Why the program does not work when I enable PLL on STM32F4?
I solved the problem by adding the code below. Thanks for all answers.
 
FLASH->ACR =
            FLASH_ACR_LATENCY_5WS               /* 6 CPU cycle wait */
          | FLASH_ACR_PRFTEN                    /* enable prefetch */
          | FLASH_ACR_ICEN                      /* instruction cache enable */
          | FLASH_ACR_DCEN;                     /* data cache enable */