STM32SYSTICK system timer tick

1. systick is 24, decrement counter

2. systick interrupt priority can be configured, by default the lowest priority

3. SysTick_Config in coe_cm3.h header file to initialize a timer for configuring

4. LOAD to reload value, NVIC_SetPriority sets the interrupt priority, the lowest priority by default

5. SysTick-> VAL = 0UL clears the counter

6. SysTick-> CTRL for controlling the register, the default system clock source, enable interrupts                

              SysTick_CTRL_CLKSOURCE_Msk|

              SysTick_CTRL_TICKINT_Msk|

              SysTick_CTRL_ENABLE_Msk