LAN9220 datasheet(24/142 Pages) MICROCHIP

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LAN9220

DS00002417A-page 24

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3.6.2

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

3.6.3

TRANSMIT CHECKSUM OFFLOAD ENGINE (TXCOE)

The transmit checksum offload engine provides assistance to the CPU by calculating a 16-bit checksum, typically for

TCP, for a transmit Ethernet frame. The TXCOE calculates the checksum and inserts the results back into the data

stream as it is transferred to the MAC.

To activate the TXCOE and perform a checksum calculation, the host must first set the TX checksum offload engine

enable bit (TXCOE_EN) in the COE_CR—Checksum Offload Engine Control Register. The host then pre-pends a 3

DWORD buffer to the data that will be transmitted. The pre-pended buffer includes a TX Command ‘A’, TX Command

‘B’, and a 32-bit TX checksum preamble. When bit 14 (CK) of the TX Command ‘B’ is set in conjunction with bit 13 (FS)

of TX Command ‘A’ and bit 16 (TXCOE_EN) of the COE_CR register, the TXCOE will perform a checksum calculation

on the associated packet. When these three bits are set, a 32-bit TX checksum preamble must be pre-pended to the

beginning of the TX packet (refer to Table 3-7). The TX checksum preamble instructs the TXCOE on the handling of the

associated packet. Bits 11:0 of the TX checksum preamble define the byte offset at which the data checksum calculation

will begin (TXCSSP). The checksum calculation will begin at this offset and will continue until the end of the packet. The

data checksum calculation must not begin in the MAC header (first 14 bytes) or in the last 4 bytes of the TX packet.

When the calculation is complete, the checksum will be inserted into the packet at the byte offset defined by bits 27:16

of the TX checksum preamble (TXCSLOC). The TX checksum cannot be inserted in the MAC header (first 14 bytes) or

in the last 4 bytes of the TX packet. If the CK bit is not set in the first TX Command ‘B’ of a packet, the packet is passed

directly through the TXCOE without modification, regardless if the TXCOE_EN is set. An example of a TX packet with

a pre-pended TX checksum preamble can be found in Section 3.12.6.3, "TX Example 3". In this example the host writes

the packet data to the ethernet controller in four fragments, the first containing the TX Checksum Preamble. Figure 3-

23 shows how these fragments are loaded into the TX Data FIFO. For more information on the TX Command ‘A’ and

TX Command ‘B’, refer to Section 3.12.2, "TX Command Format".

If the TX packet already includes a partial checksum calculation (perhaps inserted by an upper layer protocol), this

checksum can be included in the hardware checksum calculation by setting the TXCSSP field in the TX checksum pre-

amble to include the partial checksum. The partial checksum can be replaced by the completed checksum calculation

by setting the TXCSLOC pointer to point to the location of the partial checksum.

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