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Chapter 26 DMA Controller
3.DMA Controller (DMAC) Operation
Note:
• Since the register has only four bits, this function cannot be used for multiple interrupts exceeding 15 levels.
• Be sure to assign the priority of the DMA tasks at a level that is at least 15 levels higher than other interrupt
levels.
3.8 Hold Arbitration
When a device is operating in external bus extended mode, an external hold function can be
used. The relationship between external hold requests and DMA transfer requests by this mod-
ule when the hold function can be used is described below.
■ DMA Transfer Request during External Hold
DMA transfer is started when an external bus area is accessed, DMA transfer is temporarily stopped. When the
external hold is released, DMA transfer is restarted.
■ External Hold Request During DMA Transfer
The device is externally held. When an external bus area is accessed by DMA transfer, DMA transfer is
temporarily stopped. When the external hold is released, DMA transfer is restarted.
■ Simultaneous Occurrence of a DMA Transfer Request and an External Hold Request
The device is externally held and internal DMA transfer is started. When an external bus area is accessed by
DMA transfer, DMA transfer is temporarily stopped. When the external hold is released, DMA transfer is
restarted.
3.9 Operation from Starting to End/Stopping
Starting of DMA transfer is controlled independently for each channel, but before transfer starts,
the operation of all channels needs to be enabled. This section describes operation from starting
to end/stopping.
■ Operation Start
● Enabling operation for all channels
Before activating each DMAC channel, operation for all channels needs to be enabled in advance with the DMA
operation enable bit (DMAE of DMACR). All start settings and transfer requests that occurred before operation is
enabled are invalid.
● Starting transfer
The transfer operation can be started by the operation enable bit of the control register for each channel. If a
transfer request to an activated channel is accepted, the DMA transfer operation is started in the specified mode.
● Starting from a temporary stop
If a temporary stop occurs before starting with channel-by-channel or all-channel control, the temporary stopped
state is maintained even though the transfer operation is started. If transfer requests occur in the meantime, they
are accepted and retained. When temporary stopping is released, transfer is started.
■ Transfer Request Acceptance and Transfer
Sampling for transfer requests set for each channel starts after starting.