此函数把SPI的读写功能集成在一块，传递的val既是向SPI写的数据，也是从SPI读取的数据，只需要在执行读或写之前把MCU的I/O脚设置成相应的输出或输入即可。（因为在大部分应用中，是把SPI的DI和DO相连并用MCU的一个IO口来读写）。-This function of the SPI read and write functionality into a single, transmission of val is not only written to the SPI data is read from the SPI data, only need to read or write in the implementation prior to the MCU
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