定时中断。采用定时器T0，定时50ms，每次定时溢出引起中断，在中断程序中对计数器累加。每一秒LED亮0.2s，熄0.8s。

特色：低功耗。程序中技巧在于将1s分成cnt记20次，0.2秒占20 ，即中断的前4次都点亮，后16次都熄灭。-Interrupted from time to time. The use of timer T0, from time to time 50ms, each caused by overflow from time to time interrupted, interrupted proceedings in the accumulation of the counter. Seconds of each LED light 0.2s, out 0.8s. Features: low power consumption. Skills program is to 1s into cnt mind 20 times, 0.2 seconds 20 , that is, before the interruption of light 4 times, 16 times out after.