Motorola PowerQUICC II MPC8280 Series Reference Manual page 841

Freescale Semiconductor, Inc.
Hash Table Algorithm
In group address recognition, the controller determines whether the group address is a
broadcast address. If broadcast addresses are enabled, the frame is accepted, but if the group
address is not a broadcast address, address recognition can be performed on multiple group
addresses using the GADDRn hash table. In promiscuous mode, the controller receives all
incoming frames regardless of their address, unless REJECT is asserted.
If an external CAM is used for address recognition, select promiscuous mode; the frame
can be rejected by asserting REJECT while the frame is being received. The on-chip
address recognition functions can be used in addition to the external CAM address
recognition functions.
If the external CAM stores addresses that should be rejected rather than accepted, the use
of REJECT by the CAM should be logically inverted.
25.11 Hash Table Algorithm
Individual and group hash filtering operate using certain processes. The Ethernet controller
maps any 48-bit address into one of 64 bins, each represented by a bit stored in GADDRx
or IADDRx. When a command is executed, the Ethernet controller
SET GROUP ADDRESS
maps the selected 48-bit address into one of the 64 bits by passing the 48-bit address
through the on-chip 32-bit CRC generator and selecting 6 bits of the CRC-encoded result
to generate a number between 1 and 64. Bits 31–30 of the CRC result select one of the
GADDRs or IADDRs; bits 29–26 of the CRC result indicate the bit in that register.
When the Ethernet controller receives a frame, the same process is used. If the CRC
generator selects a bit that is set in the group/individual hash table, the frame is accepted.
Otherwise, it is rejected. So, if eight group addresses are stored in the hash table and random
group addresses are received, the hash table prevents roughly 56/64 (87.5%) of the group
address frames from reaching memory. Frames that reach memory must be further filtered
by the processor to determine if they contain one of the eight preferred addresses.
Better performance is achieved by using the group and individual hash tables
simultaneously. For instance, if eight group and eight physical addresses are stored in their
respective hash tables, 87.5% of all 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, thus preventing a small fraction of the frames from reaching memory.
Hash tables cannot be used to reject frames that match a set of entered addresses because
unintended addresses are mapped to the same bit in the hash table.
25.12 Interpacket Gap Time
The receiver receives back-to-back frames with a minimum interpacket spacing of 9.6 µs.
In addition, after the backoff algorithm, the transmitter waits for carrier sense to be negated
MOTOROLA Chapter 25. SCC Ethernet Mode 25-13
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