PXD20RM Datasheet, PDF (697/1628 Pages) Freescale Semiconductor, Inc – PXD20 Microcontroller

• 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 Channel 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 Channel
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 Channel 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 Channel 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.
17.5 Initialization/application information
17.5.1 Reset
The reset state of each individual bit is shown within the Register Description section (See Section 17.3.1,
Register descriptions). In summary, after reset, all channels are disabled and must be explicitly enabled
before use.
17.5.2 Low Power Considerations
The DMA Channel Mux has no direct involvement with operations to place the MCU into low power
modes, however, it is possible for the low power transition to be blocked under certain circumstances.
Specifically, the MCU may not enter a low power mode (HALT/STOP/STANDBY) when all the below
conditions are true simultaneously:
• A peripheral with DMA capability is programmed to work on a divided clock.
Freescale Semiconductor
PXD20 Microcontroller Reference Manual, Rev. 1
Preliminary—Subject to Change Without Notice
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