25.7.3.8 Start Trigger Source

The DMA provides three types of start triggers which can initiate a cell transfer, a software trigger, an event system trigger, and a peripheral DMA request.

The CHCTRLAk.SWFRC bit provides the software trigger. When software or the linked-list descriptor sets CHCTRLAk.SWFRC, the DMA issues a start trigger to the channel.

The device event system provides an event trigger for each DMA channel k where k < 24 (24 start event triggers). Setting CHCTRLBk.TRIG=1 configures the channel to use the rising edge of the event trigger as the start trigger.

A channel is also capable of using a peripheral DMA request as the start trigger. Setting CHCTRLBk.TRIG to a value greater than one will select one of the connected peripheral’s DMA request as the start trigger.

For a channel to take action on a start trigger, the CHCTRLAk.ENABLE bit must be set to one, otherwise all trigger events are ignored as shown in Block Transfer.

If a peripheral DMA request is pending when the channel is enabled, the channel will honor the request and initiate a cell transfer. This is shown in the following figure.

Start trigger events that occur during a cell transfer, CHSTATk.CELLBUSY=1, are ignored as shown by the second ignored trigger in Block Transfer.