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On the other hand, if channel 0 is operating in cycle-steal mode, channel 1 will begin operating
again after channel 0 completes the transfer of one transfer unit, and without the internal bus being
released. Transfer will then alternate between the two channels in the order channel 0, channel 1,
channel 0, channel 1, and so on. In other words, the CPU cycles following cycle-steal mode
transfer are replaced by bust mode transfer. Figure 8.12 shows an example of this. If multiple
channels are competing to execute burst mode transfers, the channel with the highest priority
performs the burst mode transfer first.
When DMA transfers are performed using multiple channels, the bus is not released to another bus
master until all of the competing burst mode transfers have completed.
CPU
DMA CH1
DMA CH1
DMA CH0
DMA CH1
DMA CH0
DMA CH1
DMA CH1
CPU
CH0
CH1
CH0
Cycle-steal mode between
DMAC CH0 and CH1
DMAC CH1
burst mode
CPU
CPU
Priority: CH0 > CH1
CH0: Cycle-steal mode
CH1: Burst mode
DMAC CH1
burst mode
Figure 8.12 Bus State when Multiple Channels are Operating
Cycle-steal mode channels and burst mode channels should not be mixed in round-robin mode.
Doing so runs the risk that priority changes may not be made properly, although the individual
channel transfer operations will be performed correctly.
8.4.5
Number of Bus Cycle States and DREQ Pin Sampling Timing
Number of Bus Cycle States: When the DMAC is the bus master, the number of bus cycle states
is controlled by the bus state controller (BSC) in the same way as when the CPU is the bus master.
For details, see section 7, Bus State Controller (BSC).
DREQ Pin Sampling Timing:
CKIO
Bus cycle
DREQ
(Rising)
DACK
(Active-high)
CPU
CPU
Acceptance start
2nd acceptance
1st acceptance
CPU
DMAC
Non sensitive period
Figure 8.13 Example of DREQ Input Detection in Cycle Steal Mode Edge Detection
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