说明：   该程序为 SPI 典型程序，主要是采用内部测试自循环模式，而且采用中断发送、接收形式，通过程序可以学习如何进行自循环测试以及如何编写 SPI 中断程序。
(The typical procedure for the SPI program, mainly from the cyclic model with internal testing, and using interrupt sending, receiving the form, through the program can learn how to self-cycle testing and how to write SPI interrupt program.)