LAN9117 Datasheet, PDF (37/114 Pages) SMSC Corporation – HIGH PERFORMANCE SINGLE-CHIP 10/100 NON-PCI ETHERNET CONTROLLER

LAN9117
3.13 TX Data Path Operation
Data is queued for transmission by writing it into the TX data FIFO. Each packet to be transmitted may be divided among
multiple buffers. Each buffer starts with a two DWORD TX command (TX command ‘A’ and TX command ‘B’). The TX
command instructs the LAN9117 on the handling of the associated buffer. Packet boundaries are delineated using con-
trol bits within the TX command.
The host provides a 16-bit Packet Tag field in the TX command. The Packet Tag value is appended to the corresponding
TX status DWORD. All Packet Tag fields must have the same value for all buffers in a given packet. If tags differ between
buffers in the same packet the TXE error will be asserted. Any value may be chosen for a Packet Tag as long as all tags
in the same Packet are identical. Packet Tags also provide a method of synchronization between transmitted packets
and their associated status. Software can use unique Packet Tags to assist with validating matching status completions.
Note 3-14 The use of packet tags is not required by the hardware. This is a software LAN driver only application
example for use of this field.
A Packet Length field in the TX command specifies the number of bytes in the associated packet. All Packet Length
fields must have the same value for all buffers in a given packet. Hardware compares the Packet Length field and the
actual amount of data received by the Ethernet controller. If the actual packet length count does not match the Packet
Length field as defined in the TX command, the Transmitter Error (TXE) flag is asserted.
The LAN9117 can be programmed to start payload transmission of a buffer on a byte boundary by setting the “Data Start
Offset” field in the TX command. The “Data Start Offset” field points to the actual start of the payload data within the first
8 DWORDs of the buffer. Data before the “Data Start Offset” pointer will be ignored. When a packet is split into multiple
buffers, each successive buffer may begin on any arbitrary byte.
The LAN9117 can be programmed to strip padding from the end of a transmit packet in the event that the end of the
packet does not align with the host burst boundary. This feature is necessary when the LAN9117 is operating in a system
that always performs multi-word bursts. In such cases the LAN9117 must ensure that it can accept data in multiples of
the Burst length regardless of the actual packet length. When configured to do so, the LAN9117 will accept extra data
at the end of the packet and will remove the extra padding before transmitting the packet. The LAN9117 automatically
removes data up to the boundary specified in the Buffer End Alignment field specified in each TX command.
The host can instruct the LAN9117 to issue an interrupt when the buffer has been fully loaded into the TX FIFO con-
tained in the LAN9117 and transmitted. This feature is enabled through the TX command ‘Interrupt on Completion’ field.
Upon completion of transmission, irrespective of success or failure, the status of the transmission is written to the TX
status FIFO. TX status is available to the host and may be read using PIO operations. An interrupt can be optionally
enabled by the host to indicate the availability of a programmable number TX status DWORDS.
Before writing the TX command and payload data to the TX FIFO, the host must check the available TX FIFO space by
performing a PIO read of the TX_FIFO_INF register. The host must ensure that it does not overfill the TX FIFO or the
TX Error (TXE) flag will be asserted.
The host proceeds to write the TX command by first writing TX command ‘A’, then TX command ‘B’. After writing the
command, the host can then move the payload data into the TX FIFO. TX status DWORD’s are stored in the TX status
FIFO to be read by the host at a later time upon completion of the data transmission onto the wire.
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DS00002267A-page 37