在编写单片机程序时,我们经常要用到延时函数,记得我们在学51单片机时,第一个延时是通过软件延时来实现的,代码如下:
void delay(volatile uint32_t cnt)
{
volatile uint32_t i = 0;
volatile uint32_t j = 0;
for(i = 0;i<cnt;i++)
for(j = 0;j<1000;j++);
}
软件延时的优点是方便,然而却不能提供很精确的延时,而且占用CPU资源,下面的以STM32L1为例,采用定时器方式实现,可以提供很精确的延时。代码如下:
/* CPU主频:32MHz Timer2_clk = 32MHz */
void Delay_Init(void)
{ /* TIM2 clock enable */
RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM2, ENABLE);
}
void Delay_ms(uint16_t ms)
{
TIM_TimeBaseInitTypeDef TIM_TimeBaseStructure;
TIM_TimeBaseStructure.TIM_Period = ms - 1;
TIM_TimeBaseStructure.TIM_Prescaler = 32000 - 1;
TIM_TimeBaseStructure.TIM_ClockDivision = TIM_CKD_DIV1;
TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
/*每当初始化计数器会使计数器更新,TIM_FLAG_Update被置位*/
TIM_TimeBaseInit(TIM2, &TIM_TimeBaseStructure);
TIM_ClearFlag(TIM2, TIM_FLAG_Update);
TIM_Cmd(TIM2, ENABLE);
while (TIM_GetFlagStatus(TIM2, TIM_FLAG_Update) == RESET);
TIM_Cmd(TIM2, DISABLE);
}
void Delay_us(uint16_t us)
{
TIM_TimeBaseInitTypeDef TIM_TimeBaseStructure; /* Time base configuration */
TIM_TimeBaseStructure.TIM_Period = us - 1;
TIM_TimeBaseStructure.TIM_Prescaler = 32 - 1;
TIM_TimeBaseStructure.TIM_ClockDivision = TIM_CKD_DIV1;
TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
TIM_TimeBaseInit(TIM2, &TIM_TimeBaseStructure);
TIM_ClearFlag(TIM2, TIM_FLAG_Update);
TIM_Cmd(TIM2, ENABLE);
while (TIM_GetFlagStatus(TIM2, TIM_FLAG_Update) == RESET);
TIM_Cmd(TIM2, DISABLE);
}
调用延时函数时,我们每次对定时器的溢出时间进行设置,我们以更新标志置位作为延时结束。注意上面函数的参数不能为1,如果为1会使函数跑死。当然这个延时函数也是阻塞式的。