PXD10RM Datasheet, PDF (458/1332 Pages) Freescale Semiconductor, Inc – PXD10 Microcontroller

Once 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, that triggered will be ignored. This
situation is illustrated in Figure 13-5.
Periph Request
Trigger
DMA Request
Figure 13-5. 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. Once 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 5s (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 (i.e.-resolution, range of values, etc.) may be
found in the Periodic Interrupt Timer (PIT) Block Guide.
13.4.2 DMA Channels with no triggering capability
The other channels of the DMA Mux provide the normal routing functionality as described in
Section 13.1.3, Modes of Operation.
13.4.3 "Always Enabled" DMA Sources
In addition to the peripherals that can be used as DMA sources, there are 4 additional DMA sources that
are "always enabled". Unlike the peripheral DMA sources, where the peripheral controls the flow of data
during DMA transfers, the "always enabled" sources provide no such "throttling" of the data transfers.
These sources are most useful in the following cases:
• Doing DMA transfers to/from GPIO - Moving data from/to one or more GPIO pins, either
un-throttled (i.e.-as fast as possible), or periodically (using the DMA triggering capability).
13-8
PXD10 Microcontroller Reference Manual, Rev. 1
Preliminary—Subject to Change Without Notice
Freescale Semiconductor