function next()
{
    for (i = 1; i < 7; i++) {
	if (document.getElementById('cell_'+i).className == 'cell_hover') {
	    if (i == 6)
		i = 1;
	    else
		i++;
	    showscreen(i, 'none');
	    break;
	}
    }
}

showscreen('1', 'none');

toto = setInterval(next, 3000);