Channel Priorities - Hitachi SH7751 Hardware Manual

14.3.3

Channel Priorities

If the DMAC receives simultaneous transfer requests on two or more channels, it selects a channel
according to a predetermined priority system, either in a fixed mode or round robin mode. The
mode is selected with priority bits PR1 and PR0 in the DMA operation register (DMAOR).
Fixed Mode: In this mode, the relative channel priorities remain fixed. The following priority
orders are available in fixed mode:


CH0 > CH1 > CH2 > CH3


CH0 > CH2 > CH3 > CH1


CH2 > CH0 > CH1 > CH3
The priority order is selected with bits PR1 and PR0 in DMAOR.
Round Robin Mode: In round robin mode, each time the transfer of one transfer unit (byte, word,
longword, quadword, or 32 bytes) ends on a given channel, that channel is assigned the lowest
priority level. This is illustrated in figure 14.3. The order of priority in round robin mode
immediately after a reset is CH0 > CH1 > CH2 > CH3.
Note: In round robin mode, if no transfer request is accepted for any channel during DMA
transfer, the priority order becomes CH0 > CH1 > CH2 > CH3.
Rev. 3.0, 04/02, page 490 of 1064