usart_dma_double_buffer_transfer
Overview
This example shows how to use the DMA driver to implement a double buffer receive scheme from the USART
The example shows the double buffer constructed using two descriptors (g_pingpong_desc). These descriptors are cycled from one to the other.
Things to note :
The descriptors of the ping pong transfer need to be aligned to size 16
The inital transfer will perform the same job as first descriptor of ping pong, so the first linkeage is to go to g_pingpong_desc[1]
g_pingpong_desc[1] then chains the g_pingpong_desc[0] as the next descriptor
The properties are set up such that g_pingpong_desc[0] (and the initial configuration uses INTA to signal back to the callback)
g_pingpong_desc[1] uses INTB to signal to the callback
The scheduled callback uses this information to know which data was last written
A note on Performance :
The intent of this example is to illustrate how a double-buffer scheme can be implemented using the dma. The performance of this example will be limited to how quickly the echo printer can read-out the data from the ping pong buffer and display it. This means that the example will work well if characters are entered at a rate where the DMA callback to echo the string can keep up with the input stream. Connecting the USART RX to a continuous fast speed will cause the DMA to fall behind.