SH7021 datasheet(206/509 Pages) RENESAS

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External-Pin Round-Robin Mode: External-pin round-robin mode switches the priority levels of

channel 0 and channel 1, which are the channels that can receive transfer requests from external

pins

DREQ0 and DREQ1. The priority levels are changed after each (byte or word) transfer on

channel 0 or channel 1 is completed. The channel which just finished the transfer rotates to the

bottom of the priority order. The priority levels of channels 2 and 3 do not change. The initial

priority order after a reset is channel 3 > channel 2 > channel 1 > channel 0.

Figure 9.5 shows how the priority order changes when channel 0 and channel 1 transfers are

requested simultaneously and a channel 0 transfer is requested again after both channels finish

their transfers. The DMAC operates as follows:

1. Transfer requests are generated simultaneously to channels 1 and 0.

2. Channel 1 has a higher priority, so the channel 1 transfer begins first (channel 0 waits for

transfer).

3. When the channel 1 transfer ends, channel 1 becomes lowest priority.

4. The channel 0 transfer begins.

5. When the channel 0 transfer ends, channel 0 becomes lowest priority.

6. A channel 0 transfer request occurs again.

7. The channel 0 transfer begins.

8. When the channel 0 transfer ends, the priority order does not change, because channel 0 is

already the lowest priority.

SH7021 Datasheet(PDF) 206 Page - Renesas Technology Corp