9.2.6 Transfer Modes
The μDMA controller supports several transfer modes. Two of the modes support simple one-time
transfers. Several complex modes support a continuous flow of data.
9.2.6.1 Stop Mode
While Stop is not actually a transfer mode, it is a valid value for the mode field of the control word.
When the mode field has this value, the μDMA controller does not perform any transfers and disables
the channel if it is enabled. At the end of a transfer, the μDMA controller updates the control word
to set the mode to Stop.
9.2.6.2 Basic Mode
In Basic mode, the μDMA controller performs transfers as long as there are more items to transfer,
and a transfer request is present. This mode is used with peripherals that assert a μDMA request
signal whenever the peripheral is ready for a data transfer. Basic mode should not be used in any
situation where the request is momentary even though the entire transfer should be completed. For
example, a software-initiated transfer creates a momentary request, and in Basic mode, only the
number of transfers specified by the ARBSIZE field in the DMA Channel Control Word (DMACHCTL)
register is transferred on a software request, even if there is more data to transfer.
When all of the items have been transferred using Basic mode, the μDMA controller sets the mode
for that channel to Stop.
BASIC mode can be programmed to ignore when XFERSIZE reaches 0x000 and continue copying
on request until the channel is stopped manually. If the NXTUSEBURST bit in the uDMA Channel
Control Word (DMACHCTL) register is set while in BASIC mode and the XFERSIZE reaches 0x000
and is not written back, transfers continue until the request is deasserted by the peripheral.
9.2.6.3 Auto Mode
Auto mode is similar to Basic mode, except that once a transfer request is received, the transfer
runs to completion, even if the μDMA request is removed. This mode is suitable for software-triggered
transfers. Generally, Auto mode is not used with a peripheral.
When all the items have been transferred using Auto mode, the μDMA controller sets the mode for
that channel to Stop.
9.2.6.4 Ping-Pong
Ping-Pong mode is used to support a continuous data flow to or from a peripheral. To use Ping-Pong
mode, both the primary and alternate data structures must be implemented. Both structures are set
up by the processor for data transfer between memory and a peripheral. The transfer is started
using the primary control structure. When the transfer using the primary control structure is complete,
the μDMA controller reads the alternate control structure for that channel to continue the transfer.
Each time this happens, an interrupt is generated, and the processor can reload the control structure
for the just-completed transfer. Data flow can continue indefinitely this way, using the primary and
alternate control structures to switch back and forth between buffers as the data flows to or from
the peripheral.
Refer to Figure 9-2 on page 686 for an example showing operation in Ping-Pong mode.
685June 18, 2014
Texas Instruments-Production Data
Tiva
™
TM4C1294NCPDT Microcontroller
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