LAN89218 datasheet(49/148 Pages) MICROCHIP

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DS60001255B-page 49

LAN89218

3.12.3.1

TX Buffer Fragmentation Rules

Transmit buffers must adhere to the following rules:

• Each buffer can start and end on any arbitrary byte alignment

• The first buffer of any transmit packet can be any length

• Middle buffers (i.e., those with First Segment = Last Segment = 0) must be greater than, or equal to 4 bytes in

length

• The final buffer of any transmit packet can be any length

The MIL operates in store-and-forward mode and has specific rules with respect to fragmented packets. The total

space consumed in the TX FIFO (MIL) must be limited to no more than 2KB - 3 DWORDs (2,036 bytes total). Any

transmit packet that is so highly fragmented that it takes more space than this must be un-fragmented (by copying

to a driver-supplied buffer) before the transmit packet can be sent to the LAN89218.

One approach to determine whether a packet is too fragmented is to calculate the actual amount of space that it will

consume, and check it against 2,036 bytes. Another approach is to check the number of buffers against a worst-case

limit of 86 (see explanation below).

3.12.3.2

Calculating Worst-Case TX FIFO (MIL) Usage

The actual space consumed by a buffer in the MIL TX FIFO consists only of any partial DWORD offsets in the first/

last DWORD of the buffer, plus all of the whole DWORDs in between. Any whole DWORD offsets and/or alignments

are stripped off before the buffer is loaded into the TX Data FIFO, and TX command words are stripped off before

the buffer is written to the MIL TX FIFO, so none of those DWORDs count as space consumed. The worst-case

overhead for a TX buffer is 6 bytes, which assumes that it started on the high byte of a DWORD and ended on the

low byte of a DWORD. A TX packet consisting of 86 such fragments would have an overhead of 516 bytes (6 * 86)

which, when added to a 1514-byte max-size transmit packet (1516 bytes, rounded up to the next whole DWORD),

would give a total space consumption of 2,032 bytes, leaving 4 bytes to spare; this is the basis for the "86 fragment"

rule mentioned above.

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