LAN89218 datasheet(46/148 Pages) MICROCHIP

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LAN89218

DS60001255B-page 46

 2008-2015 Microchip Technology Inc.

3.12.1

TX BUFFER FORMAT

TX buffers exist in the host’s memory in a given format. The host writes a TX command word into the TX data buffer

before moving the Ethernet packet data. The TX command A and command B are 32-bit values that are used by the

LAN89218 in the handling and processing of the associated Ethernet packet data buffer. Buffer alignment,

segmentation and other packet processing parameters are included in the command structure. The following diagram

illustrates the buffer format.

Figure 3-21 shows the TX Buffer as it is written into the LAN89218. It should be noted that not all of the data shown

in this diagram is actually stored in the TX data FIFO. This must be taken into account when calculating the actual

TX data FIFO usage. Refer to Section 3.12.5 for a detailed explanation on calculating the actual TX data FIFO usage.

3.12.2

TX COMMAND FORMAT

The TX command instructs the TX FIFO controller on handling the subsequent buffer. The command precedes the

data to be transmitted. The TX command is divided into two, 32-bit words; TX command ‘A’ and TX command ‘B’.

There is a 16-bit packet tag in the TX command ‘B’ command word. Packet tags may, if host software desires, be

unique for each packet (i.e., an incrementing count). The value of the tag will be returned in the RX status word for

the associated packet. The Packet tag can be used by host software to uniquely identify each status word as it is

returned to the host.

Both TX command ‘A’ and TX command ‘B’ are required for each buffer in a given packet. TX command ‘B’ must be

identical for every buffer in a given packet. If the TX command ‘B’ words do not match, the Ethernet controller will

assert the Transmitter Error (TXE) flag.

FIGURE 3-21:

TX BUFFER FORMAT

TX Command 'A'

Offset + Data DWORD0

Last Data & PAD

1st

2nd

3rd

Last

Host Write

Order

Optional Pad DWORD0

Optional Pad DWORDn

TX Command 'B'

Optional offset DWORD0

Optional offset DWORDn

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