单片机四位数码管滚动显示0－f

单片机四位数码管滚动显示0-f，求大佬帮帮忙。

#include<stdio.h>
#include<stdlib.h>
int main()
{
    int num=0,tn=0;
    char str[4] = {0};
    while (1) {
        tn = num;
        int a = 0;
        while (tn!=0)
        {
            str[a] = (char)(tn % 2);
            tn = tn / 2;
            a++;
        }
        for (int i = 3; i >= 0; i--)
        {
            printf("%d",str[i]);
        }
        num = (num + 1) % 15;
        printf("\n");
    }
    return 0;
}