这个数字为什么不改变啊

#include<reg51.h>
unsigned char sec=0;
unsigned char s1[] = {0x3f, 0x06, 0x5b, 0x4f, 
0x66, 0x6d, 0x7d, 0x07, 0x7f, 0x6f, 0x77, 
0x7c, 0x39, 0x5e, 0x79, 0x71, 0x73, 0x3e, 
0x31, 0x6e, 0x76, 0x38, 0x00};//共阴
unsigned char s2[] = {0xc0, 0xf9, 0xa4, 0xb0, 
0x99, 0x92, 0x82, 0xf8, 0x80, 0x90, 0x88, 
0x83, 0xc6, 0xa1, 0x86, 0x8e, 0x8c, 0xc1, 
0xce, 0x91, 0x89, 0xc7, 0xff};//共阳
unsigned char s3[] = {0x40,0x79,0x24,0x20,0x19,0x12,0x02,0x78,0x00,0x18}; //带小数点的从0-9
void delay(unsigned char i)
{
while(i--)
{
unsigned int t;
for(t=0;t<120;t++);
}
}
void seg()
{
    P0=0x01;
    P1=s2[sec/10];
    delay(10);
    P1=0xff;
    
    P0=0x02;
    P1=s3[sec%10];
    delay(10);
    P1=0xff;
    
    P0=0x04;
    P1=s2[sec%10];
    delay(10);
    P1=0xff;
    
    P0=0x08;
    P1=s2[sec%10];
    delay(10);
    P1=0xff;
}
void main()
{
    while(1)
    {
        unsigned char num=0;
        seg();
        //unsigned char num=0;
        if(num<50)
        num++;
        else
        {
            num=0;
            if(sec<60)
            sec++;
            else
            sec=0;
        }
    }
}

void main()
{
unsigned char num=0;
while(1)
{
seg();
if(num<50)
num++;
else
{
num=0;
if(sec<60)
sec++;
else
sec=0;
}
}

因为你的num总是0.把num定义到while循环外面去。