RM0008 Ethernet (ETH): media access control (MAC) with DMA controller
Doc ID 13902 Rev 12 987/1096
indicates the last buffer of the frame. After the last buffer of the frame has been transmitted,
the DMA writes back the final status information to the transmit descriptor 0 (TDES0) word
of the descriptor that has the last segment set in transmit descriptor 0 (TDES0[29]). At this
time, if Interrupt on Completion (TDES0[30]) is set, Transmit Interrupt (in ETH_DMASR
register [0]) is set, the next descriptor is fetched, and the process repeats. Actual frame
transmission begins after the Transmit FIFO has reached either a programmable transmit
threshold (ETH_DMAOMR register[16:14]), or a full frame is contained in the FIFO. There is
also an option for the Store and forward mode (ETH_DMAOMR register[21]). Descriptors
are released (OWN bit TDES0[31] is cleared) when the DMA finishes transferring the frame.
Transmit polling suspended
Transmit polling can be suspended by either of the following conditions:
● The DMA detects a descriptor owned by the CPU (TDES0[31]=0) and the Transmit
buffer unavailable flag is set (ETH_DMASR register[2]). To resume, the driver must give
descriptor ownership to the DMA and then issue a Poll Demand command.
● A frame transmission is aborted when a transmit error due to underflow is detected.
The appropriate Transmit Descriptor 0 (TDES0) bit is set. If the second condition
occurs, both the Abnormal Interrupt Summary (in ETH_DMASR register [15]) and
Transmit Underflow bits (in ETH_DMASR register[5]) are set, and the information is
written to Transmit Descriptor 0, causing the suspension. If the DMA goes into Suspend
state due to the first condition, then both the Normal Interrupt Summary (ETH_DMASR
register [16]) and Transmit Buffer Unavailable (ETH_DMASR register[2]) bits are set. In
both cases, the position in the transmit list is retained. The retained position is that of
the descriptor following the last descriptor closed by the DMA. The driver must explicitly
issue a Transmit Poll Demand command after rectifying the suspension cause.
Tx DMA descriptors
The descriptor structure consists of four 32-bit words as shown in Figure 354. The bit
descriptions of TDES0, TDES1, TDES2 and TDES3 are given below.
Figure 354. Transmit descriptor
TDES 3
O
W
N
Ctrl
[30:26]
Res.
24
Ctrl
[23:20]
Reserved
[19:18]
Status [16:0]
Reserved
[31:29]
Buffer 2 byte count
[28:16]
Reserved
[15:13]
Buffer 1 byte count
[12:0]
Buffer 1 address [31:0] / Time stamp low [31:0]
Buffer 2 address [31:0] or Next descriptor address [31:0] / Time stamp high [31:0]
TDES 0
TDES 1
TDES 2
31 0
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T
T
S
E
T
T
S
S
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