LAN9250 datasheet(161/421 Pages) MICROCHIP

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 2015 Microchip Technology Inc.

DS00001913A-page 161

LAN9250

11.11.1

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

device 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 buffer format is illustrated in

Figure 11-11.

Figure 11-11 shows the TX Buffer as it is written into the device. 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. Please refer to Section 11.11.5, "Calculating Actual TX Data FIFO Usage" for a detailed explanation on

calculating the actual TX Data FIFO usage.

11.11.2

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 TX 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 11-11:

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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