AsyncPostBack resets drop down selected Value

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ASP.NET

hi

i have some controls in update panel like text box, drop down list and all

i face a problem when my timer do Post back the selected value of dropdown list get reset here is my code

<asp:Timer ID="Timer1" runat="server" Interval="1500" Enabled="false">
            </asp:Timer>
    <asp:UpdatePanel ID="UpdatePanel1" runat="server">
        <Triggers>
            <asp:AsyncPostBackTrigger ControlID="Timer1" EventName="Tick" />
        </Triggers>
        <ContentTemplate>
                 Select a Device :
            <asp:DropDownList ID="ddlDevices" runat="server" Width="300px" />
time : <asp:TextBox ID="txtDate" runat="server" />
        </ContentTemplate>
        
    </asp:UpdatePanel>

 Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        If Not Page.IsPostBack Then
            txtDate.Text =now
            ' filling the Drop Down List From DataBase
            GenModule.FillCombo("MstVehicle", "nDevice_Id", "sDevice_Id", ddlDevices, False)
            
            'ddlDevices.SelectedIndex = 0

            Timer1.Enabled = True
            Timer1_Tick(Timer1, System.EventArgs.Empty)
        End If

End Sub
'---------------------------------------------------------
Protected Sub Timer1_Tick(ByVal sender As Object, ByVal e As System.EventArgs) Handles Timer1.Tick
        ' doing Some work like 
        ' retriving data from database
        ' and populating
End Sub

thats my Code

by default dropdown list selected index is 0 [thats First item in list]
when i try to select another item from list its not getting selected

can any one help me out with this plz
Thanks in Advance