Freescale Semiconductor PXR4030, Reference Manual

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Enhanced Direct Memory Access Controller (eDMA)
21-58 Freescale Semiconductor
PXR40 Microcontroller Reference Manual, Rev. 1
the major loop. Two EDMA _x _TCD.ACTIVE bits set at the same time in the overall TCD map indicates
a higher priority channel is actively preempting a lower priority channel.
21.5.7 Channel Linking
Channel linking (or chaining) is a mechanism in which one channel sets the EDMA _x _TCD.START bit
of another channel (or itself), thus initiating a service request for that channel. This operation is
automatically performed by the eDMA engine at the conclusion of the major or minor loop when properly
enabled.
The minor loop channel linking occurs at the completion of the minor loop (or one iteration of the major
loop). The EDMA
_x _TCD.CITER.E_LINK field are used to determine whether a minor loop link is
requested. When enabled, the channel link is made after each iteration of the minor loop except for the last.
When the major loop is exhausted, only the major loop channel link fields are used to determine if a
channel link should be made. For example, with the initial fields of:
EDMA _x _TCD.CITER.E_LINK = 1
EDMA _x _TCD.CITER.LINKCH = 0xC
EDMA _x _TCD.CITER value = 0x4
EDMA _x _TCD.MAJOR.E_LINK = 1
EDMA _x _TCD.MAJOR.LINKCH = 0x7
will execute as:
1. Minor loop done

set channel 12 EDMA _x _TCD.START bit
2. Minor loop done

set channel 12 EDMA _x _TCD.START bit
3. Minor loop done

set channel 12 EDMA _x _TCD.START bit
4. Minor loop done, major loop done

set channel 7 EDMA _x _TCD.START bit
When minor loop linking is enabled (EDMA _x _TCD.CITER.E_LINK = 1), the EDMA _x _TCD.CITER
field uses a nine bit vector to form the current iteration count.
When minor loop linking is disabled (EDMA _x _TCD.CITER.E_LINK = 0), the EDMA _x _TCD.CITER
field uses a 15-bit vector to form the current iteration count. The bits associated with the
EDMA
_x _TCD.CITER.LINKCH field are concatenated onto the CITER value to increase the range of the
CITER.
NOTE
After configuration, the EDMA _x _TCD.CITER.E_LINK bit and the
EDMA _x _TCD.BITER.E_LINK bit must be equal or a configuration error
is reported. The CITER and BITER vector widths must be equal to calculate
the major loop, halfway done interrupt point.
Table 21-26 summarizes how a DMA channel can link to another DMA channel, i.e, use another channel’s
TCD, at the end of a loop.