LAN9500 Datasheet, PDF (66/213 Pages) SMSC Corporation – USB 2.0 to 10/100 Ethernet Controller Promiscuous mode

LAN950x
The RXCOE resides in the RX path within the MAC. As the RXCOE receives an Ethernet frame, it calculates the 16-bit
checksum. The RXCOE passes the Ethernet frame to the RX FIFO with the checksum appended to the end of the frame.
The RXCOE inserts the checksum immediately after the last byte of the Ethernet frame and before it transmits the status
word. The packet length field in the RX Status Word (refer to Section 5.4.1.2) will indicate that the frame size has
increased by two bytes to accommodate the checksum.
Note: When enabled, the RXCOE calculates a checksum for every received frame.
Setting the RXCOE_EN bit in the Checksum Offload Engine Control Register (COE_CR) enables the RXCOE, while the
RXCOE_MODE bit selects the operating mode. When the RXCOE is disabled, the received data is simply passed
through the RXCOE unmodified.
Note 1: Software applications must stop the receiver and flush the RX data path before changing the state of the
RXCOE_EN or RXCOE_MODE bits.
2: When the RXCOE is enabled, automatic pad stripping must be disabled (bit 8 (PADSTR) of the MAC Control
Register (MAC_CR)) and vice versa. These functions cannot be enabled simultaneously.
5.5.7.1 RX Checksum Calculation
The checksum is calculated 16 bits at a time. In the case of an odd sized frame, an extra byte of zero is used to pad up
to 16 bits.
Consider the following packet: DA, SA, Type, B0, B1, B2 … BN, FCS
Let [A, B] = A*256 + B;
If the packet has an even number of octets then
checksum = [B1, B0] + C0 + [B3, B2] + C1 + … + [BN, BN-1] + CN-1
Where C0, C1, ... CN-1 are the carry out results of the intermediate sums.
If the packet has an odd number of octets then
checksum = [B1, B0] + C0 + [B3, B2] + C1 + … + [0, BN] + CN-1
5.5.8 TRANSMIT CHECKSUM OFFLOAD ENGINE (TXCOE)
The transmit checksum offload engine provides assistance to the CPU by calculating a 16-bit checksum, typically for
TCP, for a transmit Ethernet frame. The TXCOE calculates the checksum and inserts the results back into the data
stream as it is transferred to the MAC.
To activate the TXCOE and perform a checksum calculation, the Host must first set the TX Checksum Offload Engine
Enable (TX_COE_EN) bit in the Checksum Offload Engine Control Register (COE_CR). The Host then pre-pends a 3
DWORD buffer to the data that will be transmitted. The pre-pended buffer includes a TX Command A, TX Command B,
and a 32-bit TX checksum preamble (refer to Table 5-54). When the CK bit of the TX Command ‘B’ is set in conjunction
with the FS bit of TX Command ‘A’ and the TX_COE_EN bit of the COE_CR register, the TXCOE will perform a check-
sum calculation on the associated packet. The TX checksum preamble instructs the TXCOE on the handling of the asso-
ciated packet. The TXCSSP - TX Checksum Start Pointer field of the TX checksum preamble defines the byte offset at
which the data checksum calculation will begin. The checksum calculation will begin at this offset and will continue until
the end of the packet. The data checksum calculation must not begin in the MAC header (first 14 bytes) or in the last 4
bytes of the TX packet. When the calculation is complete, the checksum will be inserted into the packet at the byte offset
defined by the TXCSLOC - TX Checksum Location field of the TX checksum preamble. The TX checksum cannot be
inserted in the MAC header (first 14 bytes) or in the last 4 bytes of the TX packet. If the CK bit is not set in the first TX
Command ‘B’ of a packet, the packet is passed directly through the TXCOE without modification, regardless if the
TXCOE_EN is set. An example of a TX packet with a pre-pended TX checksum preamble can be found in Section
5.4.2.9, "TX Example 3". In this example, the Host provides the Ethernet frame to the Ethernet controller (via a USB
packet) in four fragments, the first containing the TX Checksum Preamble. Figure 5-8 shows how these fragments are
loaded into the TX Data FIFO. For more information on the TX Command ‘A’ and TX Command ‘B’, refer to Section
5.4.2.1, "TX Command Format," on page 47.
If the TX packet already includes a partial checksum calculation (perhaps inserted by an upper layer protocol), this
checksum can be included in the hardware checksum calculation by setting the TXCSSP field in the TX checksum pre-
amble to include the partial checksum. The partial checksum can be replaced by the completed checksum calculation
by setting the TXCSLOC pointer to point to the location of the partial checksum.
DS00001875A-page 66
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