NXP Semiconductors K22F series, Reference Manual, Page: 420 / 1407

21.4.3 Always-enabled DMA sources
In addition to the peripherals that can be used as DMA sources, there are four additional
DMA sources that are always enabled. Unlike the peripheral DMA sources, where the
peripheral controls the flow of data during DMA transfers, the sources that are always
enabled provide no such "throttling" of the data transfers. These sources are most useful
in the following cases:
• Performing DMA transfers to/from GPIO—Moving data from/to one or more GPIO
pins, either unthrottled (that is, as fast as possible), or periodically (using the DMA
triggering capability).
• Performing DMA transfers from memory to memory—Moving data from memory to
memory, typically as fast as possible, sometimes with software activation.
• Performing DMA transfers from memory to the external bus, or vice-versa—Similar
to memory to memory transfers, this is typically done as quickly as possible.
• Any DMA transfer that requires software activation—Any DMA transfer that should
be explicitly started by software.
In cases where software should initiate the start of a DMA transfer, an always-enabled
DMA source can be used to provide maximum flexibility. When activating a DMA
channel via software, subsequent executions of the minor loop require that a new start
event be sent. This can either be a new software activation, or a transfer request from the
DMA 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. The
disadvantage to this option is the reduced granularity in determining the load that the
DMA transfer will impose on the system. For this option, the DMA channel must be
disabled in the DMA channel MUX.
• Use explicit software reactivation.
In this option, the DMA is configured to transfer the data using both minor and major
loops, but the processor is required to reactivate the channel by writing to the DMA
registers after every minor loop. For this option, the DMA channel must be disabled
in the DMA channel MUX.
• Use an always-enabled DMA source.
Functional description
K22F Sub-Family Reference Manual, Rev. 4, 08/2016
420 NXP Semiconductors