mcf5282 Freescale Semiconductor, Inc, mcf5282 Datasheet - Page 348

• Manufacturer Part Number: mcf5282
• Manufacturer: Freescale Semiconductor, Inc

Fast Ethernet Controller (FEC)
17.5.10 Hash Algorithm
The hash table algorithm used in the group and individual hash filtering operates as follows. The 48-bit
destination address is mapped into one of 64 bits, represented by 64 bits stored in GAUR, GALR (group
address hash match), or IAUR, IALR (individual address hash match). This mapping is performed by
passing the 48-bit address through the on-chip 32-bit CRC generator and selecting the six most significant
bits of the CRC-encoded result to generate a number between 0 and 63. The msb of the CRC result selects
GAUR (msb = 1) or GALR (msb = 0). The five least significant bits of the hash result select the bit within
the selected register. If the CRC generator selects a bit set in the hash table, the frame is accepted;
otherwise, it is rejected.
For example, 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 desired addresses.
The effectiveness of the hash table declines as the number of addresses increases.
17-38
Notes:
FCE - field in RCR register (flow control enable)
I/G - Individual/Group bit in destination address (lsb in first byte received in MAC frame)
Flush from FIFO
Hash Search
Group Table
Figure 17-28. Ethernet Address Recognition—Microcode Decisions
Reject Frame
False
FCE
Match
False
?
?
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3
False
True
True
Receive Frame
Pause Address
?
Group
Receive Frame
True
Receive Address
Recognition
I/G Address
Receive Frame
?
True
Individual Table
Flush from FIFO
Individual
Hash Search
Reject Frame
Match
False
?
False
Exact Match
?
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Receive Frame
True