Motorola MPC860 PowerQUICC User Manual page 766

Part V. The Communications Processor Module
28.13 Interpacket Gap Time
The receiver receives back-to-back frames with a minimum interpacket spacing of 9.6 ms.
In addition, after the backoff algorithm, the transmitter waits for carrier sense to be negated
before resending the frame. Retransmission begins 9.6 ms after carrier sense is negated if it
stays negated for at least 6.4 ms.
28.14 Handling Collisions
If a collision occurs as a frame is being sent, the Ethernet controller continues sending for
at least 32 bit times, thus sending a JAM pattern of 32 ones. If a collision occurs during the
preamble sequence, the JAM pattern is sent at the end of the sequence.
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 ms). 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.
28.15 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. That is,
set PCDIR[15], and clear PCPAR[15], PCDAT[15], and PSMR[HBC].
In external loopback operation, the Ethernet controller listens for data being received from
the EEST at the same time that it is sending.
28.16 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).
28-18 MPC860 PowerQUICC UserÕs Manual MOTOROLA