LAN9730 datasheet(53/222 Pages) MICROCHIP

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DS00001946A-page 53

LAN9730/LAN9730i

4.4.2.1

TX Command Format

As shown in Figure 4-5, each buffer starts with a two DWORD TX Command. The TX Command instructs the FCT on

the handling of the associated 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.

Both TX Command A and TX Command B are required for each buffer in a given packet. TX Command B must be iden-

tical for every buffer in a given packet, with the exception of the TX Checksum Enable (CK) bit. If the TX Command B

DWORDs do not match, the FCT will assert the Transmitter Error (TXE) flag.

Frame boundaries are delineated using control bits within the TX Command. The Frame Length field in TX Command

B specifies the number of bytes in the associated frame. All Frame Length fields must have the same value for all buffers

in a given Frame. Hardware compares the Frame Length field and the actual amount of data received. If the actual frame

length count does not match the Frame Length field, an error has occurred.

The formats of TX Command A and TX Command B are shown in Table 4-41 and Table 4-42, respectively.

TABLE 4-41:

TX COMMAND A FORMAT

Bits

Description

31:18

RESERVED

17:16

Data Start Offset (bytes)

This field specifies the offset of the first byte of TX Data. The offset value ranges between 0 bytes and

3 bytes.

15:14

RESERVED

First Segment

When set, this bit indicates that the associated buffer is the first segment of the frame.

Last Segment

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

RESERVED

10:0

Buffer Size (bytes)

This field indicates the number of bytes contained in the buffer following the two command DWORDS

(TX Command A and TX Command B). This value, along with the Data Start Offset field, is used by the

FCT to determine how many extra bytes were added to the end of the Buffer. A running count is also

maintained in the FCT of the cumulative buffer sizes for a given frame. This cumulative value is com-

pared against the Frame Length field in the TX Command B Word and if they do not correlate, the TXE

flag is set.

The buffer size specified does not include bytes added due to the end of buffer alignment padding or

the Data Start Offset field.

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