流水灯程序，时钟频率为50MHz，控制四个led向同一个方向移动，如流水一样。没建个0.5s点亮一个灯，使灯亮朝一个方向移动-Water lights, clock frequency of 50MHz, control four led moving in the same direction, like water, like. Did not build a 0.5s lit a lamp, the lamp lights move in one direction