说明：  设计一个用于篮球比赛的定时器。要求： （1）定时时间为24秒，按递减方式计时，每隔1秒，定时器减1； （2）定时器的时间用两位数码管显示； （3）设置两个外部控制开关，开关K1控制定时器的直接复位/启动计时，开关K2控制定时器的暂停/连续计时；当定时器递减计时到零（即定时时间到）时，定时器保持零不变，同时发出报警信号，报警信号用一个发光二极管指示。 （4）输入时钟脉冲的频率为50MHz.
(Design a timer for a basketball match. Requirements: (1) the timer time is 24 seconds and the timer is reduced by 1 second every second. (2) the timer time is displayed with two digital tubes; (3) set two external control switches, switch K1 control the direct reset/start time of the timer, switch K2 control timer pause/continuous time; When the timer decreases to zero (i.e., timing time), the timer stays zero and sends an alarm signal, indicating the alarm signal with a light-emitting diode. (4) the frequency of input clock pulse is 50MHz.)