用74hc595驱动数码管的例程，程序实现一个简易抢答器的设计方案-74hc595 driver with digital control routines, simple Responder program design methods.

改VHDL代码可以实现超声波测距的功能，其精确度达到US级，可以用七段数码管显示其数值-sound ranging sound ranging sound ranging sound ranging sound ranging sound ranging sound ranging sound ranging sound ranging sound ranging sound ranging