1、使用LCD1602作为显示器。 2、使用单片机定时器作为时间基准 3、使用独立按键，切换秒表和计时器功能 4、秒表模式下以0.01秒为单位显示变化，一个按键用于开始和停止。 5、计时模式进行倒计时，使用组合按键调整计时时间长度，一秒为单位。 计时模式下使用一个按键开始和停止计时，即使时间到蜂鸣器叫，使用该按键关闭提醒，重新回到设定的时间长度显示状态。怎么编写程序

以下是一个可能的实现方案，基于STC89C52单片机和Keil C编译器：

#include <reg52.h>

// 定义LCD1602引脚
sbit RS = P2^0;
sbit EN = P2^1;
sbit D4 = P1^4;
sbit D5 = P1^5;
sbit D6 = P1^6;
sbit D7 = P1^7;

// 定义按键引脚
sbit START_STOP_BTN = P3^0;
sbit MODE_BTN = P3^1;
sbit UP_BTN = P3^2;
sbit DOWN_BTN = P3^3;

// 定义计时器变量
unsigned char timer_hours = 0;
unsigned char timer_minutes = 0;
unsigned char timer_seconds = 0;

// 定义秒表变量
unsigned int stopwatch_ticks = 0;
unsigned char stopwatch_running = 0;

// LCD1602初始化
void lcd_init() {
    delay_ms(15);
    lcd_write_command(0x02);  // 返回首页
    lcd_write_command(0x28);  // 4位数据总线，2行显示，5x8字体
    lcd_write_command(0x0C);  // 显示开，光标关，闪烁关
    lcd_write_command(0x06);  // 光标右移
    lcd_write_command(0x01);  // 清屏
    delay_ms(5);
}

// LCD1602写入命令
void lcd_write_command(unsigned char command) {
    RS = 0;
    EN = 0;
    D7 = command & 0x80;
    D6 = command & 0x40;
    D5 = command & 0x20;
    D4 = command & 0x10;
    EN = 1;
    delay_ms(1);
    EN = 0;
    D7 = command & 0x08;
    D6 = command & 0x04;
    D5 = command & 0x02;
    D4 = command & 0x01;
    EN = 1;
    delay_ms(1);
    EN = 0;
}

// LCD1602写入数据
void lcd_write_data(unsigned char data) {
    RS = 1;
    EN = 0;
    D7 = data & 0x80;
    D6 = data & 0x40;
    D5 = data & 0x20;
    D4 = data & 0x10;
    EN = 1;
    delay_ms(1);
    EN = 0;
    D7 = data & 0x08;
    D6 = data & 0x04;
    D5 = data & 0x02;
    D4 = data & 0x01;
    EN = 1;
    delay_ms(1);
    EN = 0;
}

// 显示时间或秒表计时
void display_time() {
    lcd_write_command(0x80);  // 第一行第一列
    lcd_write_data('0' + timer_hours / 10);
    lcd_write_data('0' + timer_hours % 10);
    lcd_write_data(':');
    lcd_write_data('0' + timer_minutes / 10);
    lcd_write_data('0' + timer_minutes % 10);
    lcd_write_data(':');
    lcd_write_data('0' + timer_seconds / 10);
    lcd_write_data('0' + timer_seconds % 10);
    lcd_write_command(0xC0);  // 第二行第一列
    if (stopwatch_running) {
        lcd_write_data('0' + stopwatch_ticks / 6000 % 10);
        lcd_write_data('0' + stopwatch_ticks / 100 % 60 / 10);
        lcd_write_data('.');
        lcd_write_data('0' + stopwatch_ticks / 100 % 10);
        lcd_write_data('0' + stopwatch_ticks % 100 / 10);
        lcd_write_data('0' + stopwatch_ticks % 10);
    } else {
        lcd_write_data('T');
        lcd_write_data('I');
        lcd_write_data('M');
        lcd_write_data('E');
        lcd_write_data('R');
        lcd_write_data(' ');
        lcd_write_data('S');
        lcd_write_data('T');
        lcd_write_data('O');
        lcd_write_data('P');
    }
}

// 计时器中断处理
void timer_interrupt() interrupt 1 {
    static unsigned char counter = 0;
    TH0 = 0xFC;  // 定时1ms
    TL0 = 0x67;
    counter++;
    if (counter == 100) {  // 1s
        counter = 0;
        if (stopwatch_running) {
            stopwatch_ticks++;
        } else {
            if (timer_seconds == 0) {
                if (timer_minutes == 0) {
                    if (timer_hours == 0) {
                        // 时间到
                        TR0 = 0;  // 停止计时器
                        timer_seconds = 0;
                        timer_minutes = 0;
                        timer_hours = 0;
                        while (START_STOP_BTN == 0) {  // 等待按键
                            lcd_write_command(0x01);  // 清屏
                            lcd_write_command(0x80);  // 第一行第一列
                            lcd_write_data('T');
                            lcd_write_data('I');
                            lcd_write_data('M');
                            lcd_write_data('E');
                            lcd_write_data('\'');
                            lcd_write_data('S');
                            lcd_write_data(' ');
                            lcd_write_data('U');
                            lcd_write_data('P');
                            lcd_write_data('!');
                            delay_ms(400);
                            lcd_write_command(0x01);  // 清屏
                            delay_ms(400);
                        }
                        lcd_write_command(0x01);  // 清屏
                        display_time();
                        return;
                    } else {
                        timer_hours--;
                        timer_minutes = 59;
                        timer_seconds = 59;
                    }
                } else {
                    timer_minutes--;
                    timer_seconds = 59;
                }
            } else {
                timer_seconds--;
            }
        }
    }
    display_time();
}

// 按键检测
void button_check() {
    static unsigned char last_start_stop_btn_state = 0;
    static unsigned char last_mode_btn_state = 0;
    static unsigned char last_up_btn_state = 0;
    static unsigned char last_down_btn_state = 0;
    if (START_STOP_BTN != last_start_stop_btn_state) {  // 开始/停止计时器
        last_start_stop_btn_state = START_STOP_BTN;
        if (last_start_stop_btn_state == 0) {
            stopwatch_running = !stopwatch_running;
            if (stopwatch_running) {
                TR0 = 1;  // 启动计时器
            } else {
                TR0 = 0;  // 停止计时器
            }
        }
    }
    if (MODE_BTN != last_mode_btn_state) {  // 切换模式
        last_mode_btn_state = MODE_BTN;
        if (last_mode_btn_state == 0) {
            if (stopwatch_running) {
                stopwatch_running = 0;
                TR0 = 0;  // 停止计时器
            }
            timer_hours = 0;
            timer_minutes = 0;
            timer_seconds = 0;
            lcd_write_command(0x01);  // 清屏
        }
    }
    if (UP_BTN != last_up_btn_state) {  // 调整时间或秒表计次
        last_up_btn_state = UP_BTN;
        if (last_up_btn_state == 0) {
            if (stopwatch_running) {
                stopwatch_ticks = 0;
            } else {
                if (timer_hours < 99) {
                    timer_hours++;
                }
            }
        }
    }
    if (DOWN_BTN != last_down_btn_state) {  // 调整时间
        last_down_btn_state = DOWN_BTN;
        if (last_down_btn_state == 0) {
            if (stopwatch_running) {
                stopwatch_ticks = 0;
            } else {
                if (timer_hours > 0) {
                    timer_hours--;
                }
            }
        }
    }
}

// 主函数
void main() {
    TMOD = 0x01;  // 设置计时器0为模式1
    TH0 = 0xFC;  // 定时1ms
    TL0 = 0x67;
    TR0 = 0;  // 先停止计时器
    ET0 = 1;  // 允许计时器0中断
    EA = 1;  // 允许总中断
    lcd_init();
    while (1) {
        button_check();
    }
}

注意：以上代码仅供参考，具体实现可能需要根据具体硬件和需求进行适当修改。另外，为了保证计时器的精度，建议使用外部晶体振荡器，并在程序中配置正确的时钟频率。