Datasheet
20.6.2.6 Transfer Triggers and Actions
A DMA transfer through a DMA channel can be started only when a DMA transfer request is detected,
and the DMA channel has been granted access to the DMA. A transfer request can be triggered from
software, from a peripheral, or from an event. There are dedicated Trigger Source selections for each
DMA Channel Control B (CHCTRLB.TRIGSRC).
The trigger actions are available in the Trigger Action bit group in the Channel Control B register
(CHCTRLB.TRIGACT). By default, a trigger generates a request for a block transfer operation. If a single
descriptor is defined for a channel, the channel is automatically disabled when a block transfer has been
completed. If a list of linked descriptors is defined for a channel, the channel is automatically disabled
when the last descriptor in the list is executed. If the list still has descriptors to execute, the channel will
be waiting for the next block transfer trigger. When enabled again, the channel will wait for the next block
transfer trigger. The trigger actions can also be configured to generate a request for a beat transfer
(CHCTRLB.TRIGACT=0x2) or transaction transfer (CHCTRLB.TRIGACT=0x3) instead of a block transfer
(CHCTRLB.TRIGACT=0x0).
Figure 20-7 shows an example where triggers are used with two linked block descriptors.
Figure 20-7. Trigger Action and Transfers
CHENn
Trigger
PENDCHn
BUSYCHn
Data Transfer
CHENn
Trigger
PENDCHn
BUSYCHn
Data Transfer
CHENn
Trigger
PENDCHn
BUSYCHn
Data Transfer
Block Transfer
Block Transfer
Block Transfer
Block Transfer
Block Transfer
Block Transfer
Trigger Lost
Trigger Lost
Trigger Lost
Transaction Trigger Action
Block Trigger Action
Beat Trigger Action
BEAT
BEAT
BEAT
BEAT
BEATBEAT
BEAT
BEAT
BEAT
BEAT
BEATBEAT
BEAT
BEAT
BEAT
BEAT
BEATBEAT
If the trigger source generates a transfer request for a channel during an ongoing transfer, the new
transfer request will be kept pending (CHSTATUS.PEND=1), and the new transfer can start after the
SAM D21 Family
DMAC – Direct Memory Access Controller
© 2018 Microchip Technology Inc.
Datasheet Complete
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