PXS20RM Datasheet, PDF (392/1368 Pages) Freescale Semiconductor, Inc – PXS20 Microcontroller

eDMA Channel Mux (DMA_MUX)
18.6.3 "Always Enabled" DMA sources
In addition to the peripherals that can be used as DMA sources, there are 6 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).
• Doing DMA transfers from memory to memory - Moving data from memory to memory, typically
as fast as possible, sometimes with software activation.
• Doing DMA transfers from memory to the external bus (or vice-versa) - Similar to memory to
memory transfers, this is typically done as quickly as possible.
• Any DMA transfer that requires software activation - Any DMA transfer that should be explicitly
started by software.
In cases where software should initiate the start of a DMA transfer, a "always enabled" DMA source can
be used to provide maximum flexibility. When activating a DMA channel via software, subsequent
executions of the minor loop require a new "start" event be sent. This can either be a new software
activation, or a transfer request from the DMA_MUX. The options for doing this are:
• Transfer all data in a single minor loop. By configuring the DMA to transfer all of the data in a
single minor loop (i.e.-major loop counter = 1), no re-activation of the channel is necessary. The
disadvantage to this option is the reduced granularity in determining the load that the DMA transfer
will incur on the system. For this option, the DMA channel should be disabled in the DMA_MUX.
• Use explicit software re-activation. In this option, the DMA is configured to transfer the data using
both minor and major loops, but the processor is required to re-activate the channel (by writing to
the DMA registers) after every minor loop. For this option, the DMA channel should be disabled
in the DMA_MUX.
• Use a "always enabled" DMA source. In this option, the DMA is configured to transfer the data
using both minor and major loops, and the DMA_MUX does the channel re-activation. For this
option, the DMA channel should be enabled and pointing to an "always enabled" source. Note that
the re-activation of the channel can be continuous (DMA triggering is disabled) or can use the
DMA triggering capability. In this manner, it is possible to execute periodic transfers of packets of
data from one source to another, without processor intervention.
18.7 Initialization/application Information
18.7.1 Reset
The reset state of each individual bit is shown within the Register Description section (Section 18.3.1,
Register descriptions). In summary, after reset, all channels are disabled and must be explicitly enabled
before use.
18-8
PXS20 Microcontroller Reference Manual, Rev. 1
Freescale Semiconductor