def get_firing_cells(df, cells, min_firing_frequency=1):
    """
    Only returns the cells that fire over a certain frequency
    (across entire recording)
    :param df:
    :param cells:
    :param min_firing_frequency: in hz
    :return: Cells that meet the criteria
    """
    total_time = df.index.max() - df.index.min()
    min_firing_total = min_firing_frequency * total_time

    new_cells = []
    for cell in cells:
        total = df[cell].sum()
        if total > min_firing_total:
            new_cells.append(cell)

    return new_cells