<script type="text/javascript">
        $(document).ready(function(){
            $("#formulario").on("submit", function(e){
                e.preventDefault();
                var cdb = $("#cdb").val();
                $.ajax({
                    url: "cdb.php",
                    method: "POST",
                    dataType: "html",
                    data: cdb
                }).done(function(data){
                    $("#cdb").val("");
                    $("#cdb").focus();
                    listar();
                }).fail(function(data){
                    alert("Erro");
                });
            });
        });
</script>

<form id="formulario" method="POST" action="" accept-charset="utf-8">
        <div>
            <?php echo rand(1, 100) ?>
            <input id="cdb" type="text" name="cdb" class="validate" required autofocus onfocus="this.value='';">
            <button type="submit">GRAVAR</button>
        </div>
</form>

$cdb = filter_input(INPUT_POST,'cdb');