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