Motorola PowerQUICC II MPC8280 Series Reference Manual page 1325

Better performance is achieved by using the group and individual hash tables in
combination. For instance, if eight group and eight physical addresses are stored in their
respective hash tables, 87.5% of all frames (not just group address frames) are prevented
from reaching memory.
The effectiveness of the hash table declines as the number of addresses increases. For
instance, with 128 addresses stored in a 64-bin hash table, the vast majority of the hash table
bits are set, preventing only a small fraction of frames from reaching memory. In such
instances, an external CAM is advised if the extra bus use cannot be tolerated. See
Section 36.7, "CAM Interface."
The hash tables cannot be used to reject frames that match a set
of selected addresses because unintended addresses can map to
the same bit in the hash table. Thus, an external CAM must be
used to implement this function.
36.14 Interpacket Gap Time
The minimum interpacket gap time for back-to-back transmission is 96 bit-times. The
receiver receives back-to-back frames with this minimum spacing. In addition, after the
backoff algorithm, the transmitter waits for carrier sense to be negated before retransmitting
the frame. The retransmission begins 96 bit times after carrier sense is negated if it stays
negated for at least 64 bit times. So if there is no change in the carrier sense indication
during the first 64 bit-times (16 serial clocks), the retransmission begins 96 clocks after
carrier sense is first negated
36.15 Handling Collisions
If a collision occurs during frame transmission, the Ethernet controller continues
transmission for at least 32-bit times, transmitting a jam pattern of 32 ones. If the collision
occurs during the preamble sequence, the jam pattern is sent after the sequence ends.
If a collision occurs within 64 byte times, the process is retried. The transmitter waits a
random number of slot times. (A slot time is 512 bit times.) If a collision occurs after 64
byte times, no retransmission is performed, FCCE[TXE] is set, and the buffer is closed with
a late-collision error indication in TxBD[LC]. If a collision occurs during frame reception,
reception is stopped. This error is reported only in the RxBD if the frame is at least as long
as the MINFLR or if FPSMR[RSH] = 1.
36.16 Internal and External Loopback
Both internal and external loopback are supported by the Ethernet controller. In loopback
mode, both receive and transmit FIFO buffers are used and the FCC operates in full-duplex.
MOTOROLA
Freescale Semiconductor, Inc.
NOTE
Chapter 36. Fast Ethernet Controller
For More Information On This Product,
Go to: www.freescale.com
Interpacket Gap Time
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