the frames that were flushed and then clears the TX FIFO Flush control (FTF) bit in the
EMACDMAOPMODE register. At this point, the TX/RX Controller starts accepting new frames from
the DMA.
Transmit Status Word
At the end of the transfer of the Ethernet frame to the MAC and after the MAC completes the
transmission of the frame, the TX/RX delivers a transmit status word (TDES0) to the application. If
IEEE timestamping is enabled, the TX/RX Controller returns the specific frame's 64-bit timestamp,
along with the transmit status word. The fields for the Transmit Descriptors are described in
“Enhanced and Alternate Descriptors” on page 1413.
20.3.3.2 Receive (RX) Control Path
TX/RX Controller receives frames from the MAC and pushes them into the RX FIFO. When the fill
level of the RX FIFO crosses the programmed RX Threshold, the DMA is notified.
Receive Operation
During a receive operation the TX/RX Controller is a slave to the MAC. The steps of the receive
operation are as follows:
1. The MAC receives a frame. This data, along with SOF, EOF and byte enable information is sent
to the TX/RX Controller. The TX/RX Controller accepts the data and pushes it into the RX FIFO.
After the EOF is transferred, the MAC drives the status word, which is also pushed in to the RX
FIFO.
2. When timestamp is enabled by setting the TSEN bit in the Ethernet MAC Timestamp Control
(EMACTIMSTCTRL) register, at offset 0x700, and the 64-bit timestamp is present with the
receive status, it is appended to the frame and received by the MAC and pushed into the TX
FIFO before the corresponding receive status word is written. Thus, two additional locations
per frame are taken for storing timestamp in the RX FIFO.
3. Data can be sent to the TX/RX Controller in cut-through mode or store-and-forward mode. When
the RTC bit field of the EMACDMAOPMODE register is set to 0x0 and cut-through mode is
enabled (RSF=0), the TX/RX Controller indicates availability to transfer to the DMA when 64
bytes are in the RX FIFO or a full packet of data has been received into the RX FIFO. When
the DMA initiates transfers to system memory, the TX/RX Controller continues to transfer data
from the RX FIFO until a complete packet has been transferred. When EOF has occurred, the
TX/RX Controller sends the status word to the DMA.
Note: The timestamp transfer takes two clock cycles and the lower 32-bits of the timestamp
are sent first when timestamping is enabled.
When the RSF bit is set in the EMACDMAOPMODE register, RX FIFO store-and-forward mode
is enabled and a frame is read by the DMA only after it is completely written into the RX FIFO.
In this mode, only valid frames are read and forwarded to the application. In cut-through mode,
some error frames are not dropped because the error status is received at the end of the frame
and by that time the start of that frame has already been read out of the RX FIFO.
The TX/RX Controller is capable of storing any number of frames in the RX FIFO as long as it is
not full.
June 18, 20141436
Texas Instruments-Production Data
Ethernet Controller
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