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Communication Processor Module
16-326
MPC823 USER’S MANUAL
MOTOROLA
SCC2
COMMUNICATION
16
PROCESSOR
MODULE
In the group type of address recognition, the SCC2 Ethernet controller determines whether
or not the group address is a broadcast address. If broadcast addresses are enabled, then
the frame is accepted, but if the group address is not a broadcast address, then you can
perform address recognition on multiple group addresses using the GADDR1–4 hash table.
In promiscuous mode, the SCC2 Ethernet controller receives all the incoming frames
regardless of their address.
16.9.22.11 HASH TABLE ALGORITHM. Individual and group hash filtering can be
operated using a certain process. The SCC2 Ethernet controller maps any 48-bit address
into one of 64 bins, which are represented by 64 bits stored in GADDR1–4 or IADDR1–4.
When the SET GROUP ADDRESS command is executed, the SCC2 Ethernet controller
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 four
GADDRs or IADDRs and bits 29–26 of the CRC result select the bit within the selected
register.
When the SCC2 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. The result is that 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. Those that do reach memory
must be further filtered by the processor to determine if they truly 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.
Note: The hash tables cannot be used to reject frames that match a set of entered
addresses because unintended addresses are matched to the same bit in the
hash table.