MAC7200 datasheet(319/976 Pages) FREESCALE

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The SPP DMA Controller Module (SPP_DMA2)

MAC7200 Microcontroller Family Reference Manual, Rev. 2

Freescale Semiconductor

241

12.2.6.7.2

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 dma_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 or dynamic

scatter/gather request:

1. set the TCD.major.e_link bit,

2. read back 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 attempted dynamic link did not succeed, the channel was already

retiring.

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.word7 once 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 dma_engine once a channel begins execution.

MAC7200 Datasheet(PDF) 319 Page - Freescale Semiconductor, Inc