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
channel link by enabling the TCD.major.e_link bit at the same time the eDMA engine is
retiring the channel. The TCD.major.e_link would be set in the programmer’s model, but
it would be unclear whether the actual link was made before the channel retired.
The following coherency model is recommended when executing a dynamic channel link
request.
1. Write 1 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 = 1, the dynamic link attempt was successful.
• If TCD.major.e_link = 0, the attempted dynamic link did not succeed (the
channel was already retiring).
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.
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.
Chapter 22 Enhanced Direct Memory Access (eDMA)
K22F Sub-Family Reference Manual , Rev. 3, 7/2014
Freescale Semiconductor, Inc.
497
Содержание MK22FN256VDC12
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