设计一个基于51单片机的秒表计时器，实现如下功能： 计时器的计时精度为百分之一秒，即0.01秒。 计时时间显示格式：hh.mm.ss.SS。其中：hh表示小时数，mm表示分钟数，ss表示秒数，SS表示百分秒数。如“1.59.59.99”表示1小时59分59秒99分秒。 当计时时间小于1分钟时，只显示秒和分秒数，如“59.99”表示0小时0分59秒99分秒。 当计时时间小于1小时时，只显示分钟、秒和分秒数，如“59.59.99”表示0小时59分59秒99分秒。 计时器有两种工作状态：停止状态、计时状态。 停止状态下计时器停止计时，保持当前计时时间不变。开机时默认进入该状态并初始化当前计时时间为0（0小时0分0秒0分秒）。 计时状态下计时器进行计时，每隔百分之一秒更新当前显示的计时时间。 计时器有两个按钮：启动/停止按钮、复位按钮。 在停止状态下按下“启动/停止”按钮使计时器立即进入计时状态，在当前计时时间基础上开始计时。 在计时状态下按下“启动/停止”按钮使计时器立即进入停止状态。 在计时状态下按下“复位”按钮将计时器当前计时时间重置为0，使计时器重新开始计时。 在停止状态下按下“复位”按钮时，将计时器的当前计时时间重置为0，计时器仍然保持停止状态。

以下是一个基于51单片机的秒表计时器的示例代码，实现了上述功能：

#include <reg51.h>

// 数码管位选控制
#define LED_COM P0

// 数码管段选控制
sbit LED_A = P2^2;
sbit LED_B = P2^3;
sbit LED_C = P2^4;
sbit LED_D = P2^5;
sbit LED_E = P2^6;
sbit LED_F = P2^7;
sbit LED_G = P3^0;
sbit LED_DP = P3^1;

// 定义显示的数字和位置
unsigned char led[] = {0, 0, 0, 0, 0, 0, 0, 0};
unsigned char position[] = {0xfe, 0xfd, 0xfb, 0xf7, 0xef, 0xdf, 0xbf, 0x7f};

// 定义计时器状态
#define STOP 0
#define RUNNING 1
unsigned char timer_state = STOP;

// 定义计时器时间
unsigned long timer_hour = 0;
unsigned long timer_minute = 0;
unsigned long timer_second = 0;
unsigned long timer_millisecond = 0;

// 延时函数
void delay(unsigned int t)
{
    unsigned int i, j;
    for (i = t; i > 0; i--)
    {
        for (j = 110; j > 0; j--);
    }
}

// 数码管显示函数
void display()
{
    unsigned char i;
    for (i = 0; i < 8; i++)
    {
        LED_COM = position[i];
        switch (led[i])
        {
            case 0:
                LED_A = LED_B = LED_C = LED_D = LED_E = LED_F = 1;
                LED_G = 0;
                break;
            case 1:
                LED_B = LED_C = 1;
                LED_A = LED_D = LED_E = LED_F = LED_G = 0;
                break;
            case 2:
                LED_A = LED_B = LED_D = LED_E = LED_G = 1;
                LED_C = LED_F = 0;
                break;
            case 3:
                LED_A = LED_B = LED_C = LED_D = LED_G = 1;
                LED_E = LED_F = 0;
                break;
            case 4:
                LED_B = LED_C = LED_F = LED_G = 1;
                LED_A = LED_D = LED_E = 0;
                break;
            case 5:
                LED_A = LED_C = LED_D = LED_F = LED_G = 1;
                LED_B = LED_E = 0;
                break;
            case 6:
                LED_A = LED_C = LED_D = LED_E = LED_F = LED_G = 1;
                LED_B = 0;
                break;
            case 7:
                LED_A = LED_B = LED_C = 1;
                LED_D = LED_E = LED_F = LED_G = 0;
                break;
            case 8:
                LED_A = LED_B = LED_C = LED_D = LED_E = LED_F = LED_G = 1;
                break;
            case 9:
                LED_A = LED_B = LED_C = LED_D = LED_F = LED_G = 1;
                LED_E = LED_G = 0;
                break;
            default:
                break;
        }
        delay(1); // 延时1ms
        LED_A = LED_B = LED_C = LED_D = LED_E = LED_F = LED_G = LED_DP = 0;
    }
}

// 计时器更新函数
void update_timer()
{
    timer_millisecond++;
    if (timer_millisecond >= 100)
    {
        timer_millisecond = 0;
        timer_second++;
        if (timer_second >= 60)
        {
            timer_second = 0;
            timer_minute++;
            if (timer_minute >= 60)
            {
                timer_minute = 0;
                timer_hour++;
            }
        }
    }
}

// 主函数
void main()
{
    unsigned int i;
    while (1)
    {
        // 获取当前时间
        unsigned long hour = timer_hour;
        unsigned long minute = timer_minute;
        unsigned long second = timer_second;
        unsigned long millisecond = timer_millisecond;

        // 将时间转换为数码管显示的数字
        if (hour >= 1)
        {
            led[0] = hour / 10;
            led[1] = hour % 10;
            led[2] = minute / 10;
            led[3] = minute % 10;
            led[4] = second / 10;
            led[5] = second % 10;
            led[6] = millisecond / 10;
            led[7] = millisecond % 10;
        }
        else if (minute >= 1)
        {
            led[0] = minute / 10;
            led[1] = minute % 10;
            led[2] = second / 10;
            led[3] = second % 10;
            led[4] = millisecond / 10;
            led[5] = millisecond % 10;
            led[6] = led[7] = 0;
        }
        else
        {
            led[0] = led[1] = 0;
            led[2] = second / 10;
            led[3] = second % 10;
            led[4] = millisecond / 10;
            led[5] = millisecond % 10;
            led[6] = led[7] = 0;
        }

        // 数码管显示
        display();

        // 更新计时器
        if (timer_state == RUNNING)
        {
            update_timer();
        }

        // 检测按键
        if (P1^0 == 0) // 启动/停止按钮
        {
            if (timer_state == STOP)
            {
                timer_state = RUNNING;
            }
            else
            {
                timer_state = STOP;
            }
            while (P1^0 == 0);
        }
        if (P1^1 == 0) // 复位按钮
        {
            timer_hour = timer_minute = timer_second = timer_millisecond = 0;
            while (P1^1 == 0);
        }

        // 延时
        for (i = 0; i < 10; i++)
        {
            delay(100); // 延时0.01s
        }
    }
}

需要注意的是，这段代码中的按键检测需要根据具体的硬件平台进行实现。另外，为了实现百分之一秒的计时精度，延时函数的延时时间需要调整为10，即延时0.01秒。