for the second frame. If the second frame is valid, the transmit process transfers this frame before
writing the first frame's status information.
In Operate on Second Frame (OSF) mode, the RUN state TX DMA operates in the following
sequence:
1.
The DMA operates as described in Steps 1 to 6 (see page 1424).
2.
Without closing the previous frame's last descriptor, the DMA fetches the next descriptor.
3.
If the DMA owns the acquired descriptor, the DMA decodes the transmit buffer address in this
descriptor. If the DMA does not own the descriptor, the DMA goes into SUSPEND mode and
skips to 7.
4.
The DMA fetches the Transmit frame from system memory and transfers the frame until the
end-of-frame data is reached. It closes the intermediate descriptors if this frame is split across
multiple descriptors.
5.
The DMA waits for the previous frame's transmission status and timestamp. When the status
is available, the DMA writes the timestamp to TDES6 and TDES7, if such timestamp was
captured (as indicated by a status bit). The DMA then writes the status, with a cleared OWN
bit, to the corresponding TDES0, thus closing the descriptor. If timestamping was not enabled
for the previous frame, the DMA does not alter the contents of TDES6 and TDES7.
6.
If enabled, the Transmit interrupt (
TI
) bit is set in the
EMACDMARIS
register, the DMA fetches
the next descriptor and then proceeds to 3 (when Status is normal). If the previous transmission
status shows an underflow error, the DMA goes into SUSPEND mode
7.
In SUSPEND mode, if a pending status and timestamp are received, the DMA writes the
timestamp (if enabled for the current frame) to TDES6 and TDES7, then writes the status to the
corresponding TDES0. It then sets relevant interrupts and returns to SUSPEND mode.
8.
The DMA can exit SUSPEND mode and enter the RUN state only after receiving the Transmit
Poll demand in the
EMACTXPOLLD
register.
Note:
The DMA fetches the next descriptor in advance before closing the current descriptor.
Therefore, the descriptor chain should have more than two different descriptors for correct
and proper operation.
Figure 20-6 on page 1428 shows the flow for the TX DMA Operate-On-Second-Frame (OSF) operation.
1427
June 18, 2014
Texas Instruments-Production Data
Tiva
™
TM4C1294NCPDT Microcontroller