In my application most of time the pages are getting refersh whn the control fires. for example if i calculate any calculation in textbox, whn i press tab button after refreshing the page my amswers are coming, then whn i click the cutton to invoke calender my page is refreshing, so i dont want to refresh my page all the time. would you refer any code to avoid refreshing?

<div>
    PAYMENT<br /><br />
    <table>        
        <tr>
            <td>Consulting Fees : <asp:TextBox ID="txtcon" Width="75" class="btn" runat="server"></asp:TextBox></td>
            <td>Tablet Cost : <asp:TextBox ID="txttab" Width="75" class="btn" AutoPostBack="true" OnTextChanged="txttab_TextChanged" runat="server"></asp:TextBox></td>
            <td>Total Amount : <asp:TextBox ID="txttot" Width="75" class="btn" runat="server"></asp:TextBox></td>
        </tr>
    </table> <br /><br />       
    <table> 
        <tr>
            <td>Payment Mode : <asp:DropDownList ID="pmtmode" AutoPostBack="true" OnSelectedIndexChanged="pmtmode_SelectedIndexChanged" runat="server">
            <asp:ListItem Text="Cash">Cash</asp:ListItem>
            <asp:ListItem Text="Card">Card</asp:ListItem></asp:DropDownList></td>
            <td><asp:Label ID="lblarno" Visible="false" runat="server" Text="ArNo. :"></asp:Label> <asp:TextBox ID="txtarno" Visible="false" Width="100" class="btn" runat="server"></asp:TextBox></td>
            <td>Amount Received : <asp:TextBox ID="txtamtrec" AutoPostBack="true" OnTextChanged="txtamtrec_TextChanged" Width="75" class="btn" runat="server"></asp:TextBox></td>
            <td>Balance Amount : <asp:TextBox ID="txtbalamt" Width="75" class="btn" runat="server"></asp:TextBox></td>
        </tr>
    </table> <br />       
    
</div>

 protected void txttab_TextChanged(object sender, EventArgs e)
        {
            txttot.Text = Convert.ToString(Convert.ToInt32(txtcon.Text) + Convert.ToInt32(txttab.Text));
        }

        protected void txtamtrec_TextChanged(object sender, EventArgs e)
        {
            txtbalamt.Text = Convert.ToString(Convert.ToInt32(txttot.Text) - Convert.ToInt32(txtamtrec.Text));
        }

<table border="0" cellpadding="1" cellspacing="0" style="margin:0 auto;" align="center" class="tableborder">
  <tr><td>&nbsp;</td> </tr> 
  <tr><td align="center">
   From :<asp:Button ID="btnfrmcal" OnClick="btnfrmcal_Click" runat="server" Text=">>" /><asp:Calendar ID="frmcal" OnSelectionChanged="frmcal_SelectionChanged" Visible="false" runat="server"></asp:Calendar></td>
   <td><asp:TextBox ID="txtfromDate" Width="90px" placeholder="From" name="txtfromDate" class="textboxes" Onfocus ="displayCalendar(this,'mm/dd/yyyy',this)" runat="server" ></asp:TextBox>
   <asp:RequiredFieldValidator ID="RequiredFieldValidator6" runat="server" ControlToValidate="txtfromDate" EnableClientScript="true" ValidationGroup="NeededForValidate" ForeColor="Red" Font-Bold="true" ErrorMessage="*"></asp:RequiredFieldValidator></td>
   <td>To :<asp:Button ID="btntocal" OnClick="btntocal_Click" runat="server" Text=">>" /><asp:Calendar ID="tocal" OnSelectionChanged="tocal_SelectionChanged" Visible="false" runat="server"></asp:Calendar></td>
   <td><asp:TextBox ID="txttoDate" Width="90px" placeholder="ToDate" name="txttoDate" class="textboxes" Onfocus ="displayCalendar(this,'mm/dd/yyyy',this)" runat="server" ></asp:TextBox>
   <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="txttoDate" EnableClientScript="true" ValidationGroup="NeededForValidate" ForeColor="Red" Font-Bold="true" ErrorMessage="*"></asp:RequiredFieldValidator></td>
   <td><asp:Button ID="btnfind" runat="server" Font-Bold ="true" Text="Find" ValidationGroup="NeededForValidate" CssClass="btn btn-primary" onclick="btnfind_Click"  /></td>
   </tr>
  <tr><td>&nbsp;</td> </tr> 
 </table>

 protected void frmcal_SelectionChanged(object sender, EventArgs e)
        {
            txtfromDate.Text = frmcal.SelectedDate.ToString();
            frmcal.Visible = false;
        }

        protected void tocal_SelectionChanged(object sender, EventArgs e)
        {
            txttoDate.Text = tocal.SelectedDate.ToString();
            tocal.Visible = false;
        }