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Chapter 26 DMA Controller
3.DMA Controller (DMAC) Operation
3.11 Channel Selection and Control
Up to five channels can be simultaneously set as transfer channels. In general, an independent
function can be set for each channel.
■ Priority Among Channels
Since DMA transfer is possible only on one channel at a time, priority must be set for the channels.
Two modes, fixed and rotation, are provided as the priority settings and can be selected for each channel group
(described later).
● Fixed mode
The order of priority is fixed by channel number, with priority decreasing from channel 0 to channel 4:
(ch.0 > ch.1 > ch.2 > ch.3 > ch.4)
If a transfer request with a higher priority is received during a transfer, the transfer channel becomes the channel
with the higher priority when the transfer for the transfer unit (number set in the block size specification register x
data width) ends.
When higher priority transfer is completed, transfer is restarted on the previous channel.
Figure 3-5 Timing Example in Fixed Mode
● Rotation mode (ch.0 to ch.1 only)
When operation is enabled, the initial states have the same order that they would have in fixed mode, but at the
end of each transfer operation, the priority of the channels is reversed. Thus, if more than one transfer request is
output at the same time, the channel is switched after each transfer unit.
This mode is effective when continuous or burst transfer is set.
Figure 3-6 Timing Example in Rotation Mode
CPU SA DA SA DA SA DA SA DA CPU
ch1 ch0 ch0 ch1
ch0 transfer request
ch1 transfer request
Bus operation
Transfer ch
ch0 transfer end
ch1 transfer end
CPU SA DA SA DA SA DA SA DA CPU
ch1 ch0 ch1 ch0
ch0 transfer request
ch1 transfer request
Bus operation
Transfer ch
ch0 transfer end
ch1 transfer end