Smarty模板未加载更新的php值,继续加载缓存模板

I am using smarty 3.x I have the following template:

index.tpl

<html>
<body>Time: {$time}
</body>
</html>

and the file index.php

<?php

require 'Smarty.class.php';

$smarty = new Smarty();
$smarty->debugging = true;
$smarty->caching = true;
$smarty->setTemplateDir('/www/');
$smarty->setCompileDir('/compiled/');
$smarty->setCacheDir('/cache/');

$time= date("H:i:s");
$smarty->assign("time", $time);
$smarty->display("index.tpl");
?>

When I load the page the template is loaded displaying the time, upon refreshing the page the time is not updated (if i enable the debug in smarty the var $time is updated). Basically the page is loading from cache and not updating the time variable. I have looked through the documentation, but was not clear on what configuration I should be using to ensure these values are updated and the templates are optimally loaded (ie. performance).