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DMAC-Slide15

Manual Transfer Mode is a means by which the user can initiate a DMA block transfer to a peripheral in the absence of a DMA request from the peripheral. Each DMA channel contains a FORCE bit which the user sets to initiate a DMA transfer. To see why this is useful lets review how DMA transfers start. For peripherals that are sending data to the DPSRAM using the DMA controller, the DMA data transfer starts automatically after the DMA channel and peripheral are initialized. When the peripheral is ready to move data to the DPSRAM, it will issue a DMA request. If data also needs to be sent to the peripheral at this time, the same DMA request may be used to activate another channel to read data from DPSRAM and write it to the peripheral. However, when only sending data to a peripheral from a DPSRAM buffer, an initial data load into the peripheral is required to start the process. This could, of course, be achieved using conventional software, but a more convenient approach is to simply mimic the channel DMA request by setting a bit within the selected DMA channel. The DMA channel processes the forced request as it would any other request, and the first data element is transferred, starting the sequence. When the peripheral is ready for the next piece of data, it will send a real DMA request and the next data element is sent.

PTM Published on: 2011-11-03