NXP Semiconductors MPC5644A, Reference Manual, Page: 190 / 1735

Enhanced Direct Memory Access Controller (eDMA)
MPC5644A Microcontroller Reference Manual, Rev. 6
190 Freescale Semiconductor
8.5.8.2 Dynamic scatter/gather
Dynamic scatter/gather is the process of setting the TCD.e_sg bit during channel execution. This bit is read
from the TCD local memory at the end of channel execution, thus allowing the user to enable the feature
during channel execution.
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.
8.5.8.2.1 Method 1 (channel not using major loop channel linking)
For a channel not using major loop channel linking, the coherency model in Table 8-26 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).
Table 8-26. Coherency model for method 1
Step Action
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 theTCD.d_req bit.
Note: Should a dynamic scatter/gather attempt fail, setting the d_req bit will prevent a future
hardware activation of this channel. This stops the channel from executing with a
destination address (daddr) that was calculated using a scatter/gather address
(written in the next step) instead of a dlast final offest value.
3 Write theTCD.dlast_sga field with the scatter/gather address.
4 Write 1b to the TCD.e_sg bit.