DMA Channel Mux (DMACHMUX)
MPC5606S Microcontroller Reference Manual, Rev. 7
456 Freescale Semiconductor
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 13-5.
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 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 can be found in Chapter 27, Periodic Interrupt
Timer (PIT).
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 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 always enabled sources provide no such throttling of the data transfers. These
sources are most useful in the following cases:
Peripheral request
Tri gg er
DMA Request
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