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SysTick—系统定时器是属于CM4内核中的一个外设，内嵌在NVIC中。系统定时器是一个24bit的向下递减的计数器， 计数器每计数一次的时间为1/SYSCLK，对于APM32F402，我们设置系统时钟SYSCLK等于120M。当重装载数值寄存器的值递减到0的时候，系统定时器就产生一次中断，以此循环往复。
因为SysTick是属于CM4内核的外设，系统定时器一般用于操作系统，用于产生时基，维持操作系统的心跳。
Systick在arm cortex内核特有的片上外设，实现基本的延迟控制LED闪烁也比较简单。首先修改系统时钟为120 MHz- uint32_t SystemCoreClock = 120000000;
设置Systick时基为1ms:
- SysTick_Config(SystemCoreClock/1000);
- /**
- \brief System Tick Configuration
- \details Initializes the System Timer and its interrupt, and starts the System Tick Timer.
- Counter is in free running mode to generate periodic interrupts.
- \param [in] ticks Number of ticks between two interrupts.
- \return 0 Function succeeded.
- \return 1 Function failed.
- \note When the variable <b>__Vendor_SysTickConfig</b> is set to 1, then the
- function <b>SysTick_Config</b> is not included. In this case, the file <b><i>device</i>.h</b>
- must contain a vendor-specific implementation of this function.
- */
- __STATIC_INLINE uint32_t SysTick_Config(uint32_t ticks)
- {
- if ((ticks - 1UL) > SysTick_LOAD_RELOAD_Msk)
- {
- return (1UL); /* Reload value impossible */
- }
- SysTick->LOAD = (uint32_t)(ticks - 1UL); /* set reload register */
- NVIC_SetPriority (SysTick_IRQn, (1UL << __NVIC_PRIO_BITS) - 1UL); /* set Priority for Systick Interrupt */
- SysTick->VAL = 0UL; /* Load the SysTick Counter Value */
- SysTick->CTRL = SysTick_CTRL_CLKSOURCE_Msk |
- SysTick_CTRL_TICKINT_Msk |
- SysTick_CTRL_ENABLE_Msk; /* Enable SysTick IRQ and SysTick Timer */
- return (0UL); /* Function successful */
- }
中断函数中处理计数变量：
- /*!
- * [url=home.php?mod=space&uid=247401]@brief[/url] This function handles SysTick Handler
- *
- * @param None
- *
- * @retval None
- *
- */
- void SysTick_Handler(void)
- {
- timeElasped_ms++;
- }
主循环：
- while (1)
- {
- if(timeElasped_ms%1000 == 0){
- BOARD_LED_Toggle(LED3);
- printf("\r\n timeElaspsed_ms = %d", timeElasped_ms);
- }
- }
实物演示：
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