本代码是控制SDRAM的VHDL代码，几经优化现已趋近完美，里面主要用状态机实现，现封装为entity，便于调用模块-This code is to control the SDRAM of the VHDL code, optimization has been several times closer to perfection, which is mainly used to achieve a state machine is encapsulated entity, easy to call the module