Enter key press for focus change and check input before submit form

Question

I have a asp.net form content dropdownlist, textbox and button. I want to implement Enter key press to change focus and check input before submit form. I use onkeyup="EnterPressFocusChange(this, event)" for change focus and OnClientClick="return CheckInputData(this);" for check input data before submit. When I use button property UseSubmitBehavior="false" then change focus task working but checking input function always return true and button onCkick function calls. And when I use button property UseSubmitBehavior="true" then onCkick function calls when I press Enter Key press . How can I complete both task? Below the code that I try.

<script type="text/javascript">
        function CheckInputData(a) { 
            // If valid input return true
            // else return false
        }
        function EnterPressFocusChange(a, event) {
            if (event.which == 13) {
                // Change focus
            }
        }
    </script>

    <form runat="server">
        <asp:DropDownList ID="DropDownList1" runat="server" onkeyup="EnterPressFocusChange(this, event)" AutoPostBack="true"></asp:DropDownList>
        <asp:TextBox ID="tb1" Text="0" runat="server" onkeyup="EnterPressFocusChange(this, event)"></asp:TextBox><br />
        <asp:Button ID="btnFakeButton" runat="server" Text="Fake Button" OnClientClick="return CheckInputData(this);" UseSubmitBehavior="false" OnClick="btnFakeButton_Click"/>