SMSC LAN9420 - Manual (Page 56)

Single-Chip Ethernet Controller with HP Auto-MDIX Support and PCI Interface
Datasheet
Revision 1.22 (09-25-08) 56 SMSC LAN9420/LAN9420i
DATASHEET
3.5.3.1 Perfect Filtering
This filtering mode passes only incoming frames whose destination address field exactly matches the
value programmed into the MAC address high register (refer to
Section 4.4.2, "MAC Address High
Register (ADDRH)," on page 123 ) and the MAC address low register (refer to Section 4.4.3, "MAC
Address Low Register (ADDRL)," on page 124 ). The MAC address is formed by the concatenation of
the above two registers in the MCSR.
3.5.3.2 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 concatenating the register’s multicast hash table high (refer to
Section
4.4.4, "Multicast Hash Table High Register (HASHH)," on page 125 ) and multicast hash table low (refer
to Section 4.4.5, "Multicast Hash Table Low Register (HASHL)," on page 126 ) in the MCSR to form a
64-bit hash table. The most significant bit of the CRC 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.
3.5.3.3 Hash Perfect Filtering
In hash perfect filtering, if the received frame is a physical address, the packet filter block perfect-filters
the incoming frame’s destination field with the value programmed into the MAC Address High register
(refer to
Section 4.4.2, "MAC Address High Register (ADDRH)," on page 123 ) and the MAC address
low register (refer to Section 4.4.3, "MAC Address Low Register (ADDRL)," on page 124 ). If the
incoming frame is a multicast frame, however, the 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 3.5.3.2 .
3.5.3.4 Inverse Filtering
During 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.
1 0 0 0 X Pass all multicast frames. Frames
with physical addresses are
perfect-filtered
1 0 0 1 1 Pass all multicast frames. Frames
with physical addresses are hash-
filtered
Table 3.13 Address Filtering Modes (continued)
MCPAS PRMS INVFILT HFILT HPFILT DESCRIPTION