if PSMR[RSH] = 1.

Both internal and external loopback are supported by the Ethernet controller. In loopback
mode, both of the SCC FIFOs are used and the channel actually operates in a full-duplex
fashion. Both internal and external loopback are conÞgured using combinations of
PSMR[LPB] and GSMR[DIAG]. Because of the full-duplex nature of the loopback
operation, the performance of other SCCs is degraded.

Internal loopback disconnects the SCC from the serial interface. Receive data is connected
to the transmit data and the receive clock is connected to the transmit clock. Both FIFOs
are used. Data from the transmit FIFO is received immediately into the receive FIFO. There
is no heartbeat check in this mode; conÞgure TENA as a general-purpose output. 

In external loopback operation, the Ethernet controller listens for data being received from
the EEST at the same time that it is sending.

To run full-duplex Ethernet, select loopback and full-duplex Ethernet modes in the SCCÕs
protocol-speciÞc mode register, (PSMR[LPB, FDE] = 1). The loopback mode tells the
Ethernet controller to accept received frames without signaling a collision. Setting
PSMR[FDE] tells the controller that it can send while receiving without waiting for a clear
line (carrier sense).

The Ethernet controller reports frame reception and transmission error conditions using
channel BDs, error counters, and SCCE. Table 24-4 describes transmission errors.

Transmitter underrun

If this error occurs, the channel sends 32 bits that ensures a CRC error, stops sending the 
buffer, closes it, sets the UN bit in the TxBD and SCCE[TXE]. The channel resumes 
transmission after it receives a 

RESTART

 

TRANSMIT

 command.

Carrier sense lost 
during frame 
transmission

When this error occurs and no collision is found in the frame, the channel sets the CSL bit in 
the TxBD, sets SCCE[TXE], and continues sending the buffer normally. No retries are 
performed after this error occurs. Carrier sense is the logical OR of RENA and CLSN.

Retransmission 
attempts limit expired

The channel stops sending the buffer, closes it, sets the RL bit in the TxBD and SCCE[TXE]. 
The channel resumes transmission after it receives a 

RESTART

 

TRANSMIT

 command.

Late collision

When this error occurs, the channel stops sending the buffer, closes it, sets SCCE[TXE] and 
the LC bit in the TxBD. The channel resumes transmission after it receives the 

RESTART

 

TRANSMIT

 command. This error is discussed further in the deÞnition of PSMR[LCW].