LM3S2678 Datasheet, PDF (268/730 Pages) Texas Instruments – Stellaris® LM3S2678 Microcontroller

Micro Direct Memory Access (μDMA)
In Memory Scatter-Gather mode, the primary control structure is used to program the alternate
control structure from a table in memory. The table is set up by the processor software and contains
a list of control structures, each containing the source and destination end pointers, and the control
word for a specific transfer. The mode of each control word must be set to Scatter-Gather mode.
Each entry in the table is copied in turn to the alternate structure where it is then executed. The
μDMA controller alternates between using the primary control structure to copy the next transfer
instruction from the list, and then executing the new transfer instruction. The end of the list is marked
by setting the control word for the last entry to use Basic transfer mode. Once the last transfer is
performed using Basic mode, the μDMA controller will stop. A completion interrupt will only be
generated after the last transfer. It is possible to loop the list by having the last entry copy the primary
control structure to point back to the beginning of the list (or to a new list). It is also possible to trigger
a set of other channels to perform a transfer, either directly by programming a write to the software
trigger for another channel, or indirectly by causing a peripheral action that will result in a μDMA
request.
By programming the μDMA controller using this method, a set of arbitrary transfers can be performed
based on a single DMA request.
Refer to Figure 7-3 on page 269 and Figure 7-4 on page 270, which show an example of operation
in Memory Scatter-Gather mode. This example shows a gather operation, where data in three
separate buffers in memory will be copied together into one buffer. Figure 7-3 on page 269 shows
how the application sets up a μDMA task list in memory that is used by the controller to perform
three sets of copy operations from different locations in memory. The primary control structure for
the channel that will be used for the operation is configured to copy from the task list to the alternate
control structure.
Figure 7-4 on page 270 shows the sequence as the μDMA controller peforms the three sets of copy
operations. First, using the primary control structure, the μDMA controller loads the alternate control
structure with task A. It then peforms the copy operation specified by task A, copying the data from
the source buffer A to the destination buffer. Next, the μDMA controller again uses the primary
control structure to load task B into the alternate control structure, and then performs the B operation
with the alternate control structure. The process is repeated for task C.
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November 17, 2011
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