Texas Instruments SimpleLink CC2620 Technical Reference Manual page 1047

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12.3.6.5 Memory Scatter-gather Mode
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 memory scatter-gather mode. Each entry
in the table is, in turn, copied 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. When 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.
Figure 12-3 shows 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.
shows how the application sets up a μDMA task list in memory, that is then used by the controller to
perform three sets of copy operations from different locations in memory. The primary control structure for
the channel used for the operation is configured to copy from the task list to the alternate control structure.
Figure 12-4 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.
The μDMA controller 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.
SWCU117C – February 2015 – Revised September 2015
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Functional Description
Figure 12-3
Micro Direct Memory Access (µDMA)
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