定时器一但启动，它便在原来的数值上开始加1计数，若在程序开始时，我们没有设置TH0和TL0，它们的默认值都是0，假设时钟频率为12MHz，12个时钟周期为一个机器周期，那么此时机器周期为1us，记满TH0和TL0就需要216 -1个数，再来一个脉冲计数器溢出，随即向CPU申请中断-Once the timer starts, it began on the original value plus 1 count, in the beginning of the program, we did not set TH0 and TL0, the default value is 0, and assume that the clock frequency of 12MHz, 12 clock cyclesone machine cycle, then the machine cycle is 1us, Incurred TH0 and TL0 216-1 number, return a pulse counter overflow, then apply to the CPU interrupt.