Calling Procedure to populate drop down

hi,

I want to call a Procedure to populate drop down in a form.  Right now I  am using the code like this:

<td class="style4">Choose Category:&nbsp;<font color="red">*</font></td><td class="style3"><asp:DropDownList ID="DropDownList1" runat="server" DataSourceID="cat1"
                                                            DataTextField="catname" DataValueField="catname"></asp:DropDownList><asp:SqlDataSource ID="cat1" runat="server"
                                                            ConnectionString="<%$ ConnectionStrings:billingdatabase%>"
                                                            ProviderName="System.Data.SqlClient"
                                                            SelectCommand="SELECT [catname] FROM [Category]"></asp:SqlDataSource></td>

The Procedure in DB:

go
create procedure [dbo].[spshowproducts]
as
begin
select catname from Category 
end
GO

Please tell me how to call this procedure in the aspx code to populate the dropdown.