Active Record Update Sum 1

[eluser]Fantass[/eluser]
Im trying to each time that access to the method updade +1 like a counter.. This is what i got

Code:

$this->db->where('id',$row->id);
        $update = array(
            'counter' => +1,
        );
        $this->db->update('table', $update);

But instead sum 1 write '1' In te row.
Thanks