PLL1FRACN was being set improperly. stm32h5xxx_rcc.c does not shift the value provided by board.h. So it was being set wrong. The defintions in stm32h5xxx_rcc.h shift the FRACN value and are now used by board.h. Also, board.h was not setting PLL1P properly. PLL1P can not have odd divisors. Therefore a value of 0 was invalid. Set it to a value of 1 (divide by 2), then adjust PLL1N to 31 and PLL1FRAC1 to 2048 to actually set SYSCLK to 250MHz.