calendar extender attached to textbox

I have a textbox with calendar extender
  <asp:TextBox ID="txtpickupdate" runat="server"  Width="150px"></asp:TextBox>
<asp:CalendarExtender ID="CalendarExtender1" runat="server" Format="MM/dd/yyyy" PopupButtonID="txtpickupdate" TargetControlID="txtpickupdate">     </asp:CalendarExtender>
In this textbox only could select date from calendar extender, should not allow user to enter date. How to do that?