Ethernet (ETH): media access control (MAC) with DMA controller
RM0090
1162/1731
DocID018909 Rev 11
Figure 378. TxDMA operation in OSF mode
Transmit frame processing
The transmit DMA expects that the data buffers contain complete Ethernet frames,
excluding preamble, pad bytes, and FCS fields. The DA, SA, and Type/Len fields contain
valid data. If the transmit descriptor indicates that the MAC core must disable CRC or pad
insertion, the buffer must have complete Ethernet frames (excluding preamble), including
the CRC bytes. Frames can be data-chained and span over several buffers. Frames have to
be delimited by the first descriptor (TDES0[28]) and the last descriptor (TDES0[29]). As the
transmission starts, TDES0[28] has to be set in the first descriptor. When this occurs, the
frame data are transferred from the memory buffer to the Transmit FIFO. Concurrently, if the
last descriptor (TDES0[29]) of the current frame is cleared, the transmit process attempts to
acquire the next descriptor. The transmit process expects TDES0[28] to be cleared in this
descriptor. If TDES0[29] is cleared, it indicates an intermediary buffer. If TDES0[29] is set, it
Previous frame
status availa
b
le
Start TxDMA
(Re-)fetch next
descriptor
Write status word to
prev. frame
’
s TDES0
Transfer data from
b
uffer(s)
(AHB)
error?
Own
b
it set?
(AHB)
error?
Frame xfer
complete?
Time stamp
present?
(AHB)
error?
Write time stamp to
TDES2 & TDES3
for previous frame
(AHB)
error?
Stop TxDMA
No
Yes
No
Yes
No
Start
Yes
Close intermediate
descriptor
No
No
Wait for previous
frame
’
s Tx status
Second
frame?
Yes
Yes
No
Yes
Yes
Write time stamp to
TDES2 & TDES3
for previous frame
(AHB)
error?
(AHB)
error?
Yes
Time stamp
present?
Yes
Write status word to
prev. frame
’
s TDES0
TxDMA suspended
Yes
No
Yes
No
No
Poll
demand
No
No
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