LAN9730 datasheet(64/222 Pages) MICROCHIP

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LAN9730/LAN9730i

DS00001946A-page 64

 2012-2015 Microchip Technology Inc.

4.5.3

ADDRESS FILTERING FUNCTIONAL DESCRIPTION

The Ethernet address fields of an Ethernet packet, consists of two 6-byte fields: one for the destination address and one

for the source address. The first bit of the destination address signifies whether it is a physical address or a multicast

address.

The device’s address check logic filters the frame based on the Ethernet receive filter mode that has been enabled.

Filter modes are specified based on the state of the control bits in Table 4-45, "Address Filtering Modes", which shows

the various filtering modes used by the Ethernet MAC function. These bits are defined in more detail in the MAC Control

If the frame fails the filter, the Ethernet MAC function does not receive the packet. The host has the option of accepting

or ignoring the packet.

4.5.4

FILTERING MODES

4.5.4.1

Perfect Filtering

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

grammed 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.

4.5.4.2

Hash Only Filtering Mode

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.

TABLE 4-45:

ADDRESS FILTERING MODES

MCPAS

PRMS

INVFILT

HPFILT

Description

MAC address Perfect Filtering only

for all addresses.

MAC address Perfect Filtering for

physical address and hash filtering

for multicast addresses.

Hash Filtering for physical and mul-

ticast addresses.

Inverse Filtering

Promiscuous

Pass all multicast frames. Frames

with physical addresses are per-

fect-filtered.

Pass all multicast frames. Frames

with physical addresses are hash-

filtered.

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