def find_pudc_b(db):
    """ Find PUDC_B pin func in grid, and return the tile and site prefix.

    The PUDC_B pin is a special 7-series pin that controls unused pin pullup.

    If the PUDC_B is unused, it is configured as an input with a PULLUP.

    """
    grid = db.grid()

    pudc_b_tile_site = None
    for tile in grid.tiles():
        gridinfo = grid.gridinfo_at_tilename(tile)

        for site, pin_function in gridinfo.pin_functions.items():
            if 'PUDC_B' in pin_function:
                assert pudc_b_tile_site == None, (
                    pudc_b_tile_site, (tile, site))
                iob_y = int(site[-1]) % 2

                pudc_b_tile_site = (tile, 'IOB_Y{}'.format(iob_y))

    return pudc_b_tile_site