Calling method function from Javascript/AJAX

[eluser]Fribos[/eluser]
Hi, i'm codding a CI bassed Chat but i cant update online user's column using AJAX.
User's info comes from CI session class stored on ci_session table on my Database.
Usernames are stored in data_user column as a custom session data. I used serialize PHP method to capture that array and show usernames on a list.

Every 2.5 seconds the code refresh main container loading and reading a html file with chat log (time,username and text)

I want same refresh on online user's column, but now, i must call a method like get_all() and i have no idea how to do it.

fragment from my view

Code:

//Load the file containing the chat log
        function loadLog(){
            var oldscrollHeight = $("#chatbox2").attr("scrollHeight") - 20;
            $.ajax({
                url: "chat/log.html",
                cache: false,
                success: function(html){
                    $("#chatbox2").html(html); //Insert chat log into the #chatbox div
                    var newscrollHeight = $("#chatbox2").attr("scrollHeight") - 20;
                    if(newscrollHeight > oldscrollHeight){
                        $("#chatbox2").animate({ scrollTop: newscrollHeight }, 'normal'); //Autoscroll to bottom of div
                    }
                  },
            });
        };

        //Load the users online
        function loadUsr(){
            //document.getElementById('usuarios_chateando')[removed] = '/chatfinal/usuarios_online';
        };

        setInterval (loadLog, 2500);    //Reload file every 2.5 seconds
        setInterval (loadUsr, 2500);    //Reload file every 2.5 seconds

...and this is my Chat_model (i need usuarios_online() to show users online)

Code:

class Chat_model extends Model {
    function Chat_model()
    {
        parent::Model();
    }

    function cant_usuarios_online(){
      $consulta = $this->db->query('SELECT * FROM ci_sessions WHERE user_data<>"" ');
      return $consulta->num_rows();
    }

    function usuarios_online(){

      $consulta = $this->db->query('SELECT * FROM ci_sessions WHERE user_data<>"" ');

        //por cada registro muestro username y guardo la ip
        foreach ($consulta->result_array() as $fila)
          {
          //Le doy formato de array a user_data
          $usr = unserialize($fila['user_data']);
          //guardo la IP del usuario en la variable $ip
          $ip= $fila['session_id'];
          //muestro el username
          echo '<a href="chatfinal/whois/'.$ip.'" target="_blank">'.$usr['username'].'</a><br />';
          }
    }

    function bannear($session_id){
        //capturo datos dela sesion a bannear
        $query = $this->db->query("SELECT ip_address FROM ci_sessions WHERE session_id='".$session_id."'");
        if ($query->num_rows() > 0)
          {
             $row = $query->row();
             $this->db->set('ip', $row->ip_address);
             $this->db->insert('ci_ban');
          }
        }
}

I'll accept any idea, suggestion... everything is welcome
Thank and excuse my horrible english.