Run Cronjob Codeigniter

How to execute a method of a class in Codeigniter via CronJob?

Ex: main.php accessed at link

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');

class Principal extends CI_Controller {

  public function __construct()
  {
    parent::__construct();
    //Load Dependencies     
  }

  public function listaCron()
  {     
     file_put_contents("teste.txt", "Crontab executado em: ".date(‘d/m/Y H:i’));
  }

}