A Example Heartbeat Timer

In your apps main loop

  HeartbeatTimer();

The Heartbeat Function

//*******************************
//*******************************
//********** HEARTBEAT **********
//*******************************
//*******************************
void HeartbeatTimer()
{
  static byte heartbeat_10ms_timer = 0;
  static byte heartbeat_100ms_timer = 0;
  static unsigned long heartbeat_last_1ms_time = 0;

  while ((micros() - heartbeat_last_1ms_time) >= 1000)      //micros() provides exact 1mS timing, millis() does not!
  {

    //-------------------------------
    //----- HEARTBEAT EVERY 1mS -----
    //-------------------------------
    heartbeat_last_1ms_time += 1000;


    //<<<< Do every 1mS things here


    //-- Toggle pin 13 test --
    //static byte toggle = 0;
    //toggle ^= 1;
    //digitalWrite(13, toggle);


    heartbeat_10ms_timer++;
    if (heartbeat_10ms_timer >= 10)
    {
      //-------------------------------
      //----- 10mS HEARTBEARTBEAT -----
      //-------------------------------
      heartbeat_10ms_timer = 0;
    
    
      //<<<< Do every 10mS things here
    
    
    } //if (heartbeat_10ms_timer > 10)

    heartbeat_100ms_timer++;
    if (heartbeat_100ms_timer >= 100)
    {
      //--------------------------------
      //----- 100mS HEARTBEARTBEAT -----
      //--------------------------------
      heartbeat_100ms_timer = 0;
    

    
      //<<<< Do every 100mS things here
    
    
    } //if (heartbeat_100ms_timer > 100)
  }
}