Systick就是一个定时器而已，只是它放在了NVIC中，主要的目的是为了给操作系统提供一个硬件上的中断（号称滴答中断）。滴答中断？这里来简单地解释一下。操作系统进行运转的时候，也会有“心跳”。它会根据“心跳”的节拍来工作，把整个时间段分成很多小小的时间片，每个任务每次只能运行一个“时间片”的时间长度就得退出给别的任务运行，这样可以确保任何一个任务都不会霸占整个系统不放。或者把每个定时器周期的某个时间范围赐予特定的任务等，还有操作系统提供的各种定时功能，都与这个滴答定时器有关。因此，需要一个定时器来产生周期性的中断，而且最好还让用户程序不能随意访问它的寄存器，以维持操作系统“心跳”的节律。 只要不把它在SysTick控制及状态寄存器中的使能位清除，就永不停息。(Systick is just a timer, but it is put in NVIC. Its main purpose is to provide a hardware interrupt (known as a tick interrupt) to the operating system. Tick break? Here is a brief explanation. When the operating system is running, there will be a "heartbeat". It will work according to the beat of "heartbeat" and divide the whole time period into many small time slices. Each task can only run one time slice, so it has to quit to other tasks to run, so as to ensure that no task will occupy the whole system. Or give a certain time range of each timer cycle to a specific task, and various timing functions provided by the operating system are related to the tick timer. Therefore, a timer is needed to generate periodic interrupts, and it is better to keep the operating system "heartbeat" rhythm by preventing the user program from accessing its registers at will. As long as the enable bit in systick control and status register is not cleared, it will never stop.)