Introduction - If you have any usage issues, please Google them yourself
SysTick (System Timer): SysTick timer is tied up in the NVIC, used to generate a SysTick exception (exception number: 15). In the past, the operating system and all systems using the time base must have a hardware timer to produce the desired " tick" interrupt, as the time base for the entire system. Tick interrupt the operating system is particularly important. For example, the operating system can assign multiple tasks for a different number of time slices, to ensure that no one task can occupy the system or a time range for each timer cycle given specific tasks, various timing functions provided by the operating system are related to this ticking timer. Therefore, you need a timer to generate periodic interrupts, and preferably also allows users to freely access the program can not register it, in order to maintain the operating system " heartbeat" of the rhythm.