LAN9500 Datasheet, PDF (47/213 Pages) SMSC Corporation – USB 2.0 to 10/100 Ethernet Controller Promiscuous mode

FIGURE 5-5:
TX FIFO STORAGE
LAN950x
tx_wr_ptr
tx_wr_hd_ptr
TX Command B
TX Command A
USB Packet 1
23 Byte Payload
TX Command B
TX Command A
Unused bytes
are indicated to
the TLI by
controlling the
byte enables.
USB Packet 0
17 Byte Payload
TX Command B
TX Command A
9 Byte Payload
65 Byte
Ethernet
Frame
TX Command B
TX Command A
16 Byte Payload
tx_rd_ptr
TX Command B
TX Command A
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.
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DS00001875A-page 47