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(Design an electronic clock, require a showing hours, minutes, seconds, when a 24-hour system a crystal oscillator or timing circuitry 555 provides a standard time reference signal would also like to have a Clear function extension (with the school when the function, you can separate school for hours and minutes when the right time when the campus to stop the entry points to the hours-bit, school clock source can be borrowed from time to time in the clock circuit with punctual timekeeping function.)