说明：  基于MSP430系列单片机的软件实时时钟(RTC)代码,使用定时器B为中断源,时间变量使用字符型变量储存,便于直接输出数字
(MSP430 MCU-based software in real-time clock (RTC) code, the use of Timer B interrupt source for the time variable using the character variable storage, easy to direct the output figures)