SMSC LAN9420 - Manual (Page 60)

Single-Chip Ethernet Controller with HP Auto-MDIX Support and PCI Interface
Datasheet
Revision 1.22 (09-25-08) 60 SMSC LAN9420/LAN9420i
DATASHEET
Then the MAC inspects the frame for 16 repetitions of the MAC address without any breaks or
interruptions. In case of a break in the 16 address repetitions, the MAC scans for the
48'hFF_FF_FF_FF_FF_FF pattern again in the incoming frame.
The 16 repetitions may be anywhere in the frame but must be preceded by the synchronization stream.
The device will also accept a multicast frame, as long as it detects the 16 duplications of the MAC
address. If the MAC address of a node is 00h 11h 22h 33h 44h 55h, then the MAC scans for the
following data sequence in an Ethernet frame:
Destination Address Source Address ……………FF FF FF FF FF FF
00 11 22 33 44 55 00 11 22 33 44 55 00 11 22 33 44 55 00 11 22 33 44 55
00 11 22 33 44 55 00 11 22 33 44 55 00 11 22 33 44 55 00 11 22 33 44 55
00 11 22 33 44 55 00 11 22 33 44 55 00 11 22 33 44 55 00 11 22 33 44 55
00 11 22 33 44 55 00 11 22 33 44 55 00 11 22 33 44 55 00 11 22 33 44 55
…CRC
It should be noted that Magic Packet detection can be performed when LAN9420/LAN9420i is in any
power management state.
3.5.5 Receive Checksum Offload Engine (RXCOE)
The receive checksum offload engine (RXCOE) provides assistance to the Host by calculating a 16-
bit checksum for a received Ethernet frame. The RXCOE readily supports the following IEEE802.3
frame formats:
„ Type II Ethernet frames
„ SNAP encapsulated frames
„ Support for up to 2, 802.1q VLAN tags
The resulting checksum value can also be modified by software to support other frame formats.
The RXCOE has two modes of operation. In mode 0, the RXCOE calculates the checksum between
the first 14 bytes of the Ethernet frame and the FCS. This is illustrated in
Figure 3.19 .
In mode 1, the RXCOE supports VLAN tags and a SNAP header. In this mode the RXCOE calculates
the checksum at the start of L3 packet. The VLAN1 tag register is used by the RXCOE to indicate
what protocol type is to be used to indicate the existence of a VLAN tag. This value is typically 8100h.
Figure 3.19 RXCOE Checksum Calculation
DST SRC
T
Y
P
E
Frame Data
F
C
S
Calculate Checksum