LM3S5D56 Datasheet, PDF (364/1146 Pages) Texas Instruments – Stellaris® LM3S5D56 Microcontroller

Micro Direct Memory Access (μDMA)
8.2.6.5
Memory Scatter-Gather
Memory Scatter-Gather mode is a complex mode used when data must be transferred to or from
varied locations in memory instead of a set of contiguous locations in a memory buffer. For example,
a gather μDMA operation could be used to selectively read the payload of several stored packets
of a communication protocol and store them together in sequence in a memory buffer.
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 programming the control word for the last entry to use Auto transfer mode. Once the last transfer
is performed using Auto mode, the μDMA controller stops. A completion interrupt is generated only
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 results in a μDMA request.
By programming the μDMA controller using this method, a set of arbitrary transfers can be performed
based on a single μDMA request.
Refer to Figure 8-3 on page 365 and Figure 8-4 on page 366, 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 is copied together into one buffer. Figure 8-3 on page 365 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 is used for the operation is configured to copy from the task list to the alternate control
structure.
Figure 8-4 on page 366 shows the sequence as the μDMA controller performs 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 performs 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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January 23, 2012
Texas Instruments-Production Data