LAN9500AI datasheet(58/213 Pages) MICROCHIP

58 / 213 page

LAN950x

DS00001875A-page 58

 2010 - 2015 Microchip Technology Inc.

5.5.3

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 5-45, "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 Con-

trol Register”. Please refer to Section 7.4.1, "MAC Control Register (MAC_CR)," on page 159 for more information on

this register.

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.

5.5.4

FILTERING MODES

5.5.4.1

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.

5.5.4.2

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.

5.5.4.3

Hash Perfect Filtering

In hash perfect filtering, if the received frame is a physical address, the device’s 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 device’s packet filter function performs an imper-

fect address filtering against the hash table.

The imperfect filtering against the hash table is the same imperfect filtering process described in Section 5.5.4.2, "Hash

Only Filtering Mode".

TABLE 5-45:

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

multicast addresses

Inverse Filtering

Promiscuous

Pass all multicast frames. Frames

with physical addresses are perfect-

filtered

Pass all multicast frames. Frames

with physical addresses are hash-

filtered

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