Popover in the footer.

On click of a button a popover opens using fragmentation but its opening downwards and the content is not visible.I want that popover should open toward upside. How can I do it?

<footer>

            <OverflowToolbar id="otbFooter">

                <ToolbarSpacer/>

                <OverflowToolbarButton text="Add" icon="sap-icon://add" press="popover" class="popover"/>

            </OverflowToolbar>

  </footer>