功能要求: (1) start为”1”时，步进电机转动。

start为”0”时，步进电机停转。

(2) 预设step_in为常数，当clk为上升沿，load和start为”1’加载时，电机开始转动，转动step_in次后，停止转动。当下次符合此要求时将再次转动。

(3) 通过load,，step_in同步或异步载入steps值。

-Functional requirements: (1) start to　1 , the stepper motor rotation. start for the　0　when the stepper motor to stop. (2) default step_in is a constant, when clk for the rising edge, load and start to　1 load, the motor started to rotate, turning step_in times, stop turning. When the next in line with this request will be turning again. (3) through the load,, step_in synchronous or asynchronous loading steps value.