22.5.7.3 Dynamic scatter/gather
Scatter/gather is the process of automatically loading a new TCD into a channel. It allows
a DMA channel to use multiple TCDs; this enables a DMA channel to scatter the DMA
data to multiple destinations or gather it from multiple sources.When scatter/gather is
enabled and the channel has finished its major loop, a new TCD is fetched from system
memory and loaded into that channel’s descriptor location in eDMA programmer’s
model, thus replacing the current descriptor.
Because the user is allowed to change the configuration during execution, a coherency
model is needed. Consider the scenario where the user attempts to execute a dynamic
scatter/gather operation by enabling the TCD.e_sg bit at the same time the eDMA engine
is retiring the channel. The TCD.e_sg would be set in the programmer’s model, but it
would be unclear whether the actual scatter/gather request was honored before the
channel retired.
Two methods for this coherency model are shown in the following subsections. Method 1
has the advantage of reading the major.linkch field and the e_sg bit with a single read.
For both dynamic channel linking and scatter/gather requests, the TCD local memory
controller forces the TCD.major.e_link and TCD.e_sg bits to zero on any writes to a
channel’s TCD.word7 if that channel’s TCD.done bit is set indicating the major loop is
complete.
NOTE
The user must clear the TCD.done bit before writing the
TCD.major.e_link or TCD.e_sg bits. The TCD.done bit is
cleared automatically by the eDMA engine after a channel
begins execution.
22.5.7.3.1
Method 1 (channel not using major loop channel linking)
For a channel not using major loop channel linking, the coherency model described here
may be used for a dynamic scatter/gather request.
When the TCD.major.e_link bit is zero, the TCD.major.linkch field is not used by the
eDMA. In this case, the TCD.major.linkch bits may be used for other purposes. This
method uses the TCD.major.linkch field as a TCD indentification (ID).
1. When the descriptors are built, write a unique TCD ID in the TCD.major.linkch field
for each TCD associated with a channel using dynamic scatter/gather.
2. Write 1b to the TCD.d_req bit.
Chapter 22 Enhanced Direct Memory Access (eDMA)
K22F Sub-Family Reference Manual, Rev. 4, 08/2016
NXP Semiconductors 505
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