HITACHI 189
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.