LAN9211_12 Datasheet, PDF (32/147 Pages) SMSC Corporation – High-Performance Small Form Factor Single-Chip Ethernet Controller with HP Auto-MDIX Support

High-Performance Small Form Factor Single-Chip Ethernet Controller with HP Auto-MDIX Support
Datasheet
FIELD
31:28
27:16
15:12
11:0
Table 3.7 TX Checksum Preamble
DESCRIPTION
RESERVED
TXCSLOC - TX Checksum Location
This field specifies the byte offset where the TX checksum will be inserted in the TX packet. The
checksum will replace two bytes of data starting at this offset.
Note: The TX checksum cannot be inserted in the MAC header (first 14 bytes) or in the last 4
bytes of the TX packet.
RESERVED
TXCSSP - TX Checksum Start Pointer
This field indicates start offset, in bytes, where the checksum calculation will begin in the associated
TX packet.
Note: The data checksum calculation must not begin in the MAC header (first 14 bytes) or in
the last 4 bytes of the TX packet.
Note: When the TXCOE is enabled, the third DWORD of the pre-pended packet is not transmitted.
However, 4 bytes must be added to the packet length field in TX Command ‘B’.
Note: The TX checksum preamble must be DWORD-aligned (i.e., the two least significant bits of the
Data Start Offset fields in TX Command “A” must be zero). Any valid buffer end alignment
setting can be used.
Note: Software applications must stop the transmitter and flush the TX data path before changing the
state of the TXCOE_EN bit. However, the CK bit of TX Command ‘B’ can be set or cleared on
a per-packet basis.
3.6.2.1
3.7
3.7.1
TX Checksum Calculation
The TX checksum calculation is performed using the same operation as the RX checksum shown in
Section 3.6.1.1, with the exception that the calculation starts as indicated by the preamble, and the
transmitted checksum is the one’s-compliment of the final calculation.
Note:
When the TX checksum offload feature is invoked, if the calculated checksum is 0000h, it is
left unaltered. UDP checksums are optional under IPv4, and a zero checksum calculated by
the TX checksum offload feature will erroneously indicate to the receiver that no checksum was
calculated, however, the packet will typically not be rejected by the receiver. Under IPv6,
however, according to RFC 2460, the UDP checksum is not optional. A calculated checksum
that yields a result of zero must be changed to FFFFh for insertion into the UDP header. IPv6
receivers discard UDP packets containing a zero checksum. Thus, this feature must not be
used for UDP checksum calculation under IPv6.
Host Bus Operations
Bus Writes
The host processor is required to perform two contiguous 16-bit writes to complete a single DWORD
transfer. This DWORD must begin and end on a DWORD address boundary (A[2] and higher, cannot
change during a sixteen bit write). No ordering requirements exist. The processor can access either
the low or high word first, as long as the next write is performed to the other word. If a write to the
same word is performed, the LAN9211 disregards the transfer.
Revision 2.9 (03-01-12)
32
DATASHEET
SMSC LAN9211