Motorola MPC8260 - Manual (Page 682)

24-14 MPC8260 PowerQUICC II UserÕs Manual MOTOROLA
Part IV. Communications Processor Module
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.
24.14 Internal and External Loopback
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.
24.15 Full-Duplex Ethernet Support
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).
24.16 Handling Errors in the Ethernet Controller
The Ethernet controller reports frame reception and transmission error conditions using
channel BDs, error counters, and SCCE. Table 24-4 describes transmission errors.
Table 24-4. Transmission Errors
Error Description
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].