Information

After the DMA request has been serviced, the peripheral will negate its request,
effectively resetting the gating mechanism until the peripheral re-asserts 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 20-38. DMA MUX 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 setup, 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 5μs (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.
20.4.2 DMA channels with no triggering capability
The other channels of the DMA MUX provide the normal routing functionality as
described in Modes of operation.
Chapter 20 Direct Memory Access Multiplexer (DMAMUX)
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
Freescale Semiconductor, Inc. 325