PXS20RM Datasheet, PDF (391/1368 Pages) Freescale Semiconductor, Inc – PXS20 Microcontroller

eDMA Channel Mux (DMA_MUX)
Periph Request
Trigger
DMA Request
Figure 18-4. DMA_MUX Channel Triggering: Normal Operation
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 trigger will be ignored. This
situation is illustrated in Figure 18-5.
Periph Request
Trigger
DMA Request
Figure 18-5. 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.
18.6.2 DMA channels with no triggering capability
The other channels of the DMA Mux provide the normal routing functionality as described in
Section 18.1.3, Modes of operation.
Freescale Semiconductor
PXS20 Microcontroller Reference Manual, Rev. 1
18-7