Fast Ethernet Controller (FEC)
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
45-7
Figure 45-1
shows a flowchart for address recognition on received frames.
Figure 45-2. Ethernet Address Recognition Flowchart
45.2.6
Hash Table Algorithm
This section discusses the hash table process used in group hash filtering. When the FEC receives a frame
with the destination address I/G bit set, the 48-bit address is mapped into one of 64 bins, represented by
the 64 bits in the two hash table registers. This is performed 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 0 and 63.
Bit 31 of the CRC result selects HASH_TABLE_HIGH (bit 31 = 1) or HASH_TABLE_LOW (bit 31 = 0).
Bits 30–26 of the CRC result select the bit in the selected register. If that bit is set in the 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 (or 87.5%) of the
group address frames from reaching memory. The processor must further filter those that reach memory
to determine if they truly contain one of the eight preferred addresses.
Check Address
Receive Frame
Set Miss Bit
I/G Address
?
Receive Frame
Hash Match
?
Receive Frame
Receive Frame
Promiscuous
Discard Frame
Perfect Match
?
Broadcast
Address
?
G
I
False
True
False
True
False
True (R_CNTRL[PROM] = 1)
True
False
Mode
?
(R_CNTRL[PROM] = 1)
Summary of Contents for PowerQUICC MPC870
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Page 158: ...The MPC8xx Core MPC885 PowerQUICC Family Reference Manual Rev 2 3 18 Freescale Semiconductor ...
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Page 1312: ...Byte Ordering MPC885 PowerQUICC Family Reference Manual Rev 2 A 8 Freescale Semiconductor ...
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