实现SW1键按下后，数码管显示加1,当数值加到十时，数码管显示0；当SW2键按下后，数码管数值减1，当显示为0时，再减1时，就不显示任何数值-To achieve SW1 key is pressed , the LED display plus one , when the value added to the 10 , the digital display　 When SW2 key is pressed , the digital control value by 1 , is displayed as 0:00 , the further reduction of 1:00 . not display any value