C51控制数码管显示字符“C51--4--”

首先，需要连接一个4位共阳数码管到C51单片机上，并且连接对应的引脚。然后，编写C语言程序来控制数码管显示字符“C51--4--”。

以下是一份简单的程序示例：

#include <reg51.h> // 导入C51单片机的寄存器定义

// 定义数码管的引脚
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;
sbit LED_DP = P2^7;

void delay(unsigned int x) // 延时函数，用于控制数码管的显示时间
{
    unsigned int i, j;
    for (i = x; i > 0; i--)
        for (j = 110; j > 0; j--);
}

void main()
{
    while (1) // 循环显示字符
    {
        LED_A = 1;
        LED_B = 1;
        LED_C = 1;
        LED_D = 0;
        LED_E = 0;
        LED_F = 1;
        LED_G = 1;
        LED_DP = 0;
        delay(1000); // 延时1秒

        LED_A = 0;
        LED_B = 1;
        LED_C = 1;
        LED_D = 1;
        LED_E = 0;
        LED_F = 0;
        LED_G = 1;
        LED_DP = 0;
        delay(1000); // 延时1秒

        LED_A = 0;
        LED_B = 0;
        LED_C = 1;
        LED_D = 1;
        LED_E = 1;
        LED_F = 0;
        LED_G = 1;
        LED_DP = 0;
        delay(1000); // 延时1秒

        LED_A = 1;
        LED_B = 0;
        LED_C = 1;
        LED_D = 0;
        LED_E = 0;
        LED_F = 0;
        LED_G = 1;
        LED_DP = 0;
        delay(1000); // 延时1秒

        LED_A = 1;
        LED_B = 0;
        LED_C = 1;
        LED_D = 0;
        LED_E = 0;
        LED_F = 0;
        LED_G = 1;
        LED_DP = 0;
        delay(1000); // 延时1秒

        LED_A = 0;
        LED_B = 0;
        LED_C = 1;
        LED_D = 1;
        LED_E = 1;
        LED_F = 0;
        LED_G = 1;
        LED_DP = 0;
        delay(1000); // 延时1秒

        LED_A = 0;
        LED_B = 1;
        LED_C = 1;
        LED_D = 1;
        LED_E = 0;
        LED_F = 0;
        LED_G = 1;
        LED_DP = 0;
        delay(1000); // 延时1秒
    }
}

需要注意的是，该程序只显示了字符“C51--4--”，如果需要显示其他字符，需要修改相应的引脚控制代码。同时，需要根据具体的硬件连接情况进行修改。