Table 20-2. Enhanced Transmit Descriptor 0 (TDES0) (continued)
Description
Bit
LS: Last Segment
When set, this bit indicates that the buffer contains the last segment of the frame. When this bit is set, the TBS1
or TBS2 field in TDES1 should have a non-zero value.
29
FS: First Segment
When set, this bit indicates that the buffer contains the first segment of a frame.
28
DC: Disable CRC
When set, the MAC does not append a Cyclic Redundancy Check (CRC) to the end of the transmitted frame.
This is valid only when the first segment (TDES0[28]) is set.
27
DP: Disable Padding
When set, the MAC does not automatically add padding to a frame shorter than 64 bytes. When this bit is reset,
the DMA automatically adds padding and CRC to a frame shorter than 64 bytes, and the CRC field is added
despite the state of the DC (TDES0[27]) bit. This is valid only when the first segment (TDES0[28]) is set.
26
TTSE: Transmit Timestamp Enable
When set, this bit enables IEEE1588 hardware timestamping for the transmit frame referenced by the descriptor.
This bit is only valid when the First Segment Control bit (TDES0[28] is set.
25
CRCR: CRC Replacement Control
When set, the MAC replaces the last four bytes of the transmitted packet with recalculated CRC bytes. The CPU
should ensure that the CRC bytes are present in the frame being transferred from the Transmit Buffer. CRC
replacement is done only when Bit 27 (DC) is set to 1.
24
CIC: Checksum Insertion Control
These bits control the insertion of checksums in Ethernet frames that encapsulate TCP, UDP, or ICMP over IPv4
or IPv6. This field is valid when the First Segment control bit (TDES0[28]) is set.
■
0x0 = Do nothing. Checksum Engine bypassed.
■
0x1 = Insert IPv4 header checksum. Use this value to insert IPv4 header checksum when the frame
encapsulates an IPv4 datagram.
■
0x2 = Insert TCP/UDP/ICMP checksum. The checksum is calculated over the TCP, UDP, or ICMP segment
only and the TCP, UDP, or ICMP pseudo-header checksum is assumed to be present in the corresponding
input frame's Checksum field. An IPv4 header checksum is also inserted if the encapsulated datagram
conforms to IPv4.
■
0x3 = Insert a TCP/UDP/ICMP checksum that is fully calculated in this engine. The TCP, UDP, or ICMP
pseudo-header is included in the checksum calculation, and the input frame's corresponding Checksum field
has an all-zero value. An IPv4 Header checksum is also inserted if the encapsulated datagram conforms to
IPv4.
The Checksum engine detects whether the TCP, UDP, or ICMP segment is encapsulated in IPv4 or IPv6 and
processes its data accordingly.
23:22
TER: Transmit End of Ring
When set, this bit indicates that the descriptor list reached its final descriptor. The DMA returns to the base
address of the list, creating a descriptor ring.
21
TCH: Second Address Chained
When set, this bit indicates that the second address in the descriptor is the Next Descriptor address rather than
the second buffer address. When TDES0[20] is set, TBS2 (TDES1[28:16]) is a "don’t care" value. TDES0[21]
takes precedence over TDES0[20].
20
1415
June 18, 2014
Texas Instruments-Production Data
Tiva
™
TM4C1294NCPDT Microcontroller