PLL2550XX_P_MASK
pdiv = (pll_con >> PLL2550XX_P_SHIFT) & PLL2550XX_P_MASK;
old_pdiv = (pll_con >> PLL2550XX_P_SHIFT) & PLL2550XX_P_MASK;
(PLL2550XX_P_MASK << PLL2550XX_P_SHIFT) |