Ethernet (ETH): media access control (MAC) with DMA controller
RM0090
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DocID018909 Rev 11
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TDES7: Transmit descriptor Word7
33.6.8
Rx DMA configuration
The Receive DMA engine’s reception sequence is illustrated in
and described
below:
1.
The CPU sets up Receive descriptors (RDES0-RDES3) and sets the OWN bit
(RDES0[31]).
2. Once the SR (ETH_DMAOMR register[1]) bit is set, the DMA enters the Run state.
While in the Run state, the DMA polls the receive descriptor list, attempting to acquire
free descriptors. If the fetched descriptor is not free (is owned by the CPU), the DMA
enters the Suspend state and jumps to Step 9.
3. The DMA decodes the receive data buffer address from the acquired descriptors.
4. Incoming frames are processed and placed in the acquired descriptor’s data buffers.
5. When the buffer is full or the frame transfer is complete, the Receive engine fetches the
next descriptor.
6. If the current frame transfer is complete, the DMA proceeds to step 7. If the DMA does
not own the next fetched descriptor and the frame transfer is not complete (EOF is not
yet transferred), the DMA sets the Descriptor error bit in RDES0 (unless flushing is
disabled). The DMA closes the current descriptor (clears the OWN bit) and marks it as
intermediate by clearing the Last segment (LS) bit in the RDES1 value (marks it as last
descriptor if flushing is not disabled), then proceeds to step 8. If the DMA owns the next
descriptor but the current frame transfer is not complete, the DMA closes the current
descriptor as intermediate and returns to step 4.
7. If IEEE 1588 time stamping is enabled, the DMA writes the time stamp (if available) to
the current descriptor’s RDES2 and RDES3. It then takes the received frame’s status
and writes the status word to the current descriptor’s RDES0, with the OWN bit cleared
and the Last segment bit set.
8. The Receive engine checks the latest descriptor’s OWN bit. If the CPU owns the
descriptor (OWN bit is at 0) the Receive buffer unavailable bit (in ETH_DMASR
register[7]) is set and the DMA Receive engine enters the Suspended state (step 9). If
the DMA owns the descriptor, the engine returns to step 4 and awaits the next frame.
9. Before the Receive engine enters the Suspend state, partial frames are flushed from
the Receive FIFO (you can control flushing using bit 24 in the ETH_DMAOMR
register).
10. The Receive DMA exits the Suspend state when a Receive Poll demand is given or the
start of next frame is available from the Receive FIFO. The engine proceeds to step 2
and re-fetches the next descriptor.
31 30 29 28 27 26 25
24
23 22 21 20 19 18 17 16 15 14 13 12 11 10
9
8
7
6
5
4
3
2
1
0
TTSH
rw
Bits 31:0
TTSH:
Transmit frame time stamp high
This field is updated by DMA with the 32 most significant bits of the time stamp captured for
the corresponding transmit frame. This field has the time stamp only if the Last segment control
bit (LS) in the descriptor is set.
