LAN9250 datasheet(143/421 Pages) MICROCHIP

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 2015 Microchip Technology Inc.

DS00001913A-page 143

LAN9250

11.4.2

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. The hash table is formed by merging the values in the

Host MAC Multicast Hash Table High Register (HMAC_HASHH) and the Host MAC Multicast Hash Table Low Register

(HMAC_HASHL) to form a 64-bit hash table.

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

upper 6-bits of the CRC register are used to index the contents of the hash table. The most significant bit determines

the register to be used (HMAC_HASHH or HMAC_HASHL), while the other five bits determine the bit within the register.

A value of 00000 selects Bit 0 of the HMAC_HASHL register and a value of 11111 selects Bit 31 of the HMAC_HASHH

Broadcast frames are also accepted if Disable Broadcast Frames (BCAST) is low.

11.4.3

HASH PERFECT FILTERING

In hash perfect filtering, if the received frame is a physical address, the Host MAC packet filter will perfect-filter the

incoming frame’s destination field with the value programmed into the Host MAC Address High Register (HMAC_AD-

DRH) and the Host MAC Address Low Register (HMAC_ADDRL). However, if the incoming frame is a multicast frame,

the Host MAC 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 described in Section 11.4.2, "Hash

Only Filtering".

Broadcast frames are also accepted if Disable Broadcast Frames (BCAST) is low.

Note:

If bit 31 of HMAC_HASHH is set, the Broadcast address will cause a hash match and Broadcast frames will

be accepted regardless of the setting of the BCAST bit.

11.4.4

INVERSE FILTERING

In inverse filtering, the Host MAC packet filter accepts incoming frames with a destination address not matching the per-

fect address (i.e., the value programmed into the Host MAC Address High Register (HMAC_ADDRH) and the Host MAC

Address Low Register (HMAC_ADDRL)) and rejects frames with destination addresses matching the perfect address.

Note:

If the HMAC_ADDRH and HMAC_ADDRL registers are set to the Broadcast address, Broadcast frames will

be filtered regardless of the setting of the BCAST bit.

11.4.5

PROMISCUOUS

When the Promiscuous Mode (PRMS) bit is set, all frames are accepted regardless of their destination address.

Note:

Broadcast frames will be accepted regardless of the setting of the BCAST bit.

11.4.6

PERFECT FILTERING ALL MULTICAST

If the received frame is a physical address, the Host MAC packet filter will perfect-filter the incoming frame’s destination

field with the value programmed into the Host MAC Address High Register (HMAC_ADDRH) and the Host MAC Address

Low Register (HMAC_ADDRL).

With the Pass All Multicast (MCPAS) bit of the Host MAC Control Register (HMAC_CR) set, all multicast frames are

accepted. This includes all broadcast frames as well.

11.4.7

HASH ONLY FILTERING ALL MULTICAST

If the received frame is a physical address, the Host MAC packet filter will execute an imperfect address filtering against

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

11.4.2, "Hash Only Filtering".

With the Pass All Multicast (MCPAS) bit of the Host MAC Control Register (HMAC_CR) set, all multicast frames are

accepted. This includes all broadcast frames as well.

Note:

If bit 31 of HMAC_HASHH is set, the Broadcast address will cause a hash match and Broadcast frames

will be accepted regardless of the setting of the BCAST bit.

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