STC_SYSCLK_DF_OFFSET
#define STC_SYSCLK_DF_MASK (0x1ff << STC_SYSCLK_DF_OFFSET)
#define STC_SYSCLK_DF(x) ((((x) - 1) << STC_SYSCLK_DF_OFFSET) & STC_SYSCLK_DF_MASK)