LAN9211 datasheet(31/146 Pages) SMSC

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High-Performance Small Form Factor Single-Chip Ethernet Controller with HP Auto-MDIX

Datasheet

SMSC

LAN9211

Revision 1.93 (11-27-07)

DATASHEET

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.

3.6.1.1

RX Checksum Calculation

The checksum is calculated 16 bits at a time. In the case of an odd sized frame, an extra byte of zero

is used to pad up to 16 bits.

Consider the following packet: DA, SA, Type, B0, B1, B2 … BN, FCS

Let [A, B] = A*256 + B;

If the packet has an even number of octets then

checksum = [B1, B0] + C0 + [B3, B2] + C1 + … + [BN, BN-1] + CN-1

Where C0, C1, ... CN-1 are the carry out results of the intermediate sums.

If the packet has an odd number of octets then

checksum = [B0, B1] + C0 + [B2, B3] + C1 + … + [0, BN] + CN-1

Figure 3.8 Ethernet Frame with multiple VLAN Tags and SNAP Header

DST

SRC

L3 Packet

{DSAP, SSAP, CTRL,

OUI[23:16]}

{OUI[15:0], PID[15:0]}

Calculate Checksum

1DWORD

LAN9211 Datasheet(PDF) 31 Page - SMSC Corporation

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