Making a C function using void pop_mtx(char m[N][N]) as the parameter

The description for the function is: populates square matrix of characters with random, lower-case letters a..z. This is what I have so far.

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

void pop_mtx(int N, char m[N][N])

{

 int i;
 srand (time(NULL));
 for (i=0; i<N; i++)
 {
     m = rand() % 26 + 'a';
 }



return 0;

}