利用状态机实现对tlc5620dac控制，实验时按key1,可选择DAC的通道，数码管1显示，按key2，key3可 输入8位数/模转换值，由数码管3，4显示-State machine implementation control on tlc5620dac, press key1 experiment, you can choose the DAC channels, digital tube display, press key2, key3 can enter the 8 digit