设计一个可以顺计时和倒计时的秒表。要求计时的范围为00.0S~99.9S，用三位数码管显示，用三个发光二极管正确显示以下状态：倒计时状态、顺计时状态、待机状态-Designed to be a cis-timing and countdown stopwatch. Required time ranges from 00.0S ~ 99.9S, with three digital tube display, with three light-emitting diode display cor