LAN89218 datasheet(28/148 Pages) MICROCHIP

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LAN89218

DS60001255B-page 28

 2008-2015 Microchip Technology Inc.

An example of a TX packet with a pre-pended TX checksum preamble can be found in Section 3.12.6.3. In this

example the host writes the packet data to the Ethernet controller in four fragments, the first containing the TX

Checksum Preamble. Figure 3-24 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.

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

preamble 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.

3.6.2.1

TX Checksum Calculation

The TX checksum calculation is performed using the same operation as the RX checksum shown in Section 3.6.1.1,

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

compliment 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’.

Note:

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.

Note:

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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