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7.4.9
Multiple-Channel Operation
The DMAC channel priority order is: channel 0 > channel 1 and channel A > channel B.
Table 7.12 shows the complete priority order.
Table 7.12 Channel Priority Order
Short Address Mode
Channel 0A
Channel 0B
Channel 1A
Channel 1B
If transfers are requested on two or more channels simultaneously, or if a transfer on one channel
is requested during a transfer on another channel, the DMAC operates as follows.
• When a transfer is requested, the DMAC requests the bus right. When it gets the bus right, it
starts a transfer on the highest-priority channel at that time.
• Once a transfer starts on one channel, requests to other channels are held pending until that
channel releases the bus.
• After each transfer in short address mode, and each externally-requested or cycle-steal transfer
in normal mode, the DMAC releases the bus and returns to step 1. After releasing the bus, if
there is a transfer request for another channel, the DMAC requests the bus again.
• After completion of a burst-mode transfer, or after transfer of one block in block transfer
mode, the DMAC releases the bus and returns to step 1. If there is a transfer request for a
higher-priority channel or a bus request from a higher-priority bus master, however, the
DMAC releases the bus after completing the transfer of the current byte or word. After
releasing the bus, if there is a transfer request for another channel, the DMAC requests the bus
again.
Figure 7.19 shows the timing when channel 0A is set up for I/O mode and channel 1 for burst
mode, and a transfer request for channel 0A is received while channel 1 is active.
230
Full Address Mode
Channel 0
Channel 1
Priority
High
Low
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