使用nios的io管脚模拟spi时序对tf卡进行数据读写，同时附带一个简单的用verilog写的在vga显示器上刷屏的代码-Use the io pin of nios to simulate the timing of spi to read&write date from&to tf card, and comes with a simple code written by verilog to refresh the vga monitor.