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Design a stopwatch that can be timed and counted down. The range of requirements is 00.0S~99.9 S, shown with three digital tubes.
(1) countdown: the timer time is set by a small keyboard (in seconds, the maximum time is 99.9 seconds). The timing starts and ends with the keyboard. When the set countdown reaches 00.0 S, the countdown will stop automatically and the bell will ring.
(2) timing: the initial value is 00.0 S, and the timing and the end of the timer function are realized by the keyboard implementation. After the timer is over, show the record time.
(3) use three light-emitting diodes to correctly display the following states: the countdown state, the state of the timing, the state of standby.
(4) the buzzer sounds when the effective button is received.
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