NXP Semiconductors MPC5566, Reference Manual

Page: 413 / 1268

Enhanced Direct Memory Access (eDMA)
MPC5566 Microcontroller Reference Manual, Rev. 2
9-52 Freescale Semiconductor
9.4.8 Dynamic Programming
This section provides recommended methods to change the programming model during channel execution.
9.4.8.1 Dynamic Channel Linking and Dynamic Scatter/Gather
Dynamic channel linking and dynamic scatter/gather is the process of changing the
TCD.MAJOR.E_LINK or TCD.E_SG bits during channel execution. These bits are read from the TCD
local memory at the
end of channel execution thus allowing the user to enable either 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 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 is set in the programmer’s model, but it is unclear whether the link completed
before the channel retired.
Use the following coherency model when executing a dynamic channel link or dynamic scatter/gather
request:
1. Set the TCD.MAJOR.E_LINK bit
2. Read the TCD.MAJOR.E_LINK bit
3. Test the TCD.MAJOR.E_LINK request status:
a) If the bit is set, the dynamic link attempt was successful.
b) If the bit is cleared, the channel had already retired before the dynamic link completed.
This same coherency model is true for dynamic scatter/gather operations. For both dynamic 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 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 or TCD.E_SG bits. The TCD.DONE bit is cleared
automatically by the eDMA engine after a channel begins execution.