LAN9500AI datasheet(67/213 Pages) MICROCHIP

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DS00001875A-page 67

LAN950x

Note 1: 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.

2: 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.

3: The TXCOE_MODE may only be changed if the TX path is disabled. If it is desired to change this value

during run time, it is safe to do so only after the TX Ethernet path is disabled and the TLI is empty.

4: The TX checksum preamble must be DWORD-aligned.

5: TX preamble size is accounted for in both the buffer length and packet length.

6: The first buffer, which contains the TX preamble, may not contain any Ethernet frame data

FIGURE 5-16: TX Example Illustrating a Pre-pended TX Checksum Preamble on page 68 illustrates the use of a pre-

pended checksum preamble when transmitting an Ethernet frame consisting of 3 payload buffers.

TABLE 5-54:

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.

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