#ifndef _Max7219_H_
#define _Max7219_H_

#include<intrins.h>
#include<reg51.h>
#define uchar unsigned char
#define uint  unsigned int

#define  DECODE_MODE   0x09                 //译码方式
#define  INTENSITY     0x0A
#define  SCAN_LIMIT    0x0B
#define  SHUT_DOWN     0x0C
#define  DISPLAY_TEST  0x0D

#define    delay1us         _nop_();
#define    delay2us       {delay1us;delay1us;}
#define    delay4us        {delay2us;delay2us;}
#define    delay5us        {delay4us;delay1us;}
#define    delay12us       {delay4us;delay4us;delay4us;}

sbit din=P2^0;
sbit load=P2^1;
sbit clk=P2^2;

void Max7219_Wr_byte(uchar wrdat);
void Max7219_Wr_data(uchar addr,uchar dat);
void Init_Max7219(void);

void Disp_speed(uint speed);
void Disp_set_speed(uint set_speed);

//初始化MAX7219
void Max7219_Init(void)
{
    Max7219_Wr_data(SHUT_DOWN, 0x01);   
     Max7219_Wr_data(DISPLAY_TEST, 0x00);   
     Max7219_Wr_data(DECODE_MODE, 0xff);   
     Max7219_Wr_data(SCAN_LIMIT, 0x07);   
     Max7219_Wr_data(INTENSITY, 0x0a);
}

void Max7219_Wr_byte(uchar wrdat)
{
    bit dat;
    uchar i;
    for (i=0;i<8;i++)     
    {
        clk=0;
        dat=(bit)(wrdat&0x80);
        if(dat)
            din=1;
        else
            din=0;
        wrdat<<=1;
        clk=1;
   }
}

void Max7219_Wr_data(uchar addr,uchar dat)
{
    load=0;
    Max7219_Wr_byte(addr);
    Max7219_Wr_byte(dat);
    load=1;
}

void Disp_speed(uint speed)
{
    uchar speed_buf[4];
    uchar i;
    speed_buf[0]=speed%10;
    speed_buf[1]=speed/10%10|0x80;
    speed_buf[2]=speed/100%10;
    speed_buf[3]=speed/1000;
    if(0==speed_buf[3])
    {
        speed_buf[3]=0x0f;
        if(0==speed_buf[2])
        speed_buf[2]=0x0f;
     }
    for(i=1;i<=4;i++)
        {
            Max7219_Wr_data(i,speed_buf[i-1]);
        }
}

void Disp_set_speed(uint set_speed)
{
    uchar speed_buf[4];
    uchar i;
    speed_buf[0]=set_speed%10;
    speed_buf[1]=set_speed/10%10|0x80;
    speed_buf[2]=set_speed/100%10;
    speed_buf[3]=set_speed/1000;
    if(0==speed_buf[3])
    {
        speed_buf[3]=0x0f;
        if(0==speed_buf[2])
        speed_buf[2]=0x0f;
     }
    for(i=1;i<=4;i++)
        {
            Max7219_Wr_data(i+4,speed_buf[i-1]);
        }
}

#endif


#include <reg51.h>


sbit KEY0=P2^3;
sbit KEY1=P2^4;
sbit KEY2=P2^5;
sbit KEY3=P2^6;

sbit SPK=P2^7;
sbit LED_GREEN=P1^6;
sbit LED=P1^7;

uint set_speed;
uint speed;
uchar sec_cnt;

void Init_8051(void);
void Delay(uchar time);
uint Get_key(void);
void Warn_ring(void);

void main(void)
{
    Init_8051();
    Max7219_Init();
    while(1)
    {
        set_speed=Get_key();
        Disp_speed(speed);
        Disp_set_speed(set_speed);
        Warn_ring();
    }
}

void Delay(uchar time)
{
    uchar t1,t2;
    for(t1=time;t1>0;t1--)
        for(t2=248;t2>0;t2--);
}

//初始化
void Init_8051(void)
{
    EA=0;
    TCON=0x00;
    TMOD=0x51;
    TL0=0xe0;                //定时20ms
    TH0=0xb1;                //T1计数13个脉冲
    TL1=0xf3;
    TH1=0xff;
    TR0=1;
    TR1=1;
    IE=0x8a;
    SPK=0;
    LED=0;
    LED_GREEN=1;

}

//定时器0函数，定时50*20ms=1s
void Timer0_speed(void) interrupt 1
{
    ET0=0;
    TR0=0;
    TL0=0xe0;
    TH0=0xb1;
    TR0=1;
    sec_cnt++;
    ET0=1;
}

//计数中断 计数10次  半径d=20cm
//speed=0.2*60*10/sec_cnt*36 km/h
void Counter_speed(void) interrupt 3
{   float a=6250;
    ET1=0;
    TR1=0;
    TL1=0xf3;
    TH1=0xff;
    TR1=1;
    speed=4.52*a/sec_cnt;
    sec_cnt=0;
    ET1=1;
}

//键盘处理函数
uint Get_key(void)
{
    static    uint key_value;
    if(!KEY0)
    {
        Delay(40);
        if(!KEY0)
        {
            while(KEY3)
            {
                if(!KEY1)
                {
                    Delay(40);
                    if(0==KEY1)
                    {
                        while(0==KEY1)
                        {
                            Disp_set_speed(key_value);
                        }
                        key_value+=10;
                        if(key_value==9999)
                                key_value=0;
                    }
                }
                if(!KEY2)
                {
                    Delay(40);
                    if(0==KEY2)
                    {
                        while(0==KEY2)
                        {
                            Disp_set_speed(key_value);
                        }
                        key_value-=10;
                            if(key_value==0)
                                key_value=9999;
                    }
                }
            }
        }
    }
    return (key_value);
}

//报警函数
void Warn_ring(void)
{
    if(speed > set_speed)
    {
        LED=1;
        SPK=0;
        Delay(1);
        SPK=1;
    }
    else
        LED=0;
}

太长了，又没说是神马错误

把报错贴出来look,look

把错误描述下，大家才可以帮你啊。