请问TM4C123的systick时钟如何使用

我想使用TM4C123的滴答时钟系统，但是发现找到的systick时钟的相关函数，但是我不知道计数值的对应关系
#if defined(TARGET_IS_TM4C123_RA1) ||
defined(TARGET_IS_TM4C123_RA3) ||
defined(TARGET_IS_TM4C123_RB1) ||
defined(TARGET_IS_TM4C129_RA0) ||
defined(TARGET_IS_TM4C129_RA1)
#define ROM_SysTickValueGet
((uint32_t (*)(void))ROM_SYSTICKTABLE[0])
#endif
#if defined(TARGET_IS_TM4C123_RA1) ||
defined(TARGET_IS_TM4C123_RA3) ||
defined(TARGET_IS_TM4C123_RB1) ||
defined(TARGET_IS_TM4C129_RA0) ||
defined(TARGET_IS_TM4C129_RA1)
#define ROM_SysTickEnable
((void (*)(void))ROM_SYSTICKTABLE[1])
#endif
#if defined(TARGET_IS_TM4C123_RA1) ||
defined(TARGET_IS_TM4C123_RA3) ||
defined(TARGET_IS_TM4C123_RB1) ||
defined(TARGET_IS_TM4C129_RA0) ||
defined(TARGET_IS_TM4C129_RA1)
#define ROM_SysTickDisable
((void (*)(void))ROM_SYSTICKTABLE[2])
#endif
#if defined(TARGET_IS_TM4C123_RA1) ||
defined(TARGET_IS_TM4C123_RA3) ||
defined(TARGET_IS_TM4C123_RB1) ||
defined(TARGET_IS_TM4C129_RA0) ||
defined(TARGET_IS_TM4C129_RA1)
#define ROM_SysTickIntEnable
((void (*)(void))ROM_SYSTICKTABLE[3])
#endif
#if defined(TARGET_IS_TM4C123_RA1) ||
defined(TARGET_IS_TM4C123_RA3) ||
defined(TARGET_IS_TM4C123_RB1) ||
defined(TARGET_IS_TM4C129_RA0) ||
defined(TARGET_IS_TM4C129_RA1)
#define ROM_SysTickIntDisable
((void (*)(void))ROM_SYSTICKTABLE[4])
#endif
#if defined(TARGET_IS_TM4C123_RA1) ||
defined(TARGET_IS_TM4C123_RA3) ||
defined(TARGET_IS_TM4C123_RB1) ||
defined(TARGET_IS_TM4C129_RA0) ||
defined(TARGET_IS_TM4C129_RA1)
#define ROM_SysTickPeriodSet
((void (*)(uint32_t ui32Period))ROM_SYSTICKTABLE[5])
#endif
#if defined(TARGET_IS_TM4C123_RA1) ||
defined(TARGET_IS_TM4C123_RA3) ||
defined(TARGET_IS_TM4C123_RB1) ||
defined(TARGET_IS_TM4C129_RA0) ||
defined(TARGET_IS_TM4C129_RA1)
#define ROM_SysTickPeriodGet
((uint32_t (*)(void))ROM_SYSTICKTABLE[6])
#endif
例如，装载值对应的是多长时间