TM4C129XKCZAD Datasheet, PDF (1598/2188 Pages) Texas Instruments – Tiva Microcontroller

Ethernet Controller
24.3.3.5
Enhanced and Alternate Descriptors
Enhanced Descriptors can contain up to eight words (32 bytes) and buffers of up to 8 KB (useful
for Jumbo frames). Enhanced Descriptors support IEEE 1588-2008 Advanced Timestamp and IPC
Full Checksum (Type 2) Offload. These enhanced features are enabled with the TSEN bit of the
EMACTIMSTCTRL register and the IPC bit of the EMACCFG register, respectively.
When using Enhanced Descriptors, set the descriptor size to eight words with the Alternate Descriptor
Size (ATDS) bit in the Ethernet MAC DMA Bus Mode (EMACDMABUSMOD) register. If these
enhanced features are not enabled, the extended descriptors (DES4 to DES7) are not required.
Therefore, the software can use Alternate Descriptors with a default size of 16 bytes (4 words). For
alternate descriptors, the software should clear the Alternate Descriptor Size (ATDS) bit in the
Ethernet MAC DMA Bus Mode (EMACDMABUSMOD) register.
See the section called “Enhanced Transmit Descriptor” on page 1598 and the section called “Enhanced
Receive Descriptor” on page 1603 for more details on the descriptor structure.
Enhanced Transmit Descriptor
The MAC requires at least one descriptor for a transmit frame. In addition to two buffers, two
byte-count buffers, and two address pointers, the transmit descriptor has control fields which can
be used to control the MAC operation on per-transmit frame basis. Figure 24-5 on page 1599 shows
the enhanced transmit descriptor. Software must program the control bits TDES0[31:18] during
descriptor initialization. When the DMA updates the descriptor, it writes back all the control bits to
their initialized value, clears the OWN bit and updates the status bits.
With advanced timestamp support, the snapshot of the timestamp to be taken can be enabled for
a given frame by setting Bit 25 (TTSE) of TDES0. When the descriptor is closed (that is, when the
OWN bit is cleared), the timestamp is written into TDES6 and TDES7.
Note:
When the Advanced Timestamp feature is enabled, software should set the ATDS bit of the
Ethernet MAC DMA Bus Mode (EMACDMABUSMOD) register, offset 0xC00, so that the
DMA operates with extended descriptor size. When this control bit is reset to the default
(0), the TDES4-TDES7 descriptor space is not valid and only Alternate Descriptors are
available, with a default size of 16 bytes (4 words).
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June 18, 2014