CHAPTER 10: DMAC
FM4 Peripheral Manual, Doc. No. 002-04856 Rev. *E 467
3.5 Channel Priority Control
This section explains the channel priority control.
Channel Priority Control
If multiple channels have transfer requests, DMAC switches the channel subject to the transfer among
them at the timing of the Transfer Gap of each channel. At this point, the next channel to which the
transfer will be performed is determined according to the priority control. The priority control can be
selected from either fixed priority or rotated priority by the PR. Figure 3-6 shows an explanatory diagram.
In this figure, the X axis indicates the time axis. The arrows indicate transfer timings of each channel to
perform its transfer operation when all of the channels issue transfer requests simultaneously.
Operation in Fixed Priority Mode (PR=0)
In fixed priority mode, the channel with the smallest channel number among all the channels with a
transfer request has the priority to perform transfer operation.
(Priority order: ch.0 > ch.1 > ch.2 > ch.3 > ch.4 > ch.5 > ch.6 > ch.7)
First, the channel with the highest priority performs its transfer (ch.0 in Figure 3-6). As the channel with
the highest priority halts the transfer operation at the timing of a Transfer Gap, then, the channel with the
second highest priority performs its transfer operation (ch.1 in Figure 3-6). For this reason, the channels
with the highest and the second highest priority perform the transfer operations alternately. After that,
when the channel with higher priority completes its transfer, the channel with lower priority starts its
transfer operation (ch.3 in Figure 3-6).
Operation in Rotated Priority Mode (PR=1)
In rotate priority mode, all channels perform their transfer operations equally.
Figure 3-6 Explanatory Diagram of Channel Priority Control
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