LAN9220 datasheet(25/142 Pages) MICROCHIP

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DS00002417A-page 25

LAN9220

3.6.3.1

TX Checksum Calculation

The TX checksum calculation is performed using the same operation as the RX checksum shown in Section 3.6.2, with

the exception that the calculation starts as indicated by the preamble, and the transmitted checksum is the one’s-com-

pliment of the final calculation.

TABLE 3-7:

TX CHECKSUM PREAMBLE

Field

Description

31:28

RESERVED

27:16

TXCSLOC - TX Checksum Location

This field specifies the byte offset where the TX checksum will be inserted in the TX packet. The

checksum will replace two bytes of data starting at this offset.

Note:

The TX checksum cannot be inserted in the MAC header (first 14 bytes) or in the last 4

bytes of the TX packet.

15:12

RESERVED

11:0

TXCSSP - TX Checksum Start Pointer

This field indicates start offset, in bytes, where the checksum calculation will begin in the associated

TX packet.

Note:

The data checksum calculation must not begin in the MAC header (first 14 bytes) or in the

last 4 bytes of the TX packet.

Note:

• When the TXCOE is enabled, the third DWORD of the pre-pended packet is not transmitted. However, 4 bytes

must be added to the packet length field in TX Command ‘B’.

• The TX checksum preamble must be DWORD-aligned (i.e., the two least significant bits of the Data Start Offset

fields in TX Command “A” must be zero). Any valid buffer end alignment setting can be used.

• Software applications must stop the transmitter and flush the TX data path before changing the state of the

TXCOE_EN bit. However, the CK bit of TX Command ‘B’ can be set or cleared on a per-packet basis.

Note:

When the TX checksum offload feature is invoked, if the calculated checksum is 0000h, it is left unaltered.

UDP checksums are optional under IPv4, and a zero checksum calculated by the TX checksum offload

feature will erroneously indicate to the receiver that no checksum was calculated, however, the packet will

typically not be rejected by the receiver. Under IPv6, however, according to RFC 2460, the UDP checksum

is not optional. A calculated checksum that yields a result of zero must be changed to FFFFh for insertion

into the UDP header. IPv6 receivers discard UDP packets containing a zero checksum. Thus, this feature

must not be used for UDP checksum calculation under IPv6.

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