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Chapter 16 Enhanced Direct Memory Access (eDMA)
MPC5606BK Microcontroller Reference Manual, Rev. 2
Freescale Semiconductor
293
For this request, the TCD local memory controller forces the TCD.major.e_link bit to zero on any writes
to a channel’s TCD.word7 after 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
bit. The TCD.done bit is cleared automatically by the eDMA engine after a
channel begins execution.
16.5.8.2
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.
16.5.8.2.1
Method 1 (channel not using major loop channel linking)
For a channel not using major loop channel linking, the coherency model in
a dynamic scatter/gather request.
Table 16-24. Coherency model for a dynamic channel link request
Step
Action
1
Write 1b to the TCD.major.e_link bit.
2
Read back the TCD.major.e_link bit.
3
Test the TCD.major.e_link request status:
• If TCD.major.e_link = 1b, the dynamic link attempt was successful.
• If TCD.major.e_link = 0b, the attempted dynamic link did not succeed (the channel was
already retiring).
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