51单片机数码管显示0-99，按下按键1家1，按下按键2件1，按下按键3归零

这个问题可以通过以下步骤来解决：

1. 初始化：将数码管全部显示为0。

2. 不断读取按键的状态，如果按键1被按下，则将当前数码管的值加1；如果按键2被按下，则将当前数码管的值减1；如果按键3被按下，则将数码管的值归零。

3. 每次修改数码管的值后，需要检查是否超出了0-99的范围，如果超出，则将数码管的值重置为0或99。

4. 将修改后的数码管的值显示在数码管上。

以下是代码示例（假设数码管使用共阴极）：

#include <reg51.h>

// 数码管的端口定义
sbit digit1 = P2^0;
sbit digit2 = P2^1;
sbit segmentA = P1^0;
sbit segmentB = P1^1;
sbit segmentC = P1^2;
sbit segmentD = P1^3;
sbit segmentE = P1^4;
sbit segmentF = P1^5;
sbit segmentG = P1^6;
sbit segmentDP = P1^7;

// 按键的端口定义
sbit button1 = P3^0;
sbit button2 = P3^1;
sbit button3 = P3^2;

// 数码管的显示表
unsigned char displayTable[] = {
    0x3f,  // 0
    0x06,  // 1
    0x5b,  // 2
    0x4f,  // 3
    0x66,  // 4
    0x6d,  // 5
    0x7d,  // 6
    0x07,  // 7
    0x7f,  // 8
    0x6f   // 9
};

// 当前数码管的值
unsigned char value = 0;

// 数码管的位选择
void selectDigit(unsigned char digit) {
    switch (digit) {
        case 1:
            digit1 = 0;
            digit2 = 1;
            break;
        case 2:
            digit1 = 1;
            digit2 = 0;
            break;
        default:
            digit1 = 1;
            digit2 = 1;
            break;
    }
}

// 数码管的段选择
void selectSegment(unsigned char segment) {
    switch (segment) {
        case 0:
            segmentA = 0;
            segmentB = 0;
            segmentC = 0;
            segmentD = 0;
            segmentE = 0;
            segmentF = 0;
            segmentG = 1;
            segmentDP = 1;
            break;
        case 1:
            segmentA = 1;
            segmentB = 0;
            segmentC = 0;
            segmentD = 1;
            segmentE = 1;
            segmentF = 1;
            segmentG = 1;
            segmentDP = 1;
            break;
        case 2:
            segmentA = 0;
            segmentB = 0;
            segmentC = 1;
            segmentD = 0;
            segmentE = 0;
            segmentF = 1;
            segmentG = 0;
            segmentDP = 1;
            break;
        case 3:
            segmentA = 0;
            segmentB = 0;
            segmentC = 0;
            segmentD = 0;
            segmentE = 1;
            segmentF = 1;
            segmentG = 0;
            segmentDP = 1;
            break;
        case 4:
            segmentA = 1;
            segmentB = 0;
            segmentC = 0;
            segmentD = 1;
            segmentE = 1;
            segmentF = 0;
            segmentG = 0;
            segmentDP = 1;
            break;
        case 5:
            segmentA = 0;
            segmentB = 1;
            segmentC = 0;
            segmentD = 0;
            segmentE = 1;
            segmentF = 0;
            segmentG = 0;
            segmentDP = 1;
            break;
        case 6:
            segmentA = 0;
            segmentB = 1;
            segmentC = 0;
            segmentD = 0;
            segmentE = 0;
            segmentF = 0;
            segmentG = 0;
            segmentDP = 1;
            break;
        case 7:
            segmentA = 0;
            segmentB = 0;
            segmentC = 0;
            segmentD = 1;
            segmentE = 1;
            segmentF = 1;
            segmentG = 1;
            segmentDP = 1;
            break;
        case 8:
            segmentA = 0;
            segmentB = 0;
            segmentC = 0;
            segmentD = 0;
            segmentE = 0;
            segmentF = 0;
            segmentG = 0;
            segmentDP = 1;
            break;
        case 9:
            segmentA = 0;
            segmentB = 0;
            segmentC = 0;
            segmentD = 0;
            segmentE = 1;
            segmentF = 0;
            segmentG = 0;
            segmentDP = 1;
            break;
        default:
            segmentA = 1;
            segmentB = 1;
            segmentC = 1;
            segmentD = 1;
            segmentE = 1;
            segmentF = 1;
            segmentG = 1;
            segmentDP = 0;
            break;
    }
}

// 延时
void delay(unsigned int ms) {
    unsigned int i, j;
    for (i = 0; i < ms; i++) {
        for (j = 0; j < 120; j++);
    }
}

// 主函数
void main() {
    // 初始化数码管
    selectDigit(0);
    selectSegment(10);
    delay(100);

    // 不断更新数码管的值
    while (1) {
        // 读取按键的状态
        if (!button1) {
            value++;
        } else if (!button2) {
            value--;
        } else if (!button3) {
            value = 0;
        }

        // 检查数码管的值是否超出范围
        if (value > 99) {
            value = 0;
        } else if (value < 0) {
            value = 99;
        }

        // 显示数码管的值
        selectDigit(1);
        selectSegment(displayTable[value / 10]);
        delay(5);
        selectDigit(2);
        selectSegment(displayTable[value % 10]);
        delay(5);
    }
}