this is module of spi control. implementation to vhdl.

运用VHDL语言，实现AD7793芯片的采样程序和SPI的通讯程序，可以将该子模块加载到主程序中。-VHDL language to achieve the AD7793 chip sampling procedures and SPI communication program, this sub-module is loaded into the main program.

利用VHDL语言编写的SPI主机模块，采用内部自环回已经经过测试，发送接收数据正常，里面有modelsim工程，可以验证下仿真波形-SPI host module using VHDL language, has passed internal self-loopback test, sending and receiving data normally modelsim project, which can be verified under simulation waveforms