High-Performance Small Form Factor Single-Chip Ethernet Controller with HP Auto-MDIX
Datasheet
3.6.1
Receive Checksum Offload Engine (RXCOE)
The receive checksum offload engine provides assistance to the CPU 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 3.3.
T
F
Y
P
E
DST SRC
Frame Data
C
S
Calculate Checksum
Figure 3.3 RXCOE Checksum Calculation
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.
Example frame configurations:
p
F
r
o
t
DST SRC
L3 Packet
C
S
0
1
2
3
Calculate Checksum
1DWORD
Figure 3.4 Type II Ethernet Frame
SMSC LAN9211
29
Revision 1.93 (11-27-07)
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