写一个单片机程序，共阴极数码管，前面五位固定显示208--最后一位9秒计时，倒计时结束蜂鸣器响

以下是一个基于51单片机的共阴极数码管倒计时程序：

#include <reg52.h>   // 引入51单片机头文件

#define uchar unsigned char
#define uint unsigned int

// 数码管段选对应端口
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 = P2^0;
sbit LED_G = P2^1;

// 数码管位选对应端口
sbit LED_COM1 = P0^0;
sbit LED_COM2 = P0^1;
sbit LED_COM3 = P0^2;
sbit LED_COM4 = P0^3;
sbit LED_COM5 = P0^4;

// 蜂鸣器端口
sbit BUZZER = P2^7;

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

// 数码管显示函数
void display(uchar num, uchar pos) {
    LED_A = LED_B = LED_C = LED_D = LED_E = LED_F = LED_G = 1;  // 先清空所有段选端口
    switch (num) {  // 根据数字选择对应的段选端口
        case 0: LED_A = LED_B = LED_C = LED_D = LED_E = LED_F = 0; break;
        case 1: LED_B = LED_C = 0; break;
        case 2: LED_A = LED_B = LED_D = LED_E = LED_G = 0; break;
        case 3: LED_A = LED_B = LED_C = LED_D = LED_G = 0; break;
        case 4: LED_B = LED_C = LED_F = LED_G = 0; break;
        case 5: LED_A = LED_C = LED_D = LED_F = LED_G = 0; break;
        case 6: LED_A = LED_C = LED_D = LED_E = LED_F = LED_G = 0; break;
        case 7: LED_A = LED_B = LED_C = 0; break;
        case 8: LED_A = LED_B = LED_C = LED_D = LED_E = LED_F = LED_G = 0; break;
        case 9: LED_A = LED_B = LED_C = LED_D = LED_F = LED_G = 0; break;
        default: break;
    }
    switch (pos) {  // 根据位置选择对应的位选端口
        case 1: LED_COM1 = 0; LED_COM2 = LED_COM3 = LED_COM4 = LED_COM5 = 1; break;
        case 2: LED_COM2 = 0; LED_COM1 = LED_COM3 = LED_COM4 = LED_COM5 = 1; break;
        case 3: LED_COM3 = 0; LED_COM1 = LED_COM2 = LED_COM4 = LED_COM5 = 1; break;
        case 4: LED_COM4 = 0; LED_COM1 = LED_COM2 = LED_COM3 = LED_COM5 = 1; break;
        case 5: LED_COM5 = 0; LED_COM1 = LED_COM2 = LED_COM3 = LED_COM4 = 1; break;
        default: break;
    }
    delay(1);  // 延时1ms
}

// 主函数
void main() {
    uchar i, j, k;
    while (1) {
        for (i = 2; i <= 9; i++) {  // 前5位显示208
            display(2, 1); display(0, 2); display(8, 3);
            display(i, 4); display(0, 5);
            delay(1000);  // 延时1秒
        }
        for (j = 9; j >= 0; j--) {  // 倒计时9秒
            display(2, 1); display(0, 2); display(8, 3);
            display(j, 4); display(0, 5);
            delay(1000);  // 延时1秒
        }
        for (k = 0; k < 3; k++) {  // 蜂鸣器响三声
            BUZZER = 0;  // 发出蜂鸣器声音
            delay(500);  // 延时0.5秒
            BUZZER = 1;  // 停止蜂鸣器声音
            delay(500);  // 延时0.5秒
        }
    }
}

注意，此程序中的延时函数 `delay()` 可能需要根据实际情况进行调整，以保持数码管显示的稳定性。此外，此程序中使用的蜂鸣器需要外接一个电阻，以限制电流大小，否则可能会损坏单片机。