LAN89218 datasheet(21/148 Pages) MICROCHIP

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DS60001255B-page 21

LAN89218

3.4

Filtering Modes

3.4.1

PERFECT FILTERING

This filtering mode passes only incoming frames whose destination address field exactly matches the value

programmed into the MAC Address High register and the MAC address low register. The MAC address is formed by

the concatenation of the above two registers in the MAC CSR Function.

3.4.2

HASH ONLY FILTERING

This type of filtering checks for incoming Receive packets with either multicast or physical destination addresses, and

executes an imperfect address filtering against the hash table.

During imperfect hash filtering, the destination address in the incoming frame is passed through the CRC logic and

the upper six bits of the CRC register are used to index the contents of the hash table. The hash table is formed by

merging the register’s multicast hash table high and multicast hash table low in the MAC CSR Function to form a

64-bit hash table. The most significant bit determines the register to be used (High/Low), while the other five bits

determine the bit within the register. A value of 00000 selects Bit 0 of the multicast hash table low register and a

value of 11111 selects Bit 31 of the multicast hash table high register.

3.4.2.1

Hash Perfect Filtering

In hash perfect filtering, if the received frame is a physical address, the LAN89218 Packet Filter block perfect-filters

the incoming frame’s destination field with the value programmed into the MAC Address High register and the MAC

Address Low register. If the incoming frame is a multicast frame, however, the LAN89218 packet filter function

performs an imperfect address filtering against the hash table.

The imperfect filtering against the hash table is the same imperfect filtering process as described in the Section 3.4.2.

3.4.2.2

Inverse Filtering

In inverse filtering, the Packet Filter Block accepts incoming frames with a destination address not matching the

perfect address (i.e., the value programmed into the MAC Address High register and the MAC Address Low register

in the CRC block and rejects frames with destination addresses matching the perfect address.

For all filtering modes, when MCPAS is set, all multicast frames are accepted. When the PRMS bit is set, all frames

are accepted regardless of their destination address. This includes all broadcast frames as well.

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