LAN9220 datasheet(43/142 Pages) MICROCHIP

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DS00002417A-page 43

LAN9220

3.12.2.1

TX Command ‘A’

TABLE 3-12:

TX COMMAND 'A' FORMAT

Bits

Description

Interrupt on Completion. When set, the TXDONE flag will be asserted when the current buffer has

been fully loaded into the TX FIFO. This flag may be optionally mapped to a host interrupt.

30:26

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

25:24

Buffer End Alignment. This field specifies the alignment that must be maintained on the last data

transfer of a buffer. The host will add extra DWORDs of data up to the alignment specified in the

table below. The LAN9220 will remove the extra DWORDs. This mechanism can be used to maintain

cache line alignment on host processors.

23:21

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

20:16

Data Start Offset (bytes). This field specifies the offset of the first byte of TX data. The offset value

can be anywhere from 0 bytes to 31 a Byte offset.

15:14

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

First Segment (FS). When set, this bit indicates that the associated buffer is the first segment of the

packet.

Last Segment. When set, this bit indicates that the associated buffer is the last segment of the

packet

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

10:0

Buffer Size (bytes). This field indicates the number of bytes contained in the buffer following this

command. This value, along with the Buffer End Alignment field, is read and checked by the

LAN9220 and used to determine how many extra DWORD’s were added to the end of the Buffer. A

running count is also maintained in the LAN9220 of the cumulative buffer sizes for a given packet.

This cumulative value is compared against the Packet Length field in the TX command ‘B’ word and

if they do not correlate, the TXE flag is set.

Note:

The buffer size specified does not include the buffer end alignment padding or data start

offset added to a buffer.

[25]

[24]

End Alignment

4-byte alignment

16-byte alignment

32-byte alignment

Reserved

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