Part V: STM32 system timer and general timer programming

1. Timer concept

Timer is essentially a counter , Count from a certain value , When the count value reaches a certain value, the timing time is . The timing time is determined by the counting range （ Initial count ） And the speed of counting （ Current count ） To decide . 

Use a timer to make a delay or heartbeat clock , The main steps include starting , Determine the reference clock frequency , Determine the initial count , Query timeout flag , As shown in the figure below .

 

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2.systick ---------------------- System timer

systick It's a simple timer , Belong to ARM nucleus , It is often used for precise delay , It can also be used as the system heartbeat clock .

（1）cortex-M4 System timer of kernel ( Refer to authoritative guidelines 9.5 section )

systick Is a count down ,24 position (0xffffff), With automatic reload function （ After a fixed time , Automatically write an initial count value , There is no need for developers to re initialize the assignment ） Simple timer . The clock source is provided externally , You can choose 8 Frequency division AHB The clock (21MHz) perhaps HCLK(168MHz).

Systick The register organization is as follows , Generally, the registers that need to be configured are control and status registers （CTRL）, Reload the load value register （LOAD）, Current value register （VAL） These three .

 

 

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3. Use systick Achieve accurate delay

First step , Select the reference clock （21MHzAHB Clock or HLCK The clock ）;

The second step , Set the initial count value to determine the timer cycle ( Delay Time );

The third step , Enable timer ;

Step four , The query timeout flag waits for the delay time to .

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4.Sydtick Timer code implementation

void delay_init(void)

{
         // choice 21MAHB Clock as clock source

//SYSTICK Control and status register second position 0

//SysTick->CTRL &= ~(0x01<<2);   

//SysTick->CTRL &= ~SysTick_CTRL_CLKSOURCE_Msk;

         // Library function writing

         SysTick_CLKSourceConfig(SysTick_CLKSource_HCLK_Div8);

}

// choice 21MHz, 1um=21, 1ms=21000

// Millisecond delay 1ms = 21000  n<798 ------ (2^24)/21000

  

void delay_ms(unsigned int nms)

{
         // Calculate the initial value and put it into the initial value register , Calculate from the initial value

         SysTick->LOAD = 21000*nms-1;

         SysTick->VAL = 0;// The current count value is 0

        

         // start-up systick Start timing

         SysTick->CTRL |= SysTick_CTRL_ENABLE_Msk;

        

         // The wait count is 0, Control register 16 Displacement 1

         while(!(SysTick->CTRL&SysTick_CTRL_COUNTFLAG_Msk));

        

         // close systick

         SysTick->CTRL &= ~SysTick_CTRL_ENABLE_Msk;

}

——————————————————————————————————————————————————————————————————————————————————————5. Universal timer

（1） Introduce

    The basic function of timer is timing , General timer belongs to peripheral , Usually used with other hardware , For example, periodically collect sensor data , periodic ADC transformation , Periodically report data , Combine the general timer with GPIO Combine to produce more functions (PWM, Pulse detection .....).

    stm32f407 Yes 14 Peripheral timers ,TIM2~TIM5 TIM9~TIM14 It belongs to general timer ,TIM1 and TIM8 It belongs to advanced timer ,TIM6 and TIM7 It belongs to basic timer .

With TIM2~TIM5 For example , Understand the use of general timer

 

Three counting modes ：

Count up ( Increasing )     0 ~ Initial count

Count down ( Decline )     Initial count ~ 0

Two way counting ( Increasing / Decline )     Initial count ~ 0 ~ Initial count

The original clock frequency of the general timer depends on the bus and the frequency division coefficient of the bus

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6. Timer (TIM2) Library function programming implementation

    Add the source code of timer library function in the project

 

（1） Turn on timer 2 The clock of

RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM2, ENABLE);

（2） Initialize the timer

void TIM_TimeBaseInit(TIM_TypeDef* TIMx, TIM_TimeBaseInitTypeDef* TIM_TimeBaseInitStruct); Parameters ：     TIMx - Which timer     TIM_TimeBaseInitStruct - Initialization structure      typedef struct {   uint16_t TIM_Prescaler;         /*!< The prescaled coefficients 0x0000 and 0xFFFF */   uint16_t TIM_CounterMode;       /*!< Count mode @ref TIM_Counter_Mode */   uint32_t TIM_Period;            /*!< Initial count 0x0000 and 0xFFFF.  */    uint16_t TIM_ClockDivision;     /*!< Clock factor @ref TIM_Clock_Division_CKD */   uint8_t TIM_RepetitionCounter;  /*!< Advanced timer function , Ignore */ } TIM_TimeBaseInitTypeDef;     

（3） initialization NVIC

NVIC_Init(...);

（4） Enable timer interrupt

void TIM_ITConfig(TIM_TypeDef* TIMx, uint16_t TIM_IT, FunctionalState NewState); Parameters ：     TIMx - Which timer     TIM_IT - Which interrupt TIM_IT_Update     NewState - Can make / prohibit ENABLE

（5） Implement timer interrupt processing function

void TIM2_IRQHandler(void) {     if(TIM_GetITStatus(TIM2, TIM_IT_Update)==SET){         //......                  // Clear interrupt flag         TIM_ClearITPendingBit(TIM2, TIM_IT_Update);     } }

（6） Start timer

TIM_Cmd(TIM2,ENABLE);
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