Table 20-3. Enhanced Transmit Descriptor 1 (TDES1) (continued)
Description
Bit
TBS2: Transmit Buffer 2 Size
This field indicates the second data buffer size in bytes. This field is not valid if TDES0[20] is set.
28:16
Reserved
15:13
TBS1: Transmit Buffer 1 Size
These bits indicate the first data buffer byte size, in bytes. If this field is 0, the DMA ignores this buffer and
uses Buffer 2 or the next descriptor, depending on the value of TCH (TDES0[20]).
12:0
Table 20-4. Enhanced Transmit Descriptor 2 (TDES2)
Description
Bit
Buffer 1 Address Pointer
These bits indicate the physical address of Buffer 1. There is no limitation on the buffer address
alignment. Note that the buffers are stored in SRAM.
31:0
Table 20-5. Enhanced Transmit Descriptor 3 (TDES3)
Description
Bit
Buffer 2 Address Pointer (Next Descriptor Address)
Indicates the physical address of Buffer 2 when a descriptor ring structure is used. If the Second Address
Chained (TDES0[20]) bit is set, this address contains the pointer to the physical memory where the Next
Descriptor is present. The buffer address pointer must be aligned to the bus width only when TDES0[20] is
set.
Note that the buffers are stored in SRAM.
31:0
Table 20-6. Enhanced Transmit Descriptor 6 (TDES6)
Description
Bit
TTSL: Transmit Frame Timestamp Low
This field is updated by DMA with the least significant 32 bits of the timestamp captured for the corresponding
transmit frame. This field has the timestamp only if the Last Segment bit (LS), TDES0[29], in the descriptor
is set and Timestamp status (TTSS) bit, TDES0[17], is set.
31:0
Table 20-7. Enhanced Transmit Descriptor 7 (TDES7)
Description
Bit
TTSH: Transmit Frame Timestamp High
This field is updated by DMA with the most significant 32 bits of the timestamp captured for the corresponding
receive frame. This field has the timestamp only if the Last Segment bit (LS), TDES0[29], in the descriptor
is set and Timestamp status (TTSS) bit , TDES0[17], is set.
31:0
Enhanced Receive Descriptor
The DMA requires at least two descriptors when receiving a frame. The DMA always attempts to
acquire an extra descriptor in anticipation of an incoming frame. Before the DMA closes a descriptor,
it attempts to acquire the next descriptor even if no frames are received. In a single descriptor
(receive) system, the subsystem generates a descriptor error if the receive buffer is unable to
accommodate the incoming frame and the next descriptor is not owned by the DMA. Figure
20-4 on page 1419 shows the enhanced receive descriptor. This descriptor is used when Advanced
Timestamp or the Checksum Offload Engine is enabled.
Note:
When the Advanced Timestamp or Checksum Offload Engine features are enabled, software
should set the
ATDS
bit of the
Ethernet MAC DMA Bus Mode (EMACDMABUSMOD)
June 18, 2014
1418
Texas Instruments-Production Data
Ethernet Controller