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23.2.4.3.2 Transfers through the DMA Port CAUTION To perform internal transfers through the DMA port, clear XBUSEL/RBUSEL bit to 0 in the respective XFMT/RFMT registers. Failure to do so will result in software malfunction. Typically, you will access the McASP XRBUF registers through the DMA port. To access through the DMA port, simply have the CPU or DMA access the XRBUF through its DMA port location. See your device-specific data manual for the exact memory address. Through the DMA port, the DMA/CPU can service all the serializers through a single address. The McASP automatically cycles through the appropriate serializers. For transmit operations through the DMA port, the DMA/CPU should write to the same XBUF DMA port address to service all of the active transmit serializers. In addition, the DMA/CPU should write to the XBUF for all active transmit serializers in incremental (although not necessarily consecutive) order. For example, if serializers 0, 4, 5, and 7 are set up as active transmitters, the DMA/CPU should write to the XBUF DMA port address four times with data for serializers 0, 4, 5, and 7 upon each transmit data ready event. This exact servicing order must be followed so that data appears in the appropriate serializers. Similarly, for receive operations through the DMA port, the DMA/CPU should read from the same RBUF DMA port address to service all of the active receive serializers. In addition, reads from the active receive serializers through the DMA port return data in incremental (although not necessarily consecutive) order. For example, if serializers 1, 2, 3, and 6 are set up as active receivers, the DMA/CPU should read from the RBUF DMA port address four times to obtain data for serializers 1, 2, 3, and 6 in this exact order, upon each receive data ready event. When transmitting, the DMA/CPU must write data to each serializer configured as "active" and "transmit" within each time slot. Failure to do so results in a buffer underrun condition (Section 23.2.4.7.2). Similarly, when receiving, data must be read from each serializer configured as "active" and "receive" within each time slot. Failure to do results in a buffer overrun condition (Section 23.2.4.7.3). To perform internal transfers through the DMA port, clear XBUSEL/RBUSEL bit to 0 in the respective XFMT/RFMT registers.
