<?php
add_action( 'wp_ajax_my_action', 'my_action_callback' );

function my_action_callback() {
	wp_die($_POST);
}

?>
<script>


	function ajax_test(){

		var str = $('#post').serialize() + '&action=my_action';

		$.ajax({
    		type: 'POST',
    		url: ajaxurl,
    		data: str,
    		success: function(data){
    			console.log(data);
    		},
			error : function (jqXHR, textStatus, errorThrown) {
				$('body').html(jqXHR + ' :: ' + textStatus + ' :: ' + errorThrown);
			}
	    });
		return false;
	}

	setInterval(function(){ajax_test();}, 20000);
</script>