Introduction - If you have any usage issues, please Google them yourself
Countdown stopwatch design: stopwatch after power-on/reset, comprising: a display at the beginning of 9.9 and begins the countdown,
When the timer to 9.9S, 0.0 and digital display flashing and the buzzer sounding alarm 3S, countdown error is less than 0.2 seconds.
Crystal frequency: 11.0592MHz