21.4.3 Always-enabled DMA sourcesIn addition to the peripherals that can be used as DMA sources, there are four additionalDMA sources that are always enabled. Unlike the peripheral DMA sources, where theperipheral controls the flow of data during DMA transfers, the sources that are alwaysenabled provide no such "throttling" of the data transfers. These sources are most usefulin the following cases:• Performing DMA transfers to/from GPIO—Moving data from/to one or more GPIOpins, either unthrottled (that is, as fast as possible), or periodically (using the DMAtriggering capability).• Performing DMA transfers from memory to memory—Moving data from memory tomemory, typically as fast as possible, sometimes with software activation.• Performing DMA transfers from memory to the external bus, or vice-versa—Similarto memory to memory transfers, this is typically done as quickly as possible.• Any DMA transfer that requires software activation—Any DMA transfer that shouldbe explicitly started by software.In cases where software should initiate the start of a DMA transfer, an always-enabledDMA source can be used to provide maximum flexibility. When activating a DMAchannel via software, subsequent executions of the minor loop require that a new startevent be sent. This can either be a new software activation, or a transfer request from theDMA channel MUX. The options for doing this are:• Transfer all data in a single minor loop.By configuring the DMA to transfer all of the data in a single minor loop (that is,major loop counter = 1), no reactivation of the channel is necessary. Thedisadvantage to this option is the reduced granularity in determining the load that theDMA transfer will impose on the system. For this option, the DMA channel must bedisabled in the DMA channel MUX.• Use explicit software reactivation.In this option, the DMA is configured to transfer the data using both minor and majorloops, but the processor is required to reactivate the channel by writing to the DMAregisters after every minor loop. For this option, the DMA channel must be disabledin the DMA channel MUX.• Use an always-enabled DMA source.Functional descriptionK22F Sub-Family Reference Manual, Rev. 4, 08/2016420 NXP Semiconductors
