LAN9220 datasheet(45/142 Pages) MICROCHIP

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DS00002417A-page 45

LAN9220

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 LAN9220.

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.

3.12.4

TX STATUS FORMAT

TX status is passed to the host CPU through a separate FIFO mechanism. A status word is returned for each packet

transmitted. Data transmission is suspended if the TX status FIFO becomes full. Data transmission will resume when

the host reads the TX status and there is room in the FIFO for more “TX Status” data.

The host can optionally choose to not read the TX status. The host can optionally ignore the TX status by setting the

“TX Status Discard Allow Overrun Enable” (TXSAO) bit in the TX Configuration Register (TX_CFG). If this option is cho-

sen TX status will not be written to the FIFO. Setting this bit high allows the transmitter to continue operation with a full

TX status FIFO. In this mode the status information is still available in the TX status FIFO, and TX status interrupts still

function. In the case of an overrun, the TXSUSED counter will stay at zero and no further TX status will be written to the

TX status FIFO until the host frees space by reading TX status. If TXSAO is enabled, a TXE error will not be generated

if the TX status FIFO overruns. In this mode the host is responsible for re-synchronizing TX status in the case of an

overrun.

Bits

Description

31:16

Packet TAG. Unique identifier written by the host into the Packet Tag field of the TX command ‘B’

word. This field can be used by the host to correlate TX status words with the associated TX packets.

Error Status (ES). When set, this bit indicates that the Ethernet controller has reported an error. This

bit is the logical OR of bits 11, 10, 9, 8, 2, 1 in this status word.

14:12

Reserved. These bits are reserved. Always write zeros to this field to provide future compatibility.

Loss of Carrier. When set, this bit indicates the loss of carrier during transmission.

No Carrier. When set, this bit indicates that the carrier signal from the transceiver was not present

during transmission.

Note:

During 10/100 Mbps full-duplex transmission, the value of this bit is invalid and should be

ignored.

Late Collision. When set, indicates that the packet transmission was aborted after the collision

window of 64 bytes.

Excessive Collisions. When set, this bit indicates that the transmission was aborted after 16

collisions while attempting to transmit the current packet.

Reserved. This bit is reserved. Always write zeros to this field to provide future compatibility.

6:3

Collision Count. This counter indicates the number of collisions that occurred before the packet was

transmitted. It is not valid when excessive collisions (bit 8) is also set.

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