使用KEY1输入边沿触发，用定时器递减计数，当计数值和匹配值相等时 ，进入中断：LED3的状态翻转，每按KEY1键四次，LED的状态改变一次-Using KEY1 input edge-triggered, with the timer count down, when the count value and matching values are equal, the interrupt: LED3 state flip four