unsigned 32bit output with lenghtInChars width, either filled with '0' or with ' ' depending on fillWith0. if value is bigger than the value that is possible within lenghtInChars, the maximum possible value is shown.

itoaUint32VarLen.c

char *itoaUint32VarLen(uint32_t value, uint8_t fillWith0, uint8_t lenghtInChars) {
   static uint8_t sBuf0[12];   // maximum Value -2147483649 = 12
   uint8_t *sBuf = &sBuf0[11], *sBufWerteBegrenzung;
   uint32_t valueAlt;   // Alte Wert für schneller % 10 Berechnung
   *sBuf = 0;
   if (value == 0) {
     *--sBuf = '0';
     lenghtInChars--;  
   } else while (value) {
     if (!lenghtInChars) {
       sBufWerteBegrenzung = sBuf;
       while (sBufWerteBegrenzung < &sBuf0[11])
           *sBufWerteBegrenzung++ = '9';
       break;
     }
     valueAlt   = value;
     value     /= 10;
     *--sBuf = '0' + valueAlt - (value * 10); // schneller als  *--sBuf = '0' + (valueAlt % 10)
     lenghtInChars--;
   } 
   while (sBuf > sBuf0 && lenghtInChars--)    // mit Nullen auffüllen
       *--sBuf = fillWith0 ? '0' : ' ';
   return *sBuf;
}