LAN9115 datasheet(39/114 Pages) MICROCHIP

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DS00002269A-page 39

LAN9115

Figure 3-14, "TX Buffer Format", shows the TX Buffer as it is written into the LAN9115. 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 3.13.5, "Calculating Actual TX Data FIFO Usage," on page 42

for a detailed explanation on calculating the actual TX data FIFO usage.

Note 3-15

The LAN9115 host bus interface supports 16-bit bus transfers; internally, all data paths are 32-bits

wide. Figure 3-14 describes the host write ordering for pairs of atomic 16-bit transactions.

3.13.2

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 associ-

ated 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 3-14:

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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