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Fast Ethernet Controllers (FEC0 and FEC1)
Freescale Semiconductor
26-42
Figure 26-28. Ethernet Address Recognition—Microcode Decisions
26.5.10 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
n
, GALR
n
(group
address hash match), or IAUR
n
, IALR
n
(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
n
(msb = 1) or GALR
n
(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.
Receive Address
I/G Address
?
Exact Match
?
Hash Search
Group Table
Match
?
Hash Search
Individual Table
False
Match
?
False
False
True
True
True
Individual
Group
True
False
True
False
?
Pause Address
FCE
?
Recognition
Reject Frame
Flush from FIFO
Reject Frame
Flush from FIFO
Receive Frame
Receive Frame
Receive Frame
Receive Frame
Notes:
FCE - field in RCR
n
register (flow control enable)
I/G - Individual/Group bit in destination address (lsb in first byte received in MAC frame)
Summary of Contents for MCF54455
Page 33: ...xxviii Freescale Semiconductor ...
Page 67: ...Freescale Semiconductor 1 ...
Page 125: ...Freescale Semiconductor 1 ...
Page 145: ...Enhanced Multiply Accumulate Unit EMAC 5 21 Freescale Semiconductor ...
Page 173: ...Cache 6 28 Freescale Semiconductor ...
Page 179: ...Static RAM SRAM 7 6 Freescale Semiconductor ...
Page 207: ...Power Management 9 16 Freescale Semiconductor ...
Page 323: ...Reset Controller Module 13 8 Freescale Semiconductor ...
Page 389: ...Pin Multiplexing and Control 16 44 Freescale Semiconductor ...
Page 575: ...PCI Bus Controller 22 58 Freescale Semiconductor ...
Page 600: ...Advanced Technology Attachment ATA Freescale Semiconductor 23 25 ...
Page 601: ...Freescale Semiconductor 1 ...
Page 842: ...I2 C Interface Freescale Semiconductor 33 16 ...
Page 843: ...Freescale Semiconductor 1 ...
Page 921: ...Revision History A 6 Freescale Semiconductor ...