SH7243 datasheet(456/1726 Pages) RENESAS

456 / 1726 page

Section 10 Direct Memory Access Controller (DMAC)

Rev. 1.00 Jun. 26, 2008 Page 426 of 1692

REJ09B0393-0100

(4)

Bus Mode and Channel Priority

In priority fixed mode (CH0 > CH1), when channel 1 is transferring data in burst mode and a

request arrives for transfer on channel 0, which has higher-priority, the data transfer on channel 0

will begin immediately. In this case, if the transfer on channel 0 is also in burst mode, the transfer

on channel 1 will only resume on completion of the transfer on channel 0.

When channel 0 is in cycle steal mode, one transfer-unit of data on this channel, which has the

higher priority, is transferred. Data is then transferred continuously to channel 1 without releasing

the bus. The bus mastership will then switch between the two in this order: channel 0, channel 1,

channel 0, channel 1, etc. That is, the CPU cycle after the data transfer in cycle steal mode is

replaced with a burst-mode transfer cycle (priority execution of burst-mode cycle). An example of

this is shown in figure 10.12.

When multiple channels are in burst mode, data transfer on the channel that has the highest

priority is given precedence. When DMA transfer is being performed on multiple channels, the

bus mastership is not released to another bus-master device until all of the competing burst-mode

transfers have been completed.

CPU

DMA

CH1

DMA

CH1

DMA

CH0

DMA

CH1

DMA

CH0

DMA

CH1

DMA

CH1

CPU

CH0

CH1

CH0

DMAC CH0 and CH1

Cycle steal mode

DMAC CH1

Burst mode

CPU

Priority: CH0 > CH1

CH0: Cycle steal mode

CH1: Burst mode

DMAC CH1

Burst mode

Figure 10.12 Bus State when Multiple Channels are Operating

In round-robin mode, the priority changes as shown in figure 10.3. Note that channels in cycle

steal and burst modes must not be mixed.

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