LAN9500AI datasheet(47/213 Pages) MICROCHIP

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DS00001875A-page 47

LAN950x

5.4.2.1

TX Command Format

As shown in Figure 5-5, 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 5-41 and Table 5-42, respectively.

FIGURE 5-5:

TX FIFO STORAGE

TX Command A

TX Command B

TX Command A

TX Command B

TX Command A

TX Command B

TX Command A

TX Command B

23 Byte Payload

17 Byte Payload

9 Byte Payload

16 Byte Payload

65 Byte

Ethernet

Frame

tx_wr_ptr

tx_wr_hd_ptr

USB Packet 0

USB Packet 1

TX Command A

TX Command B

tx_rd_ptr

Unused bytes

are indicated to

the TLI by

controlling the

byte enables.

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