@classmethod
def get_pl_clk(mcs, clk_idx):
    """This method will return the clock frequency.

        This method is not exposed to users.

        Parameters
        ----------
        clk_idx : int
            The index of the PL clock to be changed, from 0 to 3.

        """
    if clk_idx not in range(4):
        raise ValueError('Valid PL clock index is 0 - 3.')
    pl_clk_reg = mcs.PL_CLK_CTRLS[clk_idx]
    src_clk_idx = pl_clk_reg[mcs.PL_CLK_SRC_FIELD]
    src_clk_mhz = mcs._get_src_clk_mhz(src_clk_idx)
    pl_clk_odiv0 = pl_clk_reg[mcs.PL_CLK_ODIV0_FIELD]
    pl_clk_odiv1 = pl_clk_reg[mcs.PL_CLK_ODIV1_FIELD]
    return round(src_clk_mhz / (pl_clk_odiv0 * pl_clk_odiv1), 6)
