LAN89218AQ SMSC, LAN89218AQ Datasheet - Page 59

• Manufacturer Part Number: LAN89218AQ
• Description: Ethernet ICs High Perform Chip 10/100 NonPCI Cntrl
• Manufacturer: SMSC

Specifications of LAN89218AQ

• Ethernet Connection Type: 10BASE-T, 100BASE-TX
• Minimum Operating Temperature: - 40 C
• Mounting Style: SMD/SMT
• Product: Ethernet Controllers
• Standard Supported: IEEE802.3, IEEE802.3u
• Supply Voltage (max): 3.6 V
• Supply Voltage (min): 3 V
• Maximum Operating Temperature: + 85 C
• Package / Case: TQFP-100
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

High Performance Single-Chip 10/100 Ethernet Controller for Automotive Applications
Datasheet
SMSC LAN89218
3.12.3
3.12.3.1
3.12.3.2
Data Start Offset [1:0]:
First TX Data Byte:
TX Data Format
The TX data section begins at the third DWORD in the TX buffer (after TX command ‘A’ and TX
command ‘B’). The location of the first byte of valid buffer data to be transmitted is specified in the
“Data Start Offset” field of the TX command ‘A’ word.
correlation between the setting of the LSB’s in the “Data Start Offset” field and the byte location of the
first valid data byte. Additionally, transmit buffer data can be offset by up to 7 additional DWORDS as
indicated by the upper three MSB’s (5:2) in the “Data Start Offset” field.
TX data is contiguous until the end of the buffer. The buffer may end on a byte boundary. Unused
bytes at the end of the packet will not be sent to the MIL for transmission.
The Buffer End Alignment field in TX command ‘A’ specifies the alignment that must be maintained for
the associated buffer. End alignment may be specified as 4-, 16-, or 32-byte. The host processor is
responsible for adding the additional data to the end of the buffer. The hardware will automatically
remove this extra data.
TX Buffer Fragmentation Rules
Transmit buffers must adhere to the following rules:
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).
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.
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
D[31:24]
Table 3.15 TX DATA Start Offset
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DATASHEET
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D[23:16]
10
Table 3.15, "TX DATA Start
D[15:8]
01
Revision 1.3 (02-23-10)
Offset", shows the
D[7:0]
00