基于DMA通过UART发送和接收数据的例子，注意DMA_0为接受通道，DMA_1为发送通道。当然可以将dma的read_master和writer_master同时连在uart_0和sdram_0的从端口上，这样是可以用一个dma对两者读写操作，但是不能同时做双向传输。-Based on DMA send and receive data through the UART example, attention DMA_0 accepted channels, DMA_1 to send the channel. Can, of course dma' s read_master and writer_master the same time even in the uart_0 and sdram_0 from the port, it can both read and write on using a dma operation, but can not simultaneously do two-way transmission.