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7721 Group User’s Manual

13-21

DMA CONTROLLER

13.3 Control

13.3.3 Channel priority levels

When the DMA enable bits of several channels are “1” and their DMA request bits are set to “1,” the

request of the channel with the highest priority is accepted first.

The fixed or rotating channel priority can be selected by the priority select bit (bit 0 at address 6816). The

priority levels themselves cannot be specified arbitrary.

The channel priority levels are determined after the DMA requests are determined.

(1)

Fixed priority

The fixed priority is selected when the priority select bit (bit 0 at address 6816) = “0.”

In the fixed priority, the channel priority levels are as follows:

channel 0 > channel 1 > channel 2 > channel 3.

(2)

Rotating priority

The rotating priority is selected when the priority select bit = “1.”

After reset, the priority levels are the same descending order as in the fixed priority:

channel 0 > channel 1 > channel 2 > channel 3.

Then, after every normal termination of a DMA transfer, the priority levels rotate in such a way that

the lowest priority is given to the channel having been performed.

When DMA transfer is forced into termination, the channel priority levels does not rotate.

Figure 13.3.1 shows an example of determining the channel priority levels.

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