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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 MHzuint32_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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