SysTick功能总结

一、初始化SysTick

按1ms来设置systick，也可以除以1000000。按1us来设置

			SysTick_Config(SystemCoreClock / 1000); //SysTick开启系统tick定时器并初始化其中断，1ms

二、systick的函数

			// cycles per microsecond
			static volatile uint32_t usTicks = 0;
			// current uptime for 1kHz systick timer. will rollover after 49 days. hopefully we won't care.
			volatile uint32_t sysTickUptime = 0;

			// SysTick
			void SysTick_Handler(void)
			{
			    sysTickUptime++;
			}

			//delay ms
			void DelayMs(uint16_t nms)
			{
			    uint32_t t0=micros();
			    while(micros() - t0 < nms * 1000);
			}

			// Return system uptime in microseconds (rollover in 70minutes)
			//返回 us
			uint32_t micros(void)
			{
			    register uint32_t ms, cycle_cnt;
			    do {
			        ms = sysTickUptime;
			        cycle_cnt = SysTick->VAL;
			    } while (ms != sysTickUptime);
			    return (ms * 1000) + (usTicks * 1000 - cycle_cnt) / usTicks;
			}

			// Return system uptime in milliseconds (rollover in 49 days)
			uint32_t millis(void)
			{
			    return sysTickUptime;
			}

systick启动后会进入SysTick_Handler()的中断，

三、少了重要的一部，哭~T.T

开启RCC时钟！！！！！

			void cycleCounterInit(void)
			{
			    RCC_ClocksTypeDef clocks;
			    RCC_GetClocksFreq(&clocks);
			    usTicks = clocks.SYSCLK_Frequency / 1000000;
			}

遇到的错误：我在初始化MPU6050时，写入数据特别慢。明明计时都准的，不知道为什么特别慢。其他还都能顺利运行。
最后发现时RCC的时钟有问题。