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31
0
7
23
15
Status [16:7]
Byte Count Buffer 1 [12:0]
Byte Count Buffer2 [28:16]
TDES0
TDES1
TDES2
TDES3
Buffer1 Address [31:0]
Buffer2 Address [31:0]/Next Descriptor Address [31:0]
OWN
Transmit Timestamp High [31:0]
Transmit Timestamp Low [31:0]
Reserved
Reserved
CTRL
[30:26]
T
T
S
E
CTRL
[24:18]
T
T
S
S
CTRL/
Status
[6:3]
Status
[2:0]
CTRL
[31:29]
Reserved
TDES4
TDES5
TDES6
TDES7
Functional Description
891
SLAU723A – October 2017 – Revised October 2018
Copyright © 2017–2018, Texas Instruments Incorporated
Ethernet Controller
15.3.3.5.1 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.
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 (four words).
Figure 15-5. Enhanced Transmit Descriptor Structure
The following tables list the Enhanced Transmit Descriptors.
•
Transmit Descriptor 0 (TDES0) contains the transmitted frame status and the descriptor ownership
information (see
•
TDES1 contains the buffer sizes and other bits which control the descriptor chain or ring and the frame
being transferred (see
)
•
TDES2 contains the address pointer to the first buffer of the descriptor (see
).
•
TDES3 contains the address pointer either to the second buffer of the descriptor or the next descriptor