After the DMA request has been serviced, the peripheral will negate its request,
effectively resetting the gating mechanism until the peripheral reasserts its request and
the next trigger event is seen. This means that if a trigger is seen, but the peripheral is not
requesting a transfer, then that trigger will be ignored. This situation is illustrated in the
following figure.
DMA request
Peripheral request
Trigger
Figure 21-4. DMAMUX channel triggering: ignored trigger
This triggering capability may be used with any peripheral that supports DMA transfers,
and is most useful for two types of situations:
• Periodically polling external devices on a particular bus
As an example, the transmit side of an SPI is assigned to a DMA channel with a
trigger, as described above. After it has been set up, the SPI will request DMA
transfers, presumably from memory, as long as its transmit buffer is empty. By using
a trigger on this channel, the SPI transfers can be automatically performed every 5 μs
(as an example). On the receive side of the SPI, the SPI and DMA can be configured
to transfer receive data into memory, effectively implementing a method to
periodically read data from external devices and transfer the results into memory
without processor intervention.
• Using the GPIO ports to drive or sample waveforms
By configuring the DMA to transfer data to one or more GPIO ports, it is possible to
create complex waveforms using tabular data stored in on-chip memory. Conversely,
using the DMA to periodically transfer data from one or more GPIO ports, it is
possible to sample complex waveforms and store the results in tabular form in on-
chip memory.
A more detailed description of the capability of each trigger, including resolution, range
of values, and so on, may be found in the periodic interrupt timer section.
21.4.2
DMA channels with no triggering capability
The other channels of the DMAMUX provide the normal routing functionality as
described in Modes of operation.
Chapter 21 Direct Memory Access Multiplexer (DMAMUX)
K22F Sub-Family Reference Manual, Rev. 4, 08/2016
NXP Semiconductors 419
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