LAN9250 datasheet(152/421 Pages) MICROCHIP

152 / 421 page

LAN9250

DS00001913A-page 152

 2015 Microchip Technology Inc.

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.

Note that in all cases, the checksum calculation ends just before the frames FCS field. In the case where padding is

added to meet the minimum frame length requirement, the checksum calculation will also include the pads byte(s). This

may lead to unexpected results if the pad byte(s) are not zero.

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 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 and before it transmits the status

word. The packet length field in the RX Status Word (refer to Section 11.12.3) will indicate that the frame size has

increased by two bytes to accommodate the checksum.

Note:

When enabled, the RXCOE calculates a checksum for every received frame.

Setting the RX Checksum Offload Engine Enable (RX_COE_EN) bit in the Host MAC Checksum Offload Engine Control

selects the operating mode. When the RXCOE is disabled, 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 RX

Checksum Offload Engine Enable (RX_COE_EN) or RX Checksum Offload Engine Mode (RX_COE_-

MODE) bits.

FIGURE 11-6:

ETHERNET FRAME WITH VLAN TAG AND SNAP HEADER

FIGURE 11-7:

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

DST

SRC

L3 Packet

{DSAP, SSAP, CTRL,

OUI[23:16]}

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

Calculate Checksum

1DWORD

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