// small test program
// use unsigned char instead of unsigned to reach wrap around with 256 steps
#include <stdio.h>
#include <math.h>
void main()
{
int c =0;
unsigned char x,x0,y,z;  // char -> int in real life
x0 = x = 12;
y = 3;
z = x;

  for (int i=0; i < 500; i++)
  {
     x0 = x;
     x++;
     c++;
     if (x < x0) { // wrap
       z+=y ;
       x+=y ;
     }

     if (x-z >= y) {
       z+=y;
       printf("ping %i\n",c);
       c = 0;
a     }
     printf(" x%i y %i z %i\n",x,y,z);
  }
}

unsigned long x,x0,y,z;
x0 = x;
y = 3;
z = x;

void setup()
{
  x0 = x = z = millis();;
  y = 12; // periode
}

void loop()
{
  x0 = x;
  x++;
  if (x < x0) { // wrap
    z+=y ;
    x+=y ;
  }

  if (x-z >= y) {
    z+=y;
    // now its time for periodic code
  }
}