LAN89218AQ Standard Microsystems (SMSC), LAN89218AQ Datasheet - Page 35

• Manufacturer Part Number: LAN89218AQ
• Manufacturer: Standard Microsystems (SMSC)

Specifications of LAN89218AQ

• Lead Free Status / RoHS Status: Compliant

High Performance Single-Chip 10/100 Ethernet Controller for Automotive Applications
Datasheet
SMSC LAN89218
3.6.1.1
3.6.2
Note: Software applications must stop the receiver and flush the RX data path before changing the
Note: When the RXCOE is enabled, automatic pad stripping must be disabled (bit 8 (PADSTR) of
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
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 bit (TXCOE_EN) in the
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. When bit 14 (CK)
of the TX Command ‘B’ is set in conjunction with bit 13 (FS) of TX Command ‘A’ and bit 16
(TXCOE_EN) of the COE_CR register, the TXCOE will perform a checksum calculation on the
associated packet. When these three bits are set, a 32-bit TX checksum preamble must be pre-pended
to the beginning of the TX packet (refer to
TXCOE on the handling of the associated packet. Bits 11:0 of the TX checksum preamble define the
byte offset at which the data checksum calculation will begin (TXCSSP). The checksum calculation will
begin at this offset and will continue until the end of the packet. When the calculation is complete, the
checksum will be inserted into the packet at the byte offset defined by bits 27:16 of the TX checksum
preamble (TXCSLOC). 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.
Note: The data checksum calculation must not begin in the MAC header (first 14 bytes) or in the last
Note: The TX checksum cannot be inserted in the MAC header (first 14 bytes) or in the last 4 bytes
state of the RXCOE_EN or RXCOE_MODE bits.
the
simultaneously.
4 bytes of the TX packet.
of the TX packet.
MAC_CR—MAC Control
DATASHEET
Register) and vice versa. These functions cannot be enabled
35
COE_CR—Checksum Offload Engine Control
Table
3.7). The TX checksum preamble instructs the
Revision 1.3 (02-23-10)
Register.