LAN9220_12 Datasheet, PDF (31/151 Pages) SMSC Corporation – 16-bit Non-PCI Small Form Factor 10/100 Ethernet Controller with Variable Voltage I/O & HP Auto-MDIX Support

16-bit Non-PCI Small Form Factor 10/100 Ethernet Controller with Variable Voltage I/O & HP Auto-MDIX Support
Datasheet
{DSAP, SSAP, CTRL,
OUI[23:16]}
{OUI[15:0], PID[15:0]}
DST
SRC
8
1
0
0
VL
Ie
Dn
S
N
A
P
0
S
N
A
P
1
L3 Packet
F
C
S
0123456
1DWORD
Calculate Checksum
Figure 3.6 Ethernet Frame with VLAN Tag and SNAP Header
{DSAP, SSAP, CTRL,
OUI[23:16]}
{OUI[15:0], PID[15:0]}
DST
SRC
8
1
0
0
V
I
D
8
1
0
0
VL
Ie
Dn
S
N
A
P
0
S
N
A
P
1
L3 Packet
F
C
S
01245678
1DWORD
Calculate Checksum
Figure 3.7 Ethernet Frame with Multiple VLAN Tags and SNAP Header
The RXCOE supports a maximum of two VLAN tags. If there are more than two VLAN tags, the VLAN
protocol identifier for the third tag is treated as an Ethernet type field. The checksum calculation will
begin immediately after the type field.
The RXCOE resides in the RX path within the MAC. As the RXCOE receives an Ethernet frame it
calculates the 16-bit checksum. The RXCOE passes the Ethernet frame to the RX Data FIFO with the
checksum appended to the end of the frame. The RXCOE inserts the checksum immediately after the
last byte of the Ethernet frame. The packet length field in the RX status word (refer to Section 3.13.3)
will indicate that the frame size has increased by two bytes to accommodate the checksum.
Setting the RXCOE_EN bit in the COE_CR—Checksum Offload Engine Control Register enables the
RXCOE, while the RXCOE_MODE bit selects the operating mode. When the RXCOE is disabled, the
the received data is simply passed through the RXCOE unmodified.
Note: Software applications must stop the receiver and flush the RX data path before changing the
state of the RXCOE_EN or RXCOE_MODE bits.
Note: When the RXCOE is enabled, automatic pad stripping must be disabled (bit 8 (PADSTR) of
the MAC_CR—MAC Control Register) and vice versa. These functions cannot be enabled
simultaneously.
SMSC LAN9220
31
DATASHEET
Revision 2.9 (03-01-12)