Data Sheet

Ethernet Controller I210 —Inline Functions
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7.2.2.1 Legacy Transmit Descriptor Format
Legacy descriptors are identified by having bit 29 of the descriptor (TDESC.DEXT) set to 0b. In this
case, the descriptor format is defined as shown in Table 7-23. Note that the address and length must
be supplied by software. Also note that bits in the command byte are optional, as is the CSO field.
Note: For frames that span multiple descriptors, the VLAN, CSO, CMD.VLE, CMD.IC, and CMD.IFCS
are valid only in the first descriptors and are ignored in the subsequent ones.
7.2.2.1.1 Buffer Address (64)
Physical address of a data buffer in host memory that contains a portion of a transmit packet.
7.2.2.1.2 Length
Length (TDESC.LENGTH) specifies the length in bytes to be fetched from the buffer address provided.
The maximum length associated with any single legacy descriptor is 9728 bytes.
Descriptor length(s) might be limited by the size of the transmit FIFO. All buffers comprising a single
packet must be able to be stored simultaneously in the transmit FIFO. For any individual packet, the
sum of the individual descriptors' lengths must be below 9728 bytes.
Note: The maximum allowable packet size for transmits can change, based on the value written to
the DMA TX Max Allowable packet size (DTXMXPKTSZ) register.
Note: Descriptors with zero length (null descriptors) transfer no data. Null descriptors can only
appear between packets and must have their EOP bits set.
Note: If the TCTL.PSP bit is set, the total length of the packet transmitted, not including FCS should
be at least 17 bytes. If bit is cleared the total length of the packet transmitted, not including
FCS should be at least 60 bytes.
7.2.2.1.3 Checksum Offset and Start - CSO
A Checksum Offset (TDESC.CSO) field indicates where, relative to the start of the packet, to insert a
TCP checksum if this mode is enabled.
Table 7-23. Transmit Descriptor (TDESC) Fetch Layout - Legacy Mode
63 48 47 40 39 36 35 32 31 24 23 16 15 0
0 Buffer Address [63:0]
8 VLAN Reserved Reserved STA CMD CSO Length
Table 7-24. Transmit Descriptor (TDESC) Write-Back Layout - Legacy Mode
63 48 47 40 39 36 35 32 31 24 23 16 15 0
0 Reserved Reserved
8 VLAN Reserved Reserved STA CMD CSO Length