Freescale Semiconductor MPC8260 - Manual (Page 797)

SCC Ethernet Mode
MPC8260 PowerQUICC II Family Reference Manual, Rev. 2
Freescale Semiconductor 25-13
If a collision occurs within 64 byte times, the retry process is initiated. The transmitter waits a random
number of slot times (512 bit times or 52 µs). If a collision occurs after 64 byte times, no retransmission
is performed and the buffer is closed with an LC error indication. 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.
25.14 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 configured 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; configure
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.
25.15 Full-Duplex Ethernet Support
To run full-duplex Ethernet, select loopback and full-duplex Ethernet modes in the SCC’s
protocol-specific 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).
25.16 Handling Errors in the Ethernet Controller
The Ethernet controller reports frame reception and transmission error conditions using channel BDs, error
counters, and SCCE.
Table 25-4 describes transmission errors.
Table 25-4. 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.